why do mayflies lay their eggs en masse on dry … · why do mayflies lay their eggs en masse on...

2273The Journal of Experimental Biology 201, 2273–2286 (1998)Printed in Great Britain © The Company of Biologists Limited 1998JEB1425

WHY DO MAYFLIES LAY THEIR EGGS EN MASSEON DRY ASPHALT ROADS?WATER-IMITATING POLARIZED LIGHT REFLECTED FROM ASPHALT ATTRACTS

EPHEMEROPTERA

GYÖRGY KRISKA1, GÁBOR HORVÁTH2,* AND SÁNDOR ANDRIKOVICS3

1Group for Methodology in Biology Teaching, Eötvös University, Budapest, Hungary, 2Department of BiologicalPhysics, Eötvös University, H-1088 Budapest, Puskin u. 5-7, Hungary and 3Department of Zoology, Eszterházy

Teachers’ Training College, Eger, Hungary*Author for correspondence (e-mail: [email protected])

Accepted 20 May; published on WWW 14 July 1998

lt,

We report on dry asphalt roads acting as ‘mayfly traps’;that is, they lure swarming, mating and egg-laying mayfliesin large numbers. To explain this surprising behaviour,we performed multiple-choice experiments withEphemeroptera in the field, and measured and comparedthe reflection–polarization characteristics of an asphaltroad and a mountain creek from which mayflies emerge.We show here that Ephemeroptera can be deceived by andattracted to dry asphalt roads because of the stronglyhorizontally polarized light reflected from the surface.Asphalt surfaces can mimic a highly polarized watersurface to Ephemeroptera. The darker and smoother theasphalt surface, the higher is the degree of polarization ofreflected light and the more attractive is the road tomayflies. We show that mayflies detect water by means ofpolarotaxis; that is, on the basis of the partially andhorizontally polarized reflected light. Asphalt roads areexcellent markers for swarming Ephemeroptera because of

their conspicuous elongated form; the sky above them isusually open, which is the prerequisite of mayfly mating,and the higher temperature of the asphalt prolongs thereproductive activity of mayflies. These additional factorsenhance the attractiveness of asphalt roads to swarmingmayflies. Thus, asphalt roads near ephemeropteranemergence sites (lakes, rivers and creeks) are a greatdanger for mayflies, because eggs laid on the asphaltinevitably perish. Asphalt roads can deceive and attractmayflies en masselike the ancient tar pits and asphalt seepsor the recent crude or waste oil lakes deceive, lure and trappolarization-sensitive water-seeking insects in largenumbers.

Key words: mayfly, Ephemeroptera, reproductive behaviour, aspharoad, insect trap, water detection, polarotaxis, polarization visionreflection polarization, video-polarimetry.

Summary

ctseets

nnlyore as

ingynytheer,ds,erleshee-are

During the last decade, we have observed every year individuals of several mayfly (Ephemeroptera) specswarmed in large numbers, mated above and landed onasphalt roads in the immediate vicinity of their emergence s(mountain streamlets), and that after copulation the femalaid their eggs en masseon the roads (Fig. 1A–C) instead oovipositing them on the water surface. These observatioespecially for egg-laying by females, suggest that the mayflwere apparently deceived by and attracted to the aspsurface, which acts as an insect trap.

Previous descriptions of ephemeropteran swarming, maand egg-laying behaviour have largely ignored misinterpreted this enigmatic phenomenon. We have ofobserved that mayflies also swarm, mate above and oviposthe shiny bodywork and windscreen of cars. The sareproductive behaviour was frequently observed above andshiny black plastic sheets used in agriculture (Fig. 1D–These artificial shiny surfaces also attract many other wa

Introduction

thaties dryiteslesfns,ieshalt

tingortenit onme onI).ter

insects. There are numerous observations of water insebeing deceived by glass panes, car roofs or wet asphalt str(Fernando, 1958; Popham, 1964).

Although the above-mentioned observations oEphemeroptera are known to entomologists, they are omentioned sporadically as marginal notes in publications lectures. It has generally been assumed that the roads servmarkers for mayflies to assign the site of swarming and mat(e.g. Brodskiy, 1973; Savolainen, 1978). Oviposition bmayflies on asphalt roads is simply explained by the shiappearance of wet roads which may lure the insects like surface of real water bodies. The first interpretation, howevcannot apply to the observed egg-laying on asphalt roabecause mayflies normally oviposit exclusively on the watsurface and not on markers. However, males and femaswarming and mating above asphalt roads perform tbehavioural elements (e.g. egg-laying flight, frequent surfactouching manoeuvres and dropping onto the surface) that

2274

rs for

gecist ofheerithte.ms anassus,gioninghe

ur

G. KRISKA, G. HORVÁTH AND S. ANDRIKOVICS

Fig. 1. Examples of mayflies deceived by and attracted to a dry asphalt road (A–C) and to the shiny black plastic sheet used in agriculture (D–I)in the immediate vicinity of a mountain creek near Budapest, Hungary, during May/June 1997. (A) A male Rhithrogena semicolorata. (B) Afemale Epeorus silvicola. (C) A female and two male Epeorus silvicolaattempting to mate. (D) A male Rhithrogena semicolorata. (E) Acopulating pair of Rhithrogena semicolorata. (F) A female and two male Rhithrogena semicolorataattempting to mate. (G) An ovipositingRhithrogena semicolorata. (H) An ovipositing Ephemera danica. (I) A male Baetis rhodani.

characteristic above water surfaces. The second interpretacannot explain why egg-laying by Ephemeroptera afrequently occurs on totally dry asphalt surfaces. In the presstudy, we give a possible explanation for this surprisibehaviour.

The mating of mayflies is preceded by a peculiar swarmbehaviour, during which a group of insects maintainsstationary position with respect to an element of the landsccalled the ‘marker’. Concentration of the males in the swaand its constant position are particularly important for mayflsince their sexually mature stage is of very short durat(Brodskiy, 1973). Markers can be large objects of relativerare occurrence: the shores of lakes, roads or rows of littoplants, for example (Savolainen, 1978). Because of the shlived adult stage and the fact that the newly moulted admayflies can dry out quickly, during swarming the mayflieremain relatively close to the water basin in whicdevelopment of the nymphal stage takes place. Thus, iessential for the markers to be near water. This is why the of markers in ephemeropteran swarming has been intensistudied and why it became a widespread view that asp

tionlsoentng

ing aapermiesionlyral

ort-ultsht isrolevelyhalt

roads near ephemeropteran emergence sites can be markeswarming and mating.

Discovering the causes of the above-mentioned stranbehaviour of mayflies may be important not only for scientifistudies of Ephemeroptera, but also for the protection of thinsect group since the huge number of eggs (an egg-packea female mayfly contains 6000–9000 eggs) laid onto tasphalt roads do not survive. Mayflies are in great dangbecause their aquatic habitat is becoming more polluted wherbicides, pesticides, excess fertiliser and industrial wasAlmost all mayfly species are threatened, and many of thehave suffered a severe decline during the last decades aresult of habitat destruction by agricultural and urbadevelopment and land drainage. As a consequence, mswarming of Ephemeroptera is now a rare phenomenon. Thit is particularly important to determine whether the egg-layinof mayflies on asphalt roads can be prevented. Little attenthas been paid to this aspect of ephemeropteran swarmbehaviour despite the considerable attention paid to tscientific study of swarm formation.

In an attempt to clarify the causes of reproductive behavio

2275Mayfly egg-laying on asphalt roads

set

ofndthehe

rek

and

d

–in,s

le-edsd

ee

ekesheire.ek

edennich

anofce inly

theity in

msewg,

of mayflies on asphalt roads, a 2 year study was conductesix species of mayfly, using visual observations, vidrecordings, multiple-choice experiments and videpolarimetric measurements in the field. On the basis of investigations, we propose a new interpretation for the pecubehaviour of Ephemeroptera on asphalt roads. Our explanais that asphalt roads mimic a highly and horizontally polarizwater surface to water-seeking mayflies which, as we shhere, detect water by means of the horizontally polarizreflected light, like many other water insects (Schwind, 191991, 1995; Schwind and Horváth, 1993; Horváth, 199Horváth and Zeil, 1996; Horváth and Varjú, 1997).

Materials and methodsMultiple-choice experiments with swarming mayflies using

different test surfaces

Although we frequently observed the reproductivbehaviour of Ephemera danica(Müll.), Ecdyonurus venosus(Fabr.), Epeorus silvicola (Etn.), Baetis rhodani (Pict.),Rhithrogena semicolorata(Curt.) and Haproleptoides confusa(Hag.) above dry asphalt roads, we performed multiple-choexperiments only with Epeorus silvicolaand Rhithrogenasemicolorata. Our experiments were carried out in late Maand early June of 1996 and 1997 near the village of Dömörklocated approximately 30 km from Budapest, Hungary. Ostudy site was the bank of a typical reach of a mountain crecalled Bükkös patak, from which mayflies emerge in larnumbers and where they swarm during May and Ju(Andrikovics, 1991; Andrikovics and Kéri, 1991). In thimmediate vicinity (at a distance of 1–5 m) of the creek, asphalt road runs between trees and bushes almost paralthe water and in some places it crosses the stream over sbridges. The creek itself runs in a valley under trees and buand is usually completely shadowed by riparian vegetatiexcept where the road crosses it. The road is several mehigher than the creek, and above it the sky is open. The surof the asphalt road is relatively smooth and dark grey, but thare several patches of a lighter grey with a rougher surfac

In the multiple-choice experiments, we laid rectangular tsurfaces of different types onto the asphalt road at differreaches of the creek where mayflies swarmed. The 1 m×2 mtest surfaces were placed 0.5 m apart. The test surfaces wewere (i) shiny black plastic (polyethylene) sheet, (ii) shinwhite (milky) plastic (polyethylene) sheet, (iii) shinyaluminium foil, (iv) slightly shiny black cloth, (v) matt blackcloth and (vi) matt white cloth. To avoid the influence of coloon the choice of mayflies, the test surfaces were composeneutral grey (uncoloured) reflecting materials. On seveoccasions, we counted the number of mayflies landing on swarming immediately above (height no more than 0.1 m) am×0.1 m rectangular region of the test surface. The positionthe test surfaces with respect to each other was chanrandomly in order to avoid the possible influence of thposition on the number of mayflies attracted.

Our experiments were always carried out under clear sk

d oneoo-ourliartionedowed

85,5;

e

ice

yapuurek,

gene

eanlel tomall

sheson,tres

faceeree.estent

usedy

urd ofraland 0.1 ofged

eir

ies.

At the beginning of an experiment, the landscape wailluminated by direct light from the setting sun, and after sunsby skylight from above.

Both visual observations and video-recordings were madethe swarming behaviour of mayflies above the asphalt road athe test surfaces. We also used photographs to document landing and egg-laying of mayflies on the asphalt road and ttest surfaces.

During the experiments, we measured the watetemperature, the air temperature immediately above the creand the asphalt road, and the temperature of the asphalt test surfaces.

Video-polarimetric recordings of an asphalt road, a creek anthe test surfaces

Using video-polarimetry, we measured the reflectionpolarization characteristics of some reaches of a mountacreek (from which mayflies emerge and where they swarmmate and oviposit), an asphalt road (above which mayflieswarm every year) and the test surfaces (used in the multipchoice experiments described above). This method is describin detail by Horváth and Varjú (1997). In the case of scenewith flowing water, several digitized pictures were averageprior to the calculation of the reflection–polarizationcharacteristics in order to eliminate the effect of motion.

Using our video-polarimeter, we could measure thpolarization of light through the three colour channels of thvideo camera: red (R, wavelength λred=λmax=730±65 nm,mean ± S.D.), green (G, λgreen=600±65 nm) and blue (B,λblue=470±65 nm). Because the recorded scenes – the cre(the shore and bottom of which were covered by grey pebbland stones), asphalt and test surfaces – were colourless, tpolarization was practically independent of the spectral rangThe grey asphalt surface (wet or dry), test surfaces and crebed had the common spectral feature that they reflectapproximately equally the entire visible spectrum of thincident light, as do all neutral grey objects. Thus, whepresenting the measured reflection–polarizatiocharacteristics, we omit reference to the spectral range in whthe measurement was obtained.

ResultsThe swarming behaviour of the mayflies

Depending on the species, the swarming of mayflies begprior to and after sunset every evening from the beginning May until the end of June in both years. After the emergenof the insects from the mountain creek, the males gatheredseveral diffuse swarms in the air at a distance of approximate4–5 m from the ground. At the beginning of swarming, weobserved these relatively diffuse swarms everywhere above streamlet, asphalt road, dirt roads and clearings in the vicinof the emergence sites. Generally, these swarms developedplaces where the sky was visible. As time elapsed, the swargradually became nearer to the ground and more females flthrough them in order to copulate with the males. After matin

2276

kingss.forht

eveternasalt

al,92),andt fromeket

gg-ons,

out,ales

heir

seedndale

raningherce

ixnly

of

hite

kalt

one.nly

ingtic


Table 1.Air temperature and the number of Rhithrogenasemicolorata landing on a 0.1 m × 0.1 m area of three testsurfaces (a shiny black plastic sheet, a shiny white plastic

sheet, a shiny aluminium foil) for 30 s versus time on 23 May1996

Number of insects landing

Shiny ShinyAir black white Shiny

Time temperature plastic plastic aluminium(h) (°C) sheet sheet foil

19:06 25.5 1 0 019:09 25.5 3 1 019:12 25.0 4 0 019:32 24.0 8 0 019:35 24.0 9 2 019:41 23.5 13 1 020:09 21.5 16 0 020:20 21.0 33 1 020:25 20.5 57 1 220:33 20.0 97 1 020:40 19.0 166 0 020:48 18.0 85 0 220:56 17.0 29 2 021:02 16.0 9 1 0

the females returned to the streamlet or landed on the asproad and laid their eggs on the water or asphalt surface.

Later, as the air temperature and intensity of ambient lidecreased, the swarms gradually left the dirt roads aclearings. We then observed swarming mayflies exclusivabove the asphalt road and those reaches of the creek opthe sky. In these swarms, both the males and females periodically up and down, displaying the species-specnuptial dances (see Fischer, 1992), or flew parallel to the wor asphalt surface against the prevailing breeze. Thfrequently touched the water or asphalt surface, or droponto it for a few seconds. When the air temperature decreato below approximately 14–15 °C and the light intensity wlow, mayfly swarming suddenly ceased, and the insedisappeared from both the water and asphalt surfaces. Tthen landed on the leaves of neighbouring trees, bushesgrass in order to roost.

All six mayfly species observed showed the same behavabove and on the asphalt road as at the water surface.density of swarming, mating and ovipositing mayflies whighest above those patches of the asphalt road wheresurface was smoother and darker than the surrounding regNo reproductive behaviour occurred above the relatively liggrey or rough spots of the asphalt. One of the most typreactions of female mayflies to the smooth, black asphpatches was the following: after copulation in the air, tfemales arrived above one of these patches. First, they across the patch, then suddenly turned back at its border,in the presence of a gentle breeze, they all flew into the breFemales touched the patch several times and landed on lay their eggs. Thus, we assume that the darker and smoothe asphalt, the greater is its attractiveness to water-seemayflies. Experiments to test this hypothesis are in progre

Above the asphalt road, we observed two types of flight the six Ephemeroptera species, which are typical fligmanoeuvres usually found only above a water surface. (i) Egg-laying flight of females. The females, generally facing into thslight breeze, flew to and fro parallel to and immediately abothe asphalt surface, dancing up and down in a zig-zag patand sometimes touching the asphalt. This type of flight wshown only by females above the middle part of the asphroad. During egg-laying flight, the females showed a typicspecies-specific stereotypical flight pattern (see Fischer, 19which resembled the nuptial dance of the swarming males occurred simultaneously with it. As egg-laying flighprogressed, an increasing number of eggs was pressed outthe genitalia of the females. At the end of this flight, thfemales landed on the asphalt and laid their egg-pac(Fig. 1G). In the case of Ephemera danica, the females landedon the asphalt and remained on it until their elongated epacket had been pressed out and laid (Fig. 1H). The functiof egg-laying flight are finding an optimal site for ovipositionand/or allowing a larger number of eggs to be pressed and/or acting as a defence against attacks by swarming m(Fischer, 1992). (ii) Water-touching manoeuvres of males. Themales also periodically touched the asphalt surface during t

halt

ghtnd

elyen toflewificaterey

pedsed

asctshey

and

iour Theas theions.ht

icalalt

heflew andeze.it tother

flight, usually facing into the wind. Some of the individualtouched the asphalt periodically only with their cerci whilflying up and down immediately above the road. Others landon the asphalt, stayed on it for 1 s and then took off, to laagain some seconds later. Similar water touching by mmayflies (e.g. Baetis vernus, Ecdyonurus venosus, Rhithrogenasemicolorataand Ephemera danica) was observed by Fischer(1992) above natural water surfaces at ephemeropteemergence sites. According to Fischer (1992), such touchof the water surface by male Ephemeroptera allows them eitto drink water or to test the height above the water surfausing the cerci.

Multiple-choice experiments with swarming mayflies

We performed the multiple-choice experiments with all smayfly species; however, quantitative data were gathered ofor Rhithrogena semicolorataand Epeorus silvicola, a typicalmedium- and large-sized mayfly species, respectively.

Table 1 shows the air temperature and the number Rhithrogena semicoloratalanding on a given region of threedifferent test surfaces (a shiny black plastic sheet, a shiny wplastic sheet and a shiny aluminium foil). Rhithrogenasemicoloratais attracted almost exclusively to the shiny blacplastic sheet. At the beginning of swarming above the asphroad (at approximately 19:00 h), only a few mayflies landed the black plastic, but their number increased rapidly over timAt 20:40 h, the reproductive activity reached its maximum othis plastic. Swarming ceased suddenly approximate20–30 min after this maximum because of the decreastemperature and the low light intensity. The shiny white plas


dith

forrerved. WeTheherireds and

to

utich ofallly

trol

asr

ieses4

heee

an

Table 2.Air temperature, temperature of the test surfaces andthe number of Epeorus silvicola landing on a 0.1 m × 0.1 m

area of three test surfaces (a shiny black plastic sheet, a shinywhite plastic sheet, a shiny aluminium foil) for 30 s versus

time on 3 June 1996


Temperature Shiny ShinyAir of the test black white

Time temperature surfaces plastic plastic Aluminium(h) (oC) (oC) sheet sheet foil

19:10 25.0 27.5 11 0 019:12 25.0 27 9 0 019:15 24.5 27 9 0 019:19 24.0 26.5 24 0 019:22 23.5 26 26 1 019:25 23.0 25.5 19 1 019:29 22.0 24.5 16 0 020:03 21.5 23.5 3 0 0

Table 3.The number of Rhithrogena semicoloratalanding ona 0.1 m × 0.1 m area of three test surfaces (a shiny black

plastic sheet, a slightly shiny black cloth, a matt white cloth)for 30 s versus time on 17 June 1996


Time Shiny black Slightly shiny Matt (h) plastic sheet black cloth white cloth

19:33 25 6 019:38 18 3 019:43 20 3 219:48 25 2 019:53 23 5 119:58 22 4 020:03 24 4 020:08 16 3 020:13 23 4 020:18 21 4 0

The slightly shiny black cloth reflected partially horizontallypolarized light. Its degree of polarization was much lower than thatof the shiny black plastic sheet (see Table 6; Fig. 4).

Table 4.The number of Epeorus silvicolalanding on andflying immediately above (within a height of 0.1 m) a 0.1 m ×

0.1 m area of three test surfaces (a shiny black plastic sheet, amatt black cloth, a matt white cloth) for 30 s versus time on 6

May 1997


Time Shiny black Matt Matt (h) plastic sheet black cloth white cloth

20:12 13+50 0+3 0+020:14 20+150 1+2 0+020:20 160+170 0+0 0+020:27 32+32 0+0 0+020:30 16+10 0+0 0+0

The numbers of mayflies are given in the format a+b, where a isthe number of insects landing on the surface and b is the number ofinsects swarming above it.

sheet and the aluminium foil were not attractive to Rhithrogenasemicolorata. The very small number of mayflies observelanding on these test surfaces is negligible in comparison wthe number landing on the black plastic sheet.

As Table 2 demonstrates, similar results were obtainedEpeorus silvicola. To preclude the possibility that temperatudifferences between the test surfaces resulted in the obsepatterns, we measured the temperature of the test surfacesfound no temperature differences between the surfaces. temperature of the test surfaces was always significantly higthan the air temperature above the asphalt road (Table 2; pat-test, P<0.01). Both temperatures decreased gradually afunction of time, because both the swarming of mayflies athe multiple-choice experiments began immediately priorsunset.

In the first series of multiple-choice experiments (carried oin 1996), we found that the shiny black plastic sheet (whreflected light specularly, i.e. in such a way that the angleincidence is equal to the angle of reflection, and only a smamount of light is reflected in other directions) was the onattractive surface for all six mayfly species studied. As consurfaces, we used a slightly shiny black cloth and a matt wcloth that reflected light diffusely (that is, in all directions). Thresults of the control experiment are presented in Table 3Rhithrogena semicolorata. Again, the shiny black plastic sheewas significantly more attractive than the cloths (paired t-test,P<0.001). The white cloth was unattractive; however, the blacloth attracted a small number of mayflies. The reason for was that this black cloth was slightly shiny. This is discussbelow in detailing the reflection–polarization characteristicsthe test surfaces.

In 1997, we used totally matt black cloth as one of tcontrol surfaces. The other two test surfaces were a shiny bplastic sheet and a matt white cloth. The number of Epeorussilvicola swarming immediately above and landing on thesurfaces is given in Table 4. The matt black and white surfa

hitee

fort

ckthised of

helack

seces

were unattractive to mayflies; the shiny black plastic sheet wthe only attractive surface. A similar result was found foRhithrogena semicolorata(Table 5). In this species, themajority of mayflies observed on the cloths and the aluminiumfoil were copulating pairs; they began to mate while still in theair and dropped accidentally onto these surfaces. The mayflobserved on the black plastic sheet were mainly single malor egg-laying females, but copula were also abundant. Tablealso shows the typical pattern observed in all six species: at tbeginning of swarming, only a few mayflies landed on thshiny black plastic sheet, but later almost every member of thswarm landed on it periodically. At the end of swarming, weobserved that more individuals had settled onto the plastic thwere flying above it.

2278

o

s

e

t

in

er

ndvethernue itedeired ofor

the

ndy.ome

ndtheas

nghe

rghing°C.ed

lymaltceceseal

e).ck

hesen

heirm theeesfter


Table 5.The number of Rhithrogena semicoloratalanding ona 0.1 m × 0.1 m area of four test surfaces (a shiny black

plastic sheet, a matt black cloth, a matt white cloth, a shinaluminium foil) for 30 s versus time on 11 May 1997


Shiny Matt Matt ShinyTime black black white aluminium(h) plastic sheet cloth cloth foil

19:10 6 0 0 019:15 8 0 1 019:20 8 0 1 019:25 11 0 0 119:39 12 1 0 019:43 13 1 0 219:47 12 0 0 019:51 21 0 0 019:54 20 3 0 019:57 18 4 0 120:00 26 4 0 420:03 23 3 0 020:06 28 2 0 020:09 31 2 0 120:12 35 0 0 120:13 29 2 0 020:15 60 2 0 120:16 63 5 0 120:21 64 0 0 020:25 63 0 0 020:28 58 0 0 020:31 26 0 0 020:34 8 0 0 020:37 8 0 0 0

The landing of mayflies on the black plastic sheet wasintensive that we could hear the loud strikes of the insect bodsimilar to rain drops rattling on the plastic. If we covered apart of the black plastic sheet with a piece of any other tsurface, then reproductive activity of mayflies ceased abthis region, but not above the surrounding sheet. When piece of the other test surface was removed, reproducbehaviour of the insects above this part of the black plasheet recommenced.

Displacing the black plastic sheet

To demonstrate the strong preference of swarming mayflfor the shiny black plastic sheet, we lifted the black plassheet above which mayflies swarmed in large numbers moved it slowly such that its surface remained horizontal. Tswarming mayflies followed the slowly moving plastic. Whethe black plastic sheet with the cloud of swarming mayflies wmoved above one of the other test surfaces and then the bplastic was quickly removed, the mayfly cloud dissipatrapidly. When the black plastic was replaced on tunattractive test surface, the mayflies returned and quicdeveloped a swarm. If the black plastic was held vertically, mayflies did not swarm over or next to it, nor did they follo

soies

nyestvethetivetic

iesticandhenaslackd

heklyhew

its movement. The same was true for all other test surfacesthis experiment.

Transferring the mayflies from the black plastic sheet to othtest surfaces

Using a hand net, we captured mayflies (single males afemales, egg-laying females, copulating pairs) swarming abothe black plastic sheet and released them onto one of the otest surfaces. We observed that these mayflies did not contitheir reproductive activity on the new test surface, but leftand returned to the black plastic. However, if we transferrthem to another black plastic sheet, they began threproductive behaviour again, showing that the capturmayflies did not fly away from the new test surface becausethe netting procedure, but because of the unattractive repellant nature of the test surface.

The influence of temperature on the reaction of mayflies to test surfaces

The water temperature of the creek was between 12 a14 °C, and did not change during swarming on a given daThe air temperature above the creek (at a distance of 1 m frthe water surface) was significantly higher than that of thwater (paired t-test, P<0.001) and decreased fromapproximately 20–22 °C to 14–15 °C between the start aend of swarming each day. The air temperature above asphalt road (at a distance of 1 m from the surface) wsignificantly higher still (paired t-test, P<0.001) anddecreased from approximately 25–26 °C to 16–17 °C duriswarming (Table 1). The warmest location was always tasphalt road and the test surfaces on it (Table 2; paired t-test,P<0.01).

The swarming of mayflies began immediately prior to oafter sunset when the air temperature was still relatively hiabove both the asphalt surface and the creek. The swarmceased when the air temperature decreased below 14–16The higher air temperature above the asphalt road prolongthe reproductive behaviour of mayflies by approximate15 min in comparison with the reaches of the creek frowhich the sky was visible, presumably making the asphmore attractive to mayflies than the creek. However, sinthere was no temperature difference among the test surfainvestigated, the different reactions of mayflies to thdifferent test surfaces cannot be explained by their thermperception. Similarly, a role of olfaction in the choice of thtest surface by mayflies can be excluded (see belowMayflies must have preferred the asphalt road and the blaplastic sheet and avoided the other test surfaces because tsurfaces were visually attractive, non-attractive or everepellant.

Mayflies roosted on the leaves of trees and bushes after treproductive activity. To study the role of the substratuchosen by the insects as a roosting place, we again usedtest surfaces, which were laid onto the ground beneath trand bushes on the bank of the creek. We observed that, aswarming, the mayflies landed en massenot only on the shiny

y


sehees,.m).hemlesad

nadtedtheeemwehehtheeree

eione

theere

ery. of

gck

taltnd

e

ths.t

black plastic sheet, but also on the other test surfairrespective of their type (using paired t-test, there were nosignificant differences between surfaces chosen). Tbehaviour of roosting mayflies was, however, quite differefrom the behaviour observed during swarming. Roostimayflies did not dance, fly up and down, or oviposit on the tsurfaces, but simply settled on them and remained motionlapparently using the test surfaces as roosting places and nreproduction sites. Because of the lower temperature, roosting of mayflies on the shore of the creek began earlier tat the border of the warmer asphalt road.

Reflection–polarization characteristics of the swarming siteof mayflies

From the above observations, it is clear that mayflies sereproduction sites predominantly on the basis of visual cu(see also Discussion). We can hypothesize that the detecof water surfaces as oviposition sites occurs by polarotaxisin many other water insects (Schwind, 1985, 1991, 199Thus, using video-polarimetry, we determined threflection–polarization characteristics of several reaches omountain creek and compared them with those of an asproad and the test surfaces.

Reaches of a mountain creek

Fig. 2 shows the measured reflection–polarizatiocharacteristics of three different reaches of a mountain crfrom which mayflies emerge and where they swarm, mate oviposit yearly in large numbers. All three scenes hadslightly undulating water surface and were recorded fromdirection of view of the camera of 60 ° measured from tvertical, which is slightly larger than the Brewster angle asphalt (57.5 °) and water (53 °) with refractive indices 1.57 and 1.33, respectively. In the first reach of the creek (F2A, row 1), the water was relatively slow and calm andsmall pond was present in the shadow of trees. Throughfoliage, skylight illuminated the water surface from abovand to the right. The degree of polarization is high only those regions of the water surface that are illuminated by skylight (Fig. 2A, row 2). The other regions of the water anthe shore reflect practically unpolarized light. Because of undulation of the water surface, the degree of polarizatand the E-vector alignment (Fig. 2A, row 3) change strongfrom site to site on the water surface, giving a relatively brodistribution of these variables (Fig. 2A, rows 4, 5). The Evectors of light reflected from the water surface aapproximately horizontal but, because of the ripples on water surface, they can diverge strongly from this directi(Fig. 2A, rows 3, 5).

The second reach of the mountain creek was exposedskylight from above (Fig. 2B). The water flowed slowly amonstones and pebbles. Here, the degree of polarization of lreflected from the undulating surface of the turbulent water walso relatively low, and the dry stones and pebbles were largunpolarized (Fig. 2B, row 2). Thus, the spatial distributions the degree (Fig. 2B, row 2) and direction (Fig. 2B, row 3)

ces

hentngestess,ot asthehan

s

lectestion as5).ef ahalt

neekand a a

heofofig.

a theeinthed

theionlyad-

retheon

tog

ightaselyofof

polarization are very patchy and the histograms of thevariables are again relatively broad (Fig. 2B, rows 4, 5). In tcase of the third reach (Fig. 2C), the creek flowed under trebut its surface was illuminated by skylight from the sideConsequently, the degree of polarization of light reflected frothe water surface is relatively high (Fig. 2C, rows 2, 4However, similarly to the first and second reaches, both tdegree and direction of polarization of the light reflected frothe water surface change strongly because of the ripp(Fig. 2C, rows 2, 3), and their histograms are again bro(Fig. 2C, rows 4, 5).

Sections of an asphalt road

Fig. 3 shows the measured reflection–polarizatiocharacteristics of three different sections of the asphalt roabove and on which the investigated mayflies swarmed, maand oviposited. Analysing the patterns and histograms of degree and direction of polarization of light reflected from ththree sections of the asphalt road in Fig. 3 and comparing thwith those of the reaches of the mountain creek in Fig. 2, can establish several important points. The distribution of tdegree of polarization and the E-vector alignment of the ligreflected from the asphalt road is narrow; the E-vector of treflected light is predominantly horizontal and, apart from thlighter and rougher patches of the asphalt surface, the degof polarization is relatively high, in spite of the fact that thsurface was dry. We also measured the reflection–polarizatcharacteristics of light reflected from wet sections of thasphalt road after rain. We obtained similar results as for dry asphalt road; however, the degrees of polarization wsignificantly higher when the asphalt was wet (paired t-test,P<0.001; see Table 6; Fig. 4).

Test surfaces used in the multiple-choice experiments

Fig. 4 shows the reflection–polarization patterns of thdifferent test surfaces measured using video-polarimetTable 6 shows the measured relative brightness, degreepolarization δ and E-vector alignment α of light reflected fromthe test surfaces. From Fig. 4 and Table 6, the followinobservations can be made. Light reflected from the shiny blaplastic sheet (δ=55 %) and the wet asphalt (δ≈51 %) possessedthe highest degrees of polarization (P<0.001). The degree ofpolarization of light reflected from the dry asphalt (δ≈31 %)was still strong and much higher (P<0.001) than that from theslightly shiny black cloth (δ≈15 %), the matt black cloth(δ≈9 %) and the shiny white plastic sheet (δ=7.7 %). The mattwhite cloth (δ=3.3 %) and the shiny aluminium foil (δ=3.2 %)reflected practically unpolarized light.

Because of the approximately smooth and horizonreflecting surfaces, the direction of polarization of lighreflected from the wet and dry asphalt and the shiny black awhite plastic sheets was not significantly different fromhorizontal (P<0.001). The E-vectors of light reflected from thcloths differed significantly from the horizontal direction(P<0.001) because of the surface roughness of these cloThe shiny aluminium foil reflected the light such that it did no

2280

mlyite


change the degree and direction of polarization of the incidlight. Since the surroundings (sky and randomly oriented lblades of the vegetation) of the swarming sites and the sitthe multiple-choice experiments possessed randomly orien

enteafe ofted

E-vectors, the directions of polarization of light reflected frothe shiny aluminium foil were also random, and the relativelow degree of polarization changed strongly from site to sdepending on the direction of view.


e

r

anym.me asreofce.ealt

er

at

g

omeeed.

theing oft bealeereshesalt

ce, theis

verherd

Fig. 2. The reflection–polarization characteristics of three differereaches of a mountain creek (a typical emergence and swarmingof the mayflies studied) measured using video-polarimetry. All thrscenes with a slightly undulating water surface were recorded fromdirection of view of the camera of 60 ° measured from the vertic(A) In this relatively slow and calm reach of the creek, a small pois present, shadowed by trees. Through the foliage, skyligilluminated the water surface from above and to the right. (B) reach of the creek illuminated from above by the clear sky where water flowed slowly among stones and pebbles. (C) A reach whthe creek flowed under trees, but its surface was illuminated skylight from the side. Row 1 shows the spatial distribution of thbrightness and colour of the scene as seen through the vidpolarimetry camera. The small rectangular areas demarcatedwhite lines within the pictures in row 1 represent the regions fwhich the histograms of the degree and direction of polarization given in rows 4 and 5. Row 2 gives the patterns of the degreepolarization δ of the scenes. The colour scale is given in the top lecorner: the darker the grey tone, the higher is δ (black, δ=100 %,white, δ=0 %). Row 3 shows the patterns of the E-vector alignmentαof the scenes measured from the vertical. The darker the violet/gror red/yellow colour, the more the E-vector alignment deviates frothe horizontal or vertical, respectively (red, 0 °<α<45 °; green,45 °<α<90 °; violet, 90 °<α<135 °; yellow, 135 °<α<180 °). Row 4shows histograms (frequencies) of the distribution of the degreepolarization calculated for the rectangular windows in row 1. In ro5 are histograms (frequencies) of the distribution of the E-vecalignment calculated for the rectangular windows in row 1.

DiscussionAn appropriate explanation for the reproductive behavio

of mayflies above asphalt roads is that certain sens(olfactory, thermal or visual) cues deceive and attract theinsects. To investigate these cues, we performed multipchoice experiments with swarming mayflies. We uscolourless reflecting test surfaces because we wanted to sthe role of the brightness and polarization of reflected lightthe reproductive behaviour and detection of water by mayfliWe wished to avoid the more complex investigation of the roof colour in this behaviour. The latter is a task for futustudies.

Table 6.The reflection–polarization characteristic

S1 S2 S3Wet Matt Shiny Sh

asphalt white cloth aluminium p

Relative brightness 38.8±3.4 99.7±5.4 100±5.7(%)

Degree of polarization, 50.9±3.4 3.3±0.9 3.2±1.1δ (%)

E-vector alignment,α (degrees) 89.1±1.4 58.8±4.3 57.7±2.1

Relative brightness is calculated relative to the shiny aluminiumValues are means ±S.E.M. (N=560 × 736 = number of pixels in a vid

urorysele-d

tudy ines.lee

The role of olfaction, wind and air humidity

The asphalt road and the test surfaces did not possess characteristic smell detectable by the human olfactory systeThe black and white plastic sheets were composed of the sapolyethylene; consequently, their odour must be the same,in the case of the matt black and white cloths. Similarly, themight not be any significant difference between the smell the smooth/dark and light/rough regions of the asphalt surfaIt is, therefore, improbable that olfaction plays a role in thattractiveness of the shiny black plastic sheet and the asphsurface to mayflies. This is consistent with the results of othauthors (Schwind, 1985, 1991, 1995; Horváth et al. 1998), whofound that water-seeking insects find their aquatic habitvisually and not by means of olfaction.

Mayflies generally avoid those sites where the wind is stronand the air humidity is low (Brodskiy, 1973). Any smallpossible differences in wind velocity and relative humidityamong the test surfaces were compensated for by the randpositioning of these surfaces in the multiple-choicexperiments. Thus, a role of wind and air humidity in thattractiveness of the shiny black plastic sheet can be exclud

The role of temperature

Since there were no temperature differences between different regions of the asphalt road and the test surfaces lyon it, the attractiveness of the smoother and darker regionsthe asphalt surface and the shiny black plastic sheet cannoexplained in terms of temperature. Mayflies must have thermsensitivity in order to perceive the optimal temperature rangfor swarming (Savolainen, 1978). When the air becomes coldthan approximately 14–15 °C after sunset, swarming ceasand the mayflies roost on the leaves of grass, trees and buson the shore of their emergence site. The air above asphroads is always warmer than that above the water surfabecause the sunshine warms the dark asphalt more thanwater. This higher temperature above asphalt roads advantageous for mayflies as it prolongs their reproductiactivity. Note, however, that it is unlikely to be the highetemperature that attracts mayflies to asphalt roads. The higtemperature only affects the duration of the swarming perio

nt siteee

aal.ndhtAtheerebyeeo-

byorare offt

eenm

ofwtor

s of the test surfaces measured using video-polarimetry

S4 S5 S6 S7 S8iny white Matt Slightly shiny Shiny black Drylastic sheet black cloth black cloth plastic sheet asphalt

97.6±4.3 24.4±2.8 17.6±3.2 22.6±2.4 26.0±3.1

7.7±1.5 9.1±2.1 15.1±2.8 55.0±5.4 30.6±3.4

91.3±1.1 81.9±5.4 73.1±4.9 90.5±1.2 90.9±1.3

foil; E-vector alignment is measured with respect to the vertical. eo frame).

2282 G. KRISKA, G. HORVÁTH AND S. ANDRIKOVICS

Fig. 3. The reflection–polarization characteristics of three different sections of the asphalt road above and on which the mayflies swarmed,mated and oviposited. In each case, the asphalt surface was dry, and the scenes shown were recorded from a direction of view of the camera of60 ° with respect to the vertical. (A) A long section of the asphalt road illuminated by direct light from the setting sun under a clear sky. Thecamera viewed towards the solar meridian. (B) A short, smooth and dark section of the asphalt road illuminated by direct sunlight prior tosunset. The camera viewed towards the solar meridian. (C) A short section of the asphalt road with smooth and rough, bright and dark patchesilluminated by skylight from above after sunset. Other details are as in Fig. 2.


heilatted,heethettcenot

k

Fig. 4. The reflection–polarization characteristics of eight different test surfaces measured using video-polarimetry. The scene was illuminatedby skylight from above after sunset and recorded from a direction of view of the camera of 70 ° with respect to the vertical. The rectangularpieces of the test surfaces were laid on a dry asphalt road, a small rectangular area of which (S1) was moistened by water. S1, wet asphaltsurface; S2, matt white cloth; S3, shiny aluminium foil; S4, shiny white plastic sheet; S5, matt black cloth; S6, slightly shiny black cloth; S7,shiny black plastic sheet; S8, dry asphalt surface. (A) The colour picture of the scene as seen through the video-polarimetry camera. (B) Thepattern of the degree of polarization of the scene. (C) The pattern of the E-vector alignment of the scene. Other details are as in Fig. 2.

(approximately 15 min longer) above the asphalt rocompared with the cooler water surface. For species swarmat dusk over water, a gradual increase in swarming altitudebeen reported (Brodskiy, 1973) as the insects avoid coldnear the ground. We observed the reverse of this phenomeabove asphalt roads.

The role of colour and brightness

On the basis of the above arguments, the high attractiveof asphalt roads to mayflies can be explained only by optcues, i.e. by the colour, brightness or polarization of refleclight. Because a black or grey asphalt surface reflects the wspectrum of the incident light and its reflectivity is almoindependent of the wavelength, as for the colourless

ading

has airnon

nessicaltedholesttest

surfaces used in the experiments, the role of colour in tchoice by mayflies can be excluded. The shiny aluminium foand the plastic sheets reflected the light specularly, the mcloths reflected it diffusely. Among the test surfaces, thbrightest was the aluminium foil; the white plastic sheet anthe white matt cloth reflected a slightly, but not significantlysmaller amount of light; and the black plastic sheet and tblack cloths were the darkest (Table 6). If mayflies werattracted to the asphalt surface by positive phototaxis, then shiny aluminium foil, the shiny white plastic sheet and the mawhite cloth should have been the most attractive to them. Sinthe reverse was found, one can conclude that mayflies were guided by phototaxis to the asphalt surface.

We found that mayflies were attracted only to the shiny blac

2284

oftedle

e

se

e,helensed of

tay

ngainurhlyltual

r

nt,r ifd

arasheuselying

on.dllys.inereeg

untht

don


plastic sheet among the test surfaces. This cannot be explaby the relatively small amount of light reflected from this plassheet, because the matt black cloth, which had a similar relabrightness (using a paired t-test, there is no significantdifference in brightness; Table 6), was not attractive at all. Ispecular direction (i.e. when the angle of incidence is equathe angle of reflection), a shiny black surface reflects more lithan a matt black one; however, it was established above the amount of light reflected is not the cue used by mayflie

The role of reflection polarization

Aluminium foil does not change the degree and directionpolarization of the incident light (Horváth and Pomozi, 1997The light reflected from the plastic sheets became polariparallel to their surface, but the degree of polarization of ligreflected from the white plastic sheet (δ=7.7 %) was muchsmaller than that from the black one (δ=55 %) (Table 6). Thelight reflected from the cloths also possessed very low degof polarization; furthermore, the E-vector of light reflected frothem was not horizontal. Thus, we suggest that polarizationreflected light might be the most important variable explainithe attractiveness of the shiny black plastic sheet. We obsethat the black plastic sheet was attractive only if its surface whorizontal; the vertically oriented black plastic sheet, for whithe E-vectors of reflected light would have been vertical, wnot attractive to mayflies. Thus, we can conclude that ohorizontally polarized reflected light attracts mayflies.

This is also supported by the fact that the shiny aluminiufoil, which did not change the degree and direction polarization of reflected light, was unattractive to mayflies. Tpolarization distribution of the surroundings of the sites of ochoice experiments was generally characterized by randorientation of the E-vectors and by relatively low values of tdegree of polarization (e.g. see Fig. 3A). Thus, the ligreflected from the aluminium foil was relatively unpolarized comparison with the light reflected from the black plastic shand its E-vector was not horizontal (Table 6).

We found that the shiny black plastic sheet was moattractive to mayflies than the dry asphalt surface, and thatlatter was much more attractive than the slightly shiny blacloth. However, the smoother and darker regions of the asproad were much more attractive than the rougher and lighpatches. We found that the degree of polarization of refleclight was highest for the shiny black plastic sheet (δ=55 %);light reflected from the dry asphalt road possessed a smadegree of polarization (δ≈31 %), but a higher degree opolarization than that of the slightly shiny black clot(δ≈15 %). The degree of polarization of light reflected from trougher and lighter patches of the asphalt was lower than reflected from the smoother and darker regions of the asproad (Fig. 3C, rows 1, 2). Therefore, the higher the degreepolarization of reflected light, the greater is its attractiveneto mayflies. Hence, mayflies swarming, mating and egg-layon asphalt roads are predominantly visually deceived by attracted to the asphalt surface because the strongly horizontally polarized reflected light imitates a water surfac

inedtictive

n al toghtthats.

of).

zedht

reesm of

ngrved

aschas

nly

mofheuromhehtineet

re theckhaltterted

llerfhhethathalt ofss

ingandande.

Our results are in accordance with the earlier results Schwind (1985, 1991, 1995), whose test surfaces also attracCloeonspecies (Ephemeroptera). He found that the probabspectral range where the polarization vision system of Cloeonfunctions is between 450 and 480 nm.

We found that the slightly shiny black cloth with a degreof polarization δ of reflected light of 15 % was slightlyattractive (see Table 3), while the matt black cloth (δ=9.1 %)was relatively unattractive to the six mayfly specieinvestigated. This indicates that the threshold of thpolarization sensitivity of their visual system is between 9 %and 15 %. Apart from some anatomical studies (e.g. Horridg1976; Horridge and McLean, 1978; Burghause, 1981) on tdorsal (turban) and lateral eyes in male and femaEphemeroptera, nothing is known about the polarizatiosensitivity of the visual system in mayflies. Our observationmake it very probable that mayflies possess well-developpolarization vision and detect the water surface on the basisreflected polarized light.

Generally, aquatic insects and those living in moist substraare influenced in their choice of habitat not only bhorizontally polarized reflected light (visible from remotedistances), but also by non-optical factors; shortly after landion a substratum, they may find it unsuitable and leave ag(Schwind, 1991). However, we did not observe such behavioin the six Ephemeroptera species studied. Perhaps the higand horizontally polarized light reflected from the asphasurface or the shiny black plastic sheet was such a strong viscue that it suppressed the signals of other sensory organs.

Comparison of the attractiveness of asphalt roads and watesurfaces to mayflies

Since the asphalt is black or dark grey and non-transparean asphalt road is an efficient specular reflector and polarizeits surface is smooth; it always reflects horizontally polarizelight, the degree of polarization of which is almost 100% nethe Brewster angle (57.5°). Light penetrating into the asphalt hno effect on the polarization because it is totally absorbed. Tsituation in the case of a streamlet, however, is different, becalight reflected specularly from the water surface is horizontalpolarized, whereas light penetrating into water and emanatfrom it is vertically polarized due to refraction. This verticallypolarized component reduces the net degree of polarizatiThus, the light reflected from a brook is horizontally polarizewhen the surface-reflected light dominates and verticapolarized when the light returning from the water dominateThe greater the proportion of light returning from the water comparison with that reflected from the water surface, the lowis the net degree of polarization (Fig. 2A,B). In Fig. 2C, thdegree of polarization of light reflected from the water surfacis relatively high, because only a small amount of light is cominfrom the water (due to the sheltering vegetation), and the amoof light reflected from the water surface is high (due to the brigillumination from the side).

The highly and always horizontally polarized light reflectefrom asphalt roads and the relatively homogeneous distributi


rengteheanors ofofsts.ate

nt

tolt6)ed

edck

dpedalteneos

d

eeturisrer atckoleisof

l-

ndedDror

of the degree and direction of polarization (see Figs 3, 4) ctherefore be much more attractive to mayflies than the surfof a streamlet (Fig. 2). An asphalt road can reflect and polarthe incident light in such a way that the reflected light becoma supernormal stimulus for water-seeking mayflies comparison with the light reflected from the water. This walso observed in our multiple-choice experiments, whmayflies swarming above the asphalt road were attracted tohighly polarized light reflected from shiny black plastic shewhen it was laid onto the road. A relatively small black plastsheet (a few square metres) attracted all the mayflies swarmabove the asphalt road within several tens of metres.

According to Schwind (1991), insects inhabiting runninwaters, e.g. plecopterans living near brooks (Zwick, 199may not locate their habitats using polarization vision becaupolarization is reduced or even distorted by waves (sFig. 2B). Nevertheless, our observations and multiple-choexperiments show that this is not true of the Ephemeropteraleast for the six species studied by us.

One of the prerequisites of mayfly mating is to swarm aboplaces where the sky is visible, because the females are usudetected visually and captured by the males from belo(Brodskiy, 1973). The sky is generally open above highwaand asphalt or dirt roads; thus, in this respect, roads nearemergence site of mayflies provide a good swarming plaAfter mating, the polarotactic females return to water oviposit. Highly and horizontally polarized light reflected fromasphalt roads with a smooth and dark surface can deceiveattract them.

Hence, asphalt roads are visually attractive on several levto mayflies: the sky above them is visible, the strong ahorizontal polarization of reflected light mimics a watesurface, and they have a slightly higher temperature than surrounding areas. Asphalt roads can be much more attracfor mayflies than real creeks, because the latter frequently under trees and bushes. Mayflies do not swarm or mate abthose reaches of the creek that are in the shelter of trees;is, from which the sky is not visible. Egg-laying also takeplace on the reaches of the streamlets where the sky is visand from which polarized skylight can be reflected to guide tmayflies to the water surface.

Other choice experiments with mayflies

Using different shiny black and white plastic markeroriented horizontally or vertically, Savolainen (1978) studiethe ability of some mayfly species to recognize the markerstheir visual environment. His observation that individuals Ephemera vulgatawere deceived by horizontal plastic sheeis consistent with our results. In contrast to our choiexperiments, Fischer (1992) could attract male Baetis vernusin large numbers to a horizontally oriented matt black clolaid on the grass of a meadow, and the mayflies followed cloth if it was moved slowly. However, he carried out ncontrol experiments and gave no information about the opticharacteristics of the test surface, so that comparisons withresults are difficult.

anaceizeesinasen theeticing

g0),seee

ice, at

veallywys thece.to

and

elsndrthetiverunove

thatsiblehe

sd in

oftsce

ththeocal our

The results of Savolainen (1978) and Fischer (1992) weinterpreted regarding the effects of markers on the swarmiof mayflies. All earlier experiments studying the swarming sipreference of mayflies have concentrated exclusively on tbrightness contrast of markers; reflection polarization as optical cue was not taken into consideration. Some auth(e.g. Fischer, 1992) have hypothesized that the recognitionthe surface of creeks or lakes by the visual system Ephemeroptera is made on the basis of brightness contraOur results on the polarotaxis of Ephemeroptera demonstrthat this view is erroneous.

Asphalt roads as analogies with ancient tar seeps and receoil lakes

Our observations on mayflies deceived by and attractedhighly and horizontally polarized light reflected from aspharoads recall the earlier observations of Horváth and Zeil (199that both male and female dragonflies were visually deceivby the high and horizontal polarization of light reflected fromcrude oil lakes in the desert of Kuwait where they attemptto touch the surface or lay their eggs and perished in the blaliquid oil. Recently, G. Horváth and G. Kriska (unpublishedata) have observed that mayflies are attracted to and trapin large numbers by the waste oil lake in Budapest. Asphroads and crude or waste oil lakes resemble the Pleistocnatural tar pits and asphalt seeps in Rancho la Brea, LAngeles, USA (Akersten et al. 1983), and Starunia, WesternUkraine (Angus, 1973; Kowalski, 1997), which also trappewater insects en masseprobably because of their highreflection polarization (Horváth and Zeil, 1996).

A new method for studying ephemeropteran swarmingbehaviour in the field

Our experiments suggest that a shiny black plastic shcould be used for the investigation of reproductive behavioin Ephemeroptera. In the field, under natural conditions, it often difficult to observe mayfly swarms, because they aformed in unapproachable sites, above the water surface ohigh altitudes, for instance. The placement of a shiny blaplastic sheet of several square metres would attract the whswarm, allowing the study of the mayflies or their capture. Thsimple method could facilitate field studies by students Ephemeroptera.

This work was supported by the Hungarian NationaScience Foundation (OTKA F-014923, T-020931 and F025826). Many thanks are due to Professor Rudolf Schwiand Dr Thomas Labhart, who critically read and commenton earlier versions of the manuscript. We are grateful to Arnold Staniczek and Mrs Anikó Takács and Orsolya Rab ftheir technical assistance.

ReferencesAKERSTEN, W. A., SHAW, C. A. AND JEFFERSON, G. T. (1983). Rancho

La Brea: status and future. Paleobiology 9, 211–217.

2286

tor

ndatic

l

m

ms.

ts

e

tion

n


ANDRIKOVICS, S. (1991). On the long-term changes of the invertebrmacrofauna in the creeks of the Pilis-Visegrádi mounta(Hungary). Vh. Int. Verein. Limnol. 24, 1969–1972.

ANDRIKOVICS, S. AND KÉRI, A. (1991). Winter macroinvertebrateinvestigations along the Bükkös stream (Visegrádi MountaiHungary). Opusc. Zool. Budapest24, 57–67.

ANGUS, P. B. (1973). Pleistocene Helophorus (ColeopteHydrophilidae) from Borislav and Starunia in the Western Ukrainwith a reinterpretation of M. Somnicki’s species, description ofnew Siberian species and comparison with British Weichselfaunas. Phil. Trans. R. Soc. Lond. 265, 299–326.

BRODSKIY, A. K. (1973). The swarming behavior of mayflie(Ephemeroptera). Ent. Rev. 52, 33–39.

BURGHAUSE, F. (1981). The structure of the double-eyes of Baetisandthe uniform eyes of Ecdyonurus(Ephemeroptera). Zoomorphology98, 17–34.

FERNANDO, C. H. (1958). The colonization of small freshwatehabitats by aquatic insects. I. General discussion, methand colonization in the aquatic Coleoptera. Ceylon J. Sci. 1,116–154.

FISCHER, C. (1992). Evolution des Schwarmfluges unFlugverhalten der Ephemeropteren. PhD thesis, Num. 1992/32Friedrich Alexander University of Erlangen-Nürnberg (iGerman). 171pp.

HORRIDGE, G. A. (1976). The ommatidium of the dorsal eye of Cloeonas a specialization for photoreisomerization. Proc. R. Soc. Lond. B193, 17–29.

HORRIDGE, G. A. AND MCLEAN, M. (1978). The dorsal eye of themayfly Atalophlebia(Ephemeroptera). Proc. R. Soc. Lond. B 200,137–150.

HORVATH, G. (1995). Reflection–polarization patterns at flat watsurfaces and their relevance for insect polarization vision. J. theor.Biol. 175, 27–37.

HORVATH, G., BERNATH, B. AND MOLNAR, G. (1998). Dragonflies findcrude oil visually more attractive than water: multiple-choic

ateins

ns,

ra,e, aian

s

rods

d91,

n

er

e

experiments on dragonfly polarotaxis. Naturwissenschaften(inpress).

HORVATH, G. AND POMOZI, I. (1997). How celestial polarizationchanges due to reflection from the deflector panels used in deflecloft and mirror experiments studying avian navigation. J. theor.Biol. 184, 291–300.

HORVATH, G. AND VARJU, D. (1997). Polarization pattern offreshwater habitats recorded by video polarimetry in red, green ablue spectral ranges and its relevance for water detection by aquinsects. J. exp. Biol. 200, 1155–1163.

HORVATH, G. AND ZEIL, J. (1996). Kuwait oil lakes as insect traps.Nature379, 303–304.

KOWALSKI, K. (1997). Starunia. In Lagerstaetten of Europe.Milan:European Palaeontological Association.

POPHAM, E. J. (1964). The migration of aquatic bugs with speciareference to the Corixidae (Hemiptera, Heteroptera). Arch.Hydrobiol. 60, 450–496.

SAVOLAINEN , E. (1978). Swarming in Ephemeroptera: the mechanisof swarming and the effects of illumination and weather. Ann. Zool.Fenn. 15, 17–52.

SCHWIND, R. (1985). Sehen unter und über Wasser, Sehen voWasser: Das Sehsystem eines WasserinsekteNaturwissenschaften 72, 343–352.

SCHWIND, R. (1991). Polarization vision in water insects and insecliving on a moist substrate. J. comp. Physiol. A 169, 531–540.

SCHWIND, R. (1995). Spectral regions in which aquatic insects sereflected polarized light. J. comp. Physiol. A 177, 439–448.

SCHWIND, R. AND HORVATH, G. (1993). Reflection–polarizationpattern at water surfaces and correction of a common representaof the polarization pattern of the sky. Naturwissenschaften80,82–83.

ZWICK, P. (1990). Emergence, maturation and upstream ovipositioflights of Plecopterafrom Breitenbach, with notes on the adultphase as a possible control of stream insect populations. Hydrobiol.194, 207–223.

why do mayflies lay their eggs en masse on dry … · why do mayflies lay their eggs en masse on...

Documents