1829

Strategies for coexistence in three species of net-spinning caddisflies(Trichoptera) in second-order southern Appalachian streams

DIANE MALAS AND J. BRUCE WALLACEDepartment of Entomology, University of Georgia, Athens, GA, U.S.A. 30602

Received February 3, 1977

MALAS, D., and J. B. WALLACE. 1977. Strategies for coexistence in three species of net-spinningcaddisflies (Trichoptera) in second-order southern Appalachian streams. Can. J. Zool. 55:1829-1840.

Three species of net-spinning caddisflies, Parapsyche cardis, Diplectrona modesta, andDolophilodes distinctus were studied: Larvae of Dolophilodes are found at the lowest currentvelocities followed by Diplectrona, then Parapsyche, which prefer the higher velocities. Para-psyche larvae are most abundant on upper surfaces of stones while Diplectrona and Dolophilodes,are found primarily on undersides of stones. These distribution patterns are probably related tocapture net mesh dimensions which differ greatly for the three species, Parapsyche having thelargest and Dolophilodes the smallest meshes. Significant correlations existed for mean foregutparticle size and capture net mesh opening size between instars of all species within seasons,between instars of all species throughout the year, and between instars within species throughoutthe year. These correlations support the contention that particle size selection is an importantaspect of feeding differences between species. There are large differences between mesh openingsizes of \ast-mstarDolophilodes and first- and second-instarDJp/ecfrono larvae. However, basedon mean particle size measurements of foregut contents, there is no corresponding gap in thespectrum of particle sizes used for food. Dietary composition also varied between species.Parapsyche consumed primarily animal material. Fine particulate detritus composed over 95% ofDolophilodes gut contents and Diplectrona consumed mostly vascular plant and detritus frag-ments in late instars and fine particulate detritus in early instars.

MALAS, D., et J. B. WALLACE. 1977. Strategies for coexistence in three species of net-spinningcaddisflies (Trichoptera) in second-order southern Appalachian streams. Can. J. Zool. 55:1829-1840.

On a etudie trois especes de trichopteres a filets, Parapsyche cardis, Diplectrona modesta etDolophilodes distinctus. Les larves de Dolophilodes habitent les eaux acourants les plusfaibles,suivies de Diplectrona et de Parapsyche qui preferent les courants plus rapides. Les larves deParapsyche se retrouvent surtout a la surface superieure des pierres, alors que celles de Diplec-trona et de Dolophilodes se tiennent surtout attachees a la surface inferieure. Cette repartitions'explique probablement par les dimensions de 1'ouverture de la maille du filet de capture,differentes chez les trois especes; c'est chez Parapsyche que la maille est la plus grande et chezDolophilodes qu'elle est la plus petite. II y a correlation entre la moyenne de la taille des particulesdans 1'intestin anterieur et 1'ouverture de la maille du filet chez les divers stades de toutes lesespeces a une saison donnee, chez les stades de toutes les especes durant toute 1'annee et chez lesdivers stades au sein d'une meme espece durant toute 1'annee. Ces correlations appuient1'hypothese selon laquelle le choix de la taille des particules serait un aspect important desdifferences alimentaires entre les especes. II y a une grande difference entre les dimensions de1'ouverture de la maille chez les derniers stades de Dolophilodes et chez les premier et secondstades larvaires de Diplectrona. Cependant, d'apres la moyenne de la taille des particules dans1'intestin anterieur, il n'y a pas de grandes differences dans le spectre des tallies des particulesalimentaires. La composition de la nourriture varie chez les differentes especes. Parapsyche senourrit surtout de substance animale. Quatre-vingt-quinze pourcent de la diete de Dolophilodesse compose de fines particules de detritus, alors que Diplectrona consomme surtout des frag-ments de plantes vasculaires et de detritus, durant les derniers stades, et de fines particules dedetritus durant les premiers stades.

[Traduit par le journal]

Introduction on their biology and net structure is available inNet-spinning trichopterans of the suborder the works of Nielsen (1942), Sattler (1955, 1958,

Annulipalpia are primarily limited to flowing 1963a, 1963i), Brickenstein (1955), Kaiserwaters. Nets, spun from the silk glands, function (1965), Schuhmacher (1970), Williams andas filters for suspended particles carried by the Hynes (1973), Rhame and Stewart (1976),current. A considerable amount of information Wallace (1975a, 19756), and Wallace and Malas

1830 CAN. J. ZOOL. VOL. 55, 1977

(1976o, 19766). Some of the above authors andothers have suggested that current velocity is animportant factor in trichopteran distribution(Fremling 1960; Edington 1965, 1968; Ulfstrand1968; Philipson 1969). Wallace (19756) suggestedthat several strategies are used by net-spinningcaddisflies to partition available resources.These include (1) variations in capture net meshsize; (2) distinct microdistributional patternswith nets and retreats designed for variouscurrent velocities; and (3) temporal variations inlife cycles.

This is the first of a series of yearlong studieson streams of diverse sizes examining the re-lationships between and within species withrespect to the above three parameters. This studyis limited to two small, second-order streams ofthe southern Appalachian Mountains and isrestricted to the three species encountered mostfrequently in these streams. These include Para-psyche cardis Ross and Diplectrona modestaBanks (Hydropsychidae) and Dolophilodes dis-tinctus Walker (Philopotamidae).

Study Sites

The primary study site is a small (0.6-3 mwide) tributary of Panther Creek located inStephens County, Georgia. The stream drains asouthern Appalachian area of about 0.2 km2

and is heavily shaded by a Rhododendron under-story with deciduous trees in the upper canopy.The sampling area begins at the base of agranite wall, at the confluence of two smallwaterfalls, each about 10 m high, and extends200 m to its junction with Panther Creek. Ini-tially the stream flows through a 20 m deepshaded gorge for about 75 m over a smoothgranite outcrop which then grades into a com-bination of bedrock, sand, and gravel. The rocksin the lower two-thirds of the stream are pri-marily schistose and tend to have razorlikeedges. All the standing crop and gut analysisdata were made at this site. Microdistributionaldata were recorded at both the above site and thesecond-order stream on the lower portion ofwatershed No. 7 at the Coweeta HydrologicLaboratory in Macon County, North Carolina.The latter watershed drains an area of 0.6 km2

and had at the time of the study a hardwoodforest with a dense Rhododendron understorysurrounding the stream. This area has recentlybeen clear-cut, hence before and after compari-sons of net-spinning Trichoptera will be made at

a later date. Therefore, only microdistributionalpattern data from Coweeta are used here.

Materials and MethodsMonthly water samples were collected in dark poly-

ethylene bottles for seston analysis and were analyzedusing the method previously described by Wallace(19756). The results of monthly water samples werecombined into four seasons on the following basis:spring = April, May, and June; summer = July,August, and September; fall = October, November, andDecember; and, winter = January, February, and March.

Capture Net MeasurementsLarvae used for net measurements were obtained in

the field by handpicking. Each specimen, along with itslarval retreat, was placed in a separate vial containing80% ethanol allowing the association of a larva with itsnet. Nets were examined using both the light and scan-ning electron microscope (SEM) (Wallace and Malas1976a).Microdistribution

Current velocity readings were taken adjacent toParapsyche nets and a few Diplectrona nets and at thelower downstream side of rocks using a rubber bagcurrent meter (Gessner 1955). Most Diplectrona andDolophilodes were found on undersides of stones; forthese species velocities were recorded before removal atthe lower downstream edge, not adjacent to nets. Posi-tion of nets on rocks and maximum rock diameter werealso recorded.Larval Gut Analysis

Monthly larval collections were preserved in either80% ethanol or Kahle's fluid. Gut contents were extractedwithin a few days of collecting to avoid deterioration thatresults with storage. Gut contents of 443 larvae (73 P.cardis, 256 D. modesta, and 114 D. distinctus) wereexamined. Larval gut analyses were combined into theaforementioned seasons.

A Millipore filtration and sketching technique describedby Wallace (19756) was used for preparation and analysisof larval foreguts. The magnifications used to draw gutparticles had to be adjusted due to obvious differences inthe size of individual pieces (cf. Figs. 1-3). A Ladddigitizer gave an area estimate 20% lower than the cuttingand sketching method for smaller particle sizes (10-20urn in diameter); this may have been due to difficulty incutting precise outlines of small pieces. A total of 243 143animal, plant, and detritus fragments were measured forthe three species. Detritus was the category used for allorganic particles which could not be identified as plantor animal in origin.

Guts of Dolophilodes and early-instar Diplectrona con-tained many minute particles which were difficult to dis-tinguish using a light microscope. Five last-instar Dolo-philodes and five second-instar Diplectrona guts wereexamined using SEM. Photographs were made of the gutcontents and the area of each particle was calculatedusing a Ladd digitizer.

Estimates of the number and area of diatoms in theguts were obtained and converted to a per larva basis. Anaverage surface area for each diatom genus was obtainedusing the sketching and cutting method. Some diatom

MALAS AND WALLACE 1831

V" -" •>> : - ' *-r V7' ff- . • -jS-. ( > S c i

FIG. 1. Surface of a membrane filter containing contents of the foreguts of two last-instar larvae ofParapsyche cardis (SEM at 20 x). FIG. 2. Same as Fig. 1, of one last-instar larva of Dolophilodesdistmctus (SEM at 200 x ) . Note the numerous minute fragments from Dolophilodes guts FIG 3Same as Fig. 1, of Parapsyche cardis (SEM at 200 x). Note the relative paucity of minute fragmentson the surface of the membrane filter. One very large insect fragment occupies most of the left side ofthe photo. FIG. 4. Section of a fourth-instar larval net of Diplectrona modesta net (Dm) resting on asection of fifth-instar Dolophilodes net (Dd) (SEM at 500 x). Note the striking difference in meshopening sizes between the two species.

1832 CAN. J. ZOOL. VOL. 55, 1977

TABLE 1. Number of diatoms and number and average size of detritus, plant, and animal fragments and ash-freedry weight per litre suspended material in Panther Creek Tributary by season*

Season

SpringSummerFallWinter

Detritus,No./^

4.1xl06

3.7xl06

l .SxlO6

2.5xl06

Meansize,um2

389477590397

Plantfragments,

No./ <?

3.8x10*3.5xl04

1.4x10*3.2x10*

Meansize,um2

22140196491477320500

Animalfragments,

No./<?

S . l x l O 3

6.0xl03

5.8xl03

6.0xl03

Meansize,um2

170651103401518915155

Diatoms,No./<?

6.9x10*4.9x10*7.7xl03

1.7x10*

Ash-freedry weight,

mg/<?

3.32.21.20.7

•Based on examination at 100 x magnification. The true mean particle size is probably much smaller.

TABLE 2. Percentage of animal and plant fragments, detritus, diatoms, and filamentousalgae per litre stream water in Panther Creek Tributary by season. All percentages are

based on the area (um2) of the particles present

Season

SpringSummerFallWinter

Animalfragments

2.29.68.63.0

Plantfragments

38.930.724.845.6

Detritus

57.658.866.150.7

Diatoms

1.30.80.50.7

Filamentousalgae

0.00.10.00.0

genera were represented by several species differingwidely in size, in which case an average surface area wasdetermined from a representative sample of all sizeclasses.

Areas of filamentous algae were obtained by tracingeach algal filament using a drawing tube. Length wasmeasured with a map reader, and length and width con-versions were made using a stage micrometer.Benthic Samples

Three benthic samples were taken during the spring,early summer, late summer, and winter. All rockswithin an area 50 x 50 cm were cleaned and the under-lying substrate disturbed so that all dislodged materialwas washed into a drift net (280 um mesh). Samples werepreserved in 8% formaldehyde with the dye, phloxine B,added to facilitate sorting of early instars (Mason andYevich 1967).

Results and DiscussionStream Seston Analysis

Table 1 indicates that the stream seston con-tained many more detritus fragments thanrecognizable plant or animal fragments. Detritus(at 100 x) comprises 51% or more of theseparticles on a surface area basis throughout theentire year (Table 2).

Methods used did not provide an estimate ofthe extent or frequency of transport of largeparticles (e.g. leaf fragments, intact leaves, andsmall twigs). At 100 x the many minute frag-ments suspended in the water column were alsomissed. Undoubtedly the true mean was muchless than that shown in Table 1. This problem isdiscussed with respect to Dolophilodes under thegut analysis section of this manuscript.

Capture Nets, Descriptions and Mesh SizesFigure 4 illustrates the range of net mesh

dimensions found in Panther Creek Tributary.Net mesh dimensions observed in this study aregiven in Table 3. Meshes within any given net arevariable in size; the range of mesh size found inthe nets of each instar is shown in Table 3. Thelargest meshes measured (237 x 382 jim) werein the nets of fifth-instar P. cardis and thesmallest (0.2 x 2.5 urn) in second-instar Dolo-philodes. Data in Table 3 are in agreement withthe reports that hydropsychid net mesh dimen-sions increase with successive instar (Sattler19636; Kaiser 1965; Williams and Hynes 1973;Wallace 1975a). Wallace and Malas (19766)have shown the same for D. distinctus.

Average mesh dimensions in mature larvaefor both P. cardis and D. modesta (Table 3) aresmaller than those found in the Tallulah River,50 mi (1 mi = 1.609 km) from this site (Wallaceet al. 1977). It is not known if this represents anecological adaptation to stream size and flowpatterns.

The overall net structure may represent anadaptation to current velocity. In both P. cardisand D. modesta, the smallest net meshes arefound at the base of the net, i.e., the area closestto the substrate. This area is probably subjectedto lower current velocities due to a boundarylayer effect (Ambiihl 1962). Mesh size andirregularity of shape increase toward the periph-ery of the net. These peripheral areas with

MALAS AND WALLACE 1833

TABLE 3. Range of capture net mesh opening sizes in various instars of the three species inthis study

Species

Parapsyche cardis

Diplectrona modesta

Dolophilodes distinctus'f

Larvalinstar

I*II*IIIIVVIIIIIIIVVIIIIIIIVV

Larger meshaverage, um

50x6580x100

134x184174x288237x382

36x5144x6878x98

113x147140x219No data0 .4x2.50 .6x3 .20.8x4.0

1.75x5.5

Smaller meshaverage, um

35x4540x7073x127

101 x 203166x31430x4540x6068x7795x121

131x172

0.2x2.5No data

0.38x4.00.5x5.5

*Meshes of instars I and II of P. cardis were measured from the Coweeta site.tFrom Wallace and Malas (19766).

larger mesh are probably exposed to highercurrent velocities than the smaller mesh nearthe base.

Table 3 shows that these three species presenta wide variety of mesh sizes when instar varia-tions are included. The capacity for capturing adiverse range of seston particle sizes is thus veryhigh. The mesh opening sizes of last-instarParapsyche are 181 000 x larger than second-instar Dolophilodes nets. Particles carried down-stream can pass through larger meshed nets andstill be available to other species and smallerinstars of the same species.

MicrodistributionMicrodistribution data are shown in Fig. 5

for both study sites. Parapsyche cardis is foundprimarily on the top surfaces of rocks in areas ofhigher velocities whereas Diplectrona modestaand Dolophilodes distinctus are found under-neath rocks (Fig. 5). Current velocities forspecimens inhabiting the under surfaces of rocksare approximations since measurements were ob-tained from the back lower edge of rocks beforestones were disturbed.

Diplectrona modesta occurs more frequentlyon the sides and tops of rocks at Coweeta thanat Panther Creek Tributary. The schistose rocksof the Panther Creek Tributary have knifelikesides which leave little room for larvae to con-struct nets. Diplectrona larvae found on tops ofrocks at Coweeta usually construct their re-treats within moss which is rare in PantherCreek Tributary.

Figure 6 shows cumulative percentage of eachspecies occurring with current velocity. It isevident (Fig. 6) that the larger meshed nets ofParapsyche occur over a wider range of velocitiesthan the other two species which have smallercapture net mesh openings. Dolophilodes, whichhas extremely small net openings, is limited tolower velocities. The horizontal line on eitherside of the diagonal cumulative distributionindicates one standard deviation on either sideof the mean. Although overlaps exist in Fig. 6,the microdistribution pattern shown here resultsin utilization of a broad spectrum of streamvelocities.

Others have reported that larger meshed netsare found in areas of higher current velocity(Kaiser 1965; Wallace 1975a; Wallace et al.1977). Edington (1968) found nets of Hydro-psyche instabilis Curt, in areas of high currentvelocity and small meshed nets of Plectrocnemiaconspersa (Curt.) (Polycentropodidae) in lowercurrent velocities.

Wallace et al. (1977) suggested several reasonsfor the above distribution pattern. Smaller meshednets are more efficient in extracting the numeroussmall particles suspended in the water. Thus,these require less water to be filtered. Minutemeshed nets of Dolophilodes are quickly tornfrom their sites of attachment when exposed tocurrent velocities in the range of those toleratedby Parapsyche. Conversely, larger meshes offerless resistance to flow. Undoubtedly, the lowertrapping efficiency of large meshes is compen-sated by a higher filtering rate.

180

170

160

150

140

130

120

110CO1005y>_90u3 80>

70

60

50

40

30

20

10

PANTHER CREEK TRIBUTARY '8°170

160

150

PARAPSYCHE CARDIS = * woDIPLECTRONA MODESTA=«DOLOPHILODES DISTlNCTUS=o I3O

120

110A * SA J 100

A AA O* > 90

A A UA 6^ 2 80

A » >AA 70

' 60xx* 50

x* A 40

x x 30X * * X Ax x^x x

• oo 5 w ox x x 20* ?b° ? *

" 0° °0 " 0 '°0°°f

ooCOWEETA WS if? *

APARAPSYCHE CARDIS - » *DIPLECTRONA HODESTA-« iDOLOPHILODES DISTINCTUS = o ft

A^ A

A A "A^

AA A

t "'' A " 5^ A A A A

A AX*A A A NA Q* A ' 'A 9A A%& •

** <OA rX Xx x a (2x A^*

jf&& ~A * *-J

w AA A' X xx * x

x^x «• x5 X xx x x x x*** **xJ* *o "^"x" * 8 x x j

o *oq,"xD X X x x xX>c *

- *0 ooo o g ° o o O

frw 25CM SOcp^j CM 25cM 50CMBOTTOM OF ROCKS -|CM 25cM 50cM - BOTTOM OF HOCKS '|CM 25cM 50cM

' SIDE OF HOCKS -|CM 25CM 5OCM * SIDE OF ROCKS '|CM 25CM 50CMTOP OF HOCKS TOP OF ROCKS

FIG. 5. Microdistribution patterns of three species of net-spinning caddisflies in two different watersheds. The vertical axis is current velocity and the hori-zontal axis represents location on rocks of various diameters. Each point represents the location of one larval net.

MALAS AND WALLACE 1835

PARAPSVCHE CAROISDIPLECTRONA MODESTADOLOPHILODES DIST1NCTUS

0-10 II-JO 31-30 31-40 41-30 JI-60 61-70 71-80 Bl-VO 91-100101-110 111-130 \11-I30131^140 Ml- ISO 131-160 161^170CURRENT VELOCITY (CM/S)

FIG. 6. Cumulative percentage of three species of net-spinning caddisflies occurring at variouscurrent velocities. The horizontal line shown for each species represents one standard deviation oneither side of the mean.

Gut AnalysisPercentage composition of the foregut con-

tents and the mean particle size for the variousinstars by season are shown in Fig. 7. Diatomsare not included in the mean particle size mea-surements because (1) they are often observedgrowing in clusters on nets of some Parapsycheand Diplectrona; (2) a number were associatedwith detritus and vascular plant fragments inboth microseston and gut samples; and (3) theyrarely composed more than 1% of gut contentson an area basis.

Mean particle size, with four exceptions, in-creases with successive instar (Fig. 7). Excep-tions occur in summer with instars III and IV ofDolophilodes distinctus and instars IV and V ofParapsyche cardis. Diplectrona modesta instarsI and II contained smaller particles than Dolo-philodes; however, this was not true for gutcontents examined with SEM. A major problemwith Dolophilodes gut contents is that smalldetritus particles are compacted in the gut.Many particles remain clumped together on gutslides, resulting in relatively large mean particlesizes. The compound microscope does notsuffice to measure the small size of many Dolo-philodes gut particles. Membrane filters withDolophilodes gut contents examined with ascanning electron microscope at 1000 x showedmany particles less than 2 jam diameter. Theseparticles are overlooked at 200 or 400 x . Meanparticle size consumed by Dolophilodes is con-siderably smaller than our compound micro-scope measurements indicate since we omit manyminute particles.

Gut particle sizes measured with the SEMresulted in much lower mean particle sizes inboth fifth-instar Dolophilodes and second-instarDiplectrona than shown in Fig. 7. Figure 8 shows

the results of SEM measurements. Eleven percentof Dolophilodes gut particles measured were lessthan 1 urn2 and 50% of the particles consumedwere less than 5 urn2. No particles larger than800 um2 were measured in Dolophilodes guts.These results are consistent with the largestparticle sizes in Dolophilodes guts found withthe light microscope. Half of the second-instarDiplectrona gut particles were smaller than16.5 um2 and the largest particle measured was3200 urn2. Particle numbers, within certain sizeclasses, overlap extensively between these twospecies. About 39% of Dolophilodes gut particlesare smaller than those found in guts of second-instar Diplectrona, whereas 2.5% of Diplectronaparticles are larger than those consumed byDolophilodes (Fig. 8). Thus there is about 58.5%overlap in particle sizes consumed by the twospecies. However, when particles are consideredon a weighted basis of their contribution to thetotal area of gut contents (sum of the particlenumbers x size in each category), the results arequite different. The 2.5% of Diplectrona particleslarger than those consumed by Dolophilodesrepresent about 50% of the total area of particlesin Diplectrona guts (Fig. 8). About 12% of thetotal gut particle area measured in Dolophilodesis in a range smaller than those particles con-sumed by second-instar Diplectrona. Thus whenfigured on a total area basis the overlap betweenparticle sizes measured in the guts of these twospecies is about 38%. Undoubtedly on a volumebasis these results would have been much morepronounced since the larger particles aregenerally much thicker than the smaller ones.

Dolophilodes larvae lack suitable mandibles orother apparent structures for masticating food(Wallace and Malas 19766). Conversely, Dip-lectrona larvae have teeth situated along most of

1836 CAN. J. ZOOL. VOL. 55, 1977

PANTHER CREEK TRIBUTARY

f

:

J__, , 1 u_100 1 100 300 500u

J' V

7000 1300Q a500 3JIi

ANIMAL • •PLANT • 0DETRITUS-a

MEAN GUTPARTICLE,SIZE \i(nz

too% eoGUTCONTENTS 60

JOJO

DIATOMS 100Su? »»

\.

i

00

1

1

1

j1\\00

1 [1

i tI. . . .300 500 10

j

1

IJ •00 1500 3000 7500 MEAN GUTPARTICLESIZE urn2

n

100 300 5<JO

[

L

1000 1000 1500 MEAN GUTPARTICLESIZE ijm2

FIG. 7. Average particle size and percentage composition based on an area basis of gut contentsof various larval instars in each of four seasons. Dd, Dolophilodes distinctus', Dm, Diplectrona modesta;PC, Parapsyche cardis; I, first instar; II, second instar; III, third instar; IV, fourth instar; and, V, fifthinstar. Diatoms were counted separately and rarely composed over 1% of the gut contents on an areabasis (see text).

the mesal margin of each mandible (Ross 1944,Fig. 286). Thus Diplectrona larvae probablymasticate their food to some extent before itreaches the foregut. If this is the case thendifferences between these two species would begreater than data in Fig. 8 indicate.

Seasonal linear regression analysis of meangut particle sizes derived by combining animal,plant, and detritus particles (Fig. 7) versus meancapture net mesh opening sizes of individualinstars (Table 3) yielded the following positive

correlation coefficients: spring, r = 0.978 (p <0.0005); summer, r = 0.829 (p < 0.005); fall,r = 0.982 (p < 0.0005); and winter, r = 0.99(p < 0.0005). Correlation coefficients for theabove items for individual species across allseasons were as follows: Parapsyche cardis,r = 0.67; Diplectrona modesta, r = 0.90; and,Dolophilodes distinctus, r = 0.77. These resultsare shown in Fig. 9. Cummins (1973) suggestedthat particle size was an important criterion infood selection by insects. These results, based on

MALAS AND WALLACE 1837

- cumulative % of tolal particle numbers^ Dolophilodes - cumulative % ot t o t a l part icle areaipleclrona~ - cumulat ive % of to ta l part icle area

s

j? 50-1

' 'l6o 200 i -i • • i •I(j0^- J Q ' Q Q ' 40'00Particle Size (pm2)

FIG. 8. Foregut particle sizes of last-instar Dolophilodes distinctus and second-instar Diplectronamodesta as measured with SEM. The solid lines are the cumulative percentage of particles by numbersin various size categories. Number of particles measured: Dolophilodes = 795 and Diplectrona = 558.

243 143 gut fragments, certainly support thecontention that the size of the capture net meshopenings are related to particle sizes consumedby the larvae.

Figure 9 shows there is a broad gap in capturenet mesh sizes in these streams but there is nosuch corresponding gap in particle sizes con-sumed by different species and instars. Thesethree species are capable of consuming a widearray of seston particle sizes. Such a continuousparticle size utilization spectrum has importantconsequences for stream ecosystems that will bediscussed later. Results shown in Fig. 9 werebased on measurements made with a compoundmicroscope which overlooks the minute frag-ments. SEM measurements shown in Fig. 8would extend the mean particle size down to lessthan 5 urn2 for Dolophilodes with some gutfragments less than 1 urn2. The largest particlemeasured was a 264 000 urn2 animal fragment ina last-instar Parapsyche gut.

The average size particle found in the gutchanges from instar to instar and from species

to species; however, type of food ingested alsovaries among species (Fig. 7). Parapsyche cardishas much more animal material in its diet thanthe other two species. Parapsyche guts contain aconsiderable amount of detritus and vascularplant fragments; but, the gut contents of her-bivorous prey probably contribute substantiallyto these two categories. Late instars of Diplec-trona consume more recognizable vascular plantfragments and are the most omnivorous of thethree species. Dolophilodes is a specialized fineparticle feeder that ingests primarily very smalldetritus particles. Thus, types of particles con-sumed are another important difference betweenspecies.

Diatoms were more prevalent in the watercolumn during spring than the other seasons butrarely composed more than 1% of gut contentson an area basis. The exceptions were last-instarD. distinctus in spring (3.3%) and winter (2.1%);fourth-instar D. distinctus in spring (2.7%); andthird-instar D. distinctus in spring (1.2%). Thehigh fat content of diatoms (Prescott 1968) may

1838 CAN. J. ZOOL. VOL. 55, 1977

10*

DD= Dolophilodes distinctusDM=Diplectrona modestaPC=Parapsyche card isI-2=Larval instarSpring = oSummer= •Fall = «Winter; x

Y =51.69 *13.19XY=-57.45*0.0442XY=850.22*0.014X

DM I DM IIDDIII

DDII

IN 1MX Capture Net Mesh Size (urn2)

10000 100000

FIG. 9. Linear regression of mean gut particle size vs. capture net mesh opening size for threespecies of net-spinning caddisflies. These particle sizes were measured with a compound light micro-scope and minute fragments were not measured.

provide the animal a much larger intake ofnutritionally useful compounds than indicatedon an area basis. The same is probably true foranimal remains in the guts since assimilationefficiencies are considerably higher for predatorsthan detritivores and herbivores (McDiffett1970; Winterbourn 1971).

Filamentous algae often composed 1-2% ofDiplectrona gut contents and less than 1% forthe other two species. Although filamentousalgae were found in water samples only in spring,it was sometimes found on the nets of bothDiplectrona and Parapsyche in other seasons.This component may not be a free-floating form.Nets, which are situated in flowing waters, mayprovide a suitable substrate for algal attachmentand growth. In Panther Creek Tributary a fewnets were found that contained a large amountof algae.

Standing Crop and Temporal VariationsDolophilodes was the most abundant (52.1%)

of the filter feeding caddisflies studied with amean standing crop of 389/m2 (Fig. 10). Thisprobably reflects the availability of fine particu-

late detritus, the most abundant component ofthe stream seston. Field collections at severalsites indicate Dolophilodes emerge throughoutthe year. Pupae were most abundant in the latefall and early winter. Larvae are very abundant instreams from late spring to late fall.

Diplectrona modesta, the second most abun-dant (46.7%) of the three species, had a meanstanding crop of 349/m2. The cellular plantremains, which were abundant in their guts,appeared to be fragments of terrestrial leaf litter.These fragments probably represent egestedfeces from leaf shredders.

Parapsyche cardis, with a standing crop ofonly 9/m2, composed only 1.2% of the threespecies. They ate more animal material, theleast abundant component of the stream seston.The low standing crop is not surprising sinceParapsyche appears to be largely predaceous andfewer predators would be supported per unitarea.

A rather wide range of instars is present in thestream at a given time (Fig. 10). Since variousinstars tend to ingest different sizes and types ofparticles (Figs. 7 and 10), this represents an

MALAS AND WALLACE 1839

PAHAPSYCHE CARDISN-23 N-7

i-J

N-0 \—I*

DIPLECTRONA MODESTAN-108 N-85

DOLOPHILODES DISTINCTUS

FIG. 10. Percentage of larvae in various instars on eachof four dates in the Panther Creek Tributary. N =number per metre square (based on three 50 x 50 cmsamples on each date). The asterisk by Parapsyche cardisin April indicates a few last instars were collected on thisdate but they did not appear in the above samples onthis date.

important mechanism for reducing both inter -and intra-specific competition for food. Ulf-strand (1975) has suggested a similar mechanismfor some mayflies. Early instars (I-III) ofDolopModes and Diplectrona are most abun-dant (Fig. 10). Less food competition would beexpected in early instars since seasonally 51% ormore of the stream seston is comprised of thesmall particles used by these organisms for food(Table 2).

Last instars of P. cardis and D. modesta reachtheir maximum abundance at different times ofthe year (Fig. 10). This temporal separationprobably serves to reduce food competitionbetween late instars of these two larger particlefeeders. Recently Oswood (1976) has suggestedsuch a pattern in hydropsychids may lessencompetition by separating periods of high energydemand in each species life cycle.

Summary

There are several important mechanisms usedby the three species of net-spinning caddisflies inthis study which enable their coexistence in smallsouthern Appalachian watersheds. These includedistinct differences in capture net mesh sizewhich are linked with microdistributional pref-erences for various current velocities; temporalvariations in life cycles; and ingestion of dif-ferent sizes and types of seston between speciesand between instars within species.

The evolutionary diversity of filter feeders hasimportant consequences for stream ecosystems.Filter feeders restrict or impede downstreamenergy and nutrient losses since they collectmaterials that would otherwise be lost in theseunidirectional flow ecosystems (Wallace et al.1977). Even materials egested by filter feedersreenter the downstream detritus pool and areavailable for reingestion. Webster (1975) pro-posed the term spiralling for this ingestion-egestion of particulate organic materials becauseof the spatial aspect involved in flowing waters.Wallace et al. (1977) suggested filter feedersproduce tighter spirals of particulate organicmaterials in downstream transport and thereforeincrease the ability of a given section of streamto process organic materials. Smith (1975) hasstated "it is not correct to say that much of theevolution of species is irrelevant to ecosystemfunction." The evolutionary diversity of thesefilter-feeding insects certainly supports Smith'sstatement. Evolution of individual species,feeding on somewhat different types and sizes ofseston, has resulted in much more efficientutilization of particulate organic materials indownstream transit.

Acknowledgements

This work was supported by grants no.DEB 74-00618 A01 and no. BMS 74-12088 A01from the National Science Foundation. Wethank Ms. Kay Campbell for technical assist-ance.

AMBUHL, H. 1962. Die Besonderheiten der Was-serstromung in physikalischer, chemischer undbiologischerHinsicht. Schweiz. Z. Hydrol. 24:367-382.

BRICKENSTEIN, C. 1955. Uber den Netzbau der Larve vonNeureclipsis bimaculata L. (Trichopt. Polycen-tropidae). Abh. Bayer. Akad. Wiss. Math. Naturwiss.K1.,N.F.H.69: 1-44.

CUMMINS, K. W. 1973. Trophic relations of aquaticinsects. Annu. Rev. Entomol. 18: 183-206.

EDINGTON, J. M. 1965. The effect of waterflow on popula-tions of net-spinning Trichoptera. Mitt. Int. Ver. Theor.Angew. Limnol. 13: 40-48.

1968. Habitat preference in net-spinning caddislarvae with special reference to the influence of runningwater. J. Anim. Ecol. 37: 675-692.

FREMLING, C. R. 1960. Biology and possible control ofnuisance caddisflies of the upper Mississippi River. Res.Bull. Agric. Home Econ. Stn. Iowa State Univ. 483:856-879.

GESSNER, F. 1955. Hydrobotanik I. Energiehaushalt.Deutsch. Ver. Wissensch., Berlin.

KAISER, P. 1965. Uber Netzbau und Stromungssinn beiden Larven der Gattung Hydropsyche Pict. (Ins.,

1840 CAN. J. ZOOL. VOL. 55, 1977

Trichoptera). Int. Rev. Gasamten Hydrobiol. SO:169-224.

McDiFFETT, W. F. 1970. The transformation of energy bya stream detritivore Pteronarcys scotti (Plecoptera).Ecology, 51: 975-988.

MASON, W. T., and P. P. YEVICH. 1967. The use ofphloxine B and rose bengal stains to facilitate sortingbenthic samples. Trans. Am. Micros. Soc. 86: 221-223.

NIELSEN, A. 1942. Uber die Entwicklung und Biologie derTrichopteren. Arch. Hydrobiol. Suppl. 17: 255-631.

OSWOOD, M. W. 1976. Comparative life histories of theHydropsychidae (Trichoptera) in a Montana lake outlet.Am. Midi. Nat. 96: 493-497.

PHILIPSON, G. N. 1969. Some factors affecting the net-spinning of caddisfly Hydropsyche instabilis Curtis(Trichoptera, Hydropsychidae). Hydrobiologia, 34:369-377.

PRESCOTT, G. W. 1968. The algae: a review. HoughtonMifflinCo., Boston.

RHAME, R. E., and K. W. STEWART. 1976. Life cycles andfood habits of three Hydropsychidae (Trichoptera)species in the Brazos River, Texas. Trans. Am. En-tomol. Soc. 102: 65-99.

Ross, H. H. 1944. The caddis flies or Trichoptera of Il-linois. Bull. 111. Nat. Hist. Surv. 23, 1-326.

SATTLER, W. 1955. Uber den Netzbau der Larve von Hy-dropsyche angustipennis Curt. Naturwissenschaften,42: 186-187.

1958. Beitrage zur Kenntnis von Lebensweise undKorperbau der Larve und Puppe von Hydropsyche Pict.(Trichoptera) mil besonderer Beriicksichtigung desNetzbaues. Z. Morphol. Oekol. Tiere, 47: 115-119.

1963a. Die Larven- und Puppenbauten von Diplec-trona felix McLach. (Trichoptera). Zool. Anz. 170:53-55.

19636. Uber den Korperbau, die Okologie undEthologie der Larve und Puppe von Macronema Pict.(Hydropsychidae), ein als Larve sich von "Mikro-Drift"ernahrendesTrichopterausdemAmazonasgebeit. Arch.Hydrobiol. 59: 26-60.

SCHUHMACHER, H. 1970. Untersuchungen zur Taxon-

omie, Biologie und Okologie einiger Kocherfliegenartender Gattung Hydropsyche Pict. (Insecta, Trichoptera).Int. Rev. Gesamten Hydrobiol. 55: 511-557.

SMITH, F. E. 1975. Ecosystems and evolution. Bull. Ecol.Soc. Am. 56: 2-6.

ULFSTRAND, S. 1968. Benthic animal communities in Lap-land streams. A field study with particular reference toEphemeroptera, Plecoptera, Trichoptera and Diptera:Simuliidae. Oikos Suppl. 10.

1975. Diversity and some other parameters ofEphemeroptera and Plecoptera communities in subarc-tic running waters. Arch. Hydrobiol. 76: 499-520.

WALLACE, J. B. 1975a. The larval retreat and food ofArctopsyche; with phylogenetic notes on feeding adapta-tions in Hydropsychidae larvae (Trichoptera). Ann. En-tomol. Soc. Am. 68: 167-173.

1975&. Food partitioning in net-spinning Trichop-tera larvae: Hydropsyche venularis, Cheumatopsycheetrona, and Macronema zebratum (Hydropsychidae).Ann. Entomol. Soc. Am. 68: 463-472.

WALLACE, J.B., and D.MALAS. 1976a. The significance ofthe elongate rectangular mesh found in capture nets offine particle filter feeding Trichoptera larvae. Arch.Hydrobiol. 77: 205-212.

1976&. The fine structure of capture nets of larvalPhilopotamidae (Trichoptera): with special emphasis onDolophilodes distinctus. Can. J. Zool. 54: 1788-1802.

WALLACE, J. B., J. R. WEBSTER, and W. R. WOODALL.1977. Role of filter feeders in flowing waters. Arch. Hy-drobiol. 79: 506-532.

WEBSTER, J. R. 1975. Analysis of potassium and calciumdynamics in stream ecosystems on three southern Ap-palachian watersheds of contrasting vegetation. Ph.D.thesis, Univ. of Georgia, Athens.

WILLIAMS, N. E., and H. B. N. HYNES. 1973. Microdis-tribution and feeding of the net-spinning caddisflies(Trichoptera) of a Canadian stream. Oikos, 24: 73-84.

WINTERBOURN, M. J. 1971. An ecological study ofBanksiola crotchi Banks (Trichoptera: Phryganeidae) inMarion Lake, British Columbia. Can. J. Zool. 49:637-645.