observations on the sticky trap predator zelus luridus st…l

Introduction

Predatory Reduviidae, or assassin bugs,show a substantial range of methods to cap-ture prey and a great diversity of structuresinvolved in this activity. The taxon Phy-matinae includes species that possess sub-chelate or even chelate raptorial legs (HAN-DLIRSCH 1897), in many Emesinae prey or-ganisms may be secured between the spinytibia and femur of the fore leg (WYGODZIN-SKY 1966), and the “fossula spongiosa“ pres-ent in many taxa of Reduviidae is a flexiblecushion-like structure on the tip of the tibiathat assists in the handling of prey organ-isms (MILLER 1942; EDWARDS 1962). Moreunusual than these diverse types of raptoriallegs are the preying methods of some Holop-tilinae, where secretions are used to paralyze

ants before piercing them (JACOBSON 1911).In at least some Apiomerini, Ectinoderiniand Harpactorini, sticky secretions that areeither secreted by the insect itself (BARTH

1952; EDWARDS 1966), or acquired fromplants with viscous exudates, are applied tothe legs (ROEPKE 1932; EISNER 1988), andthis sticky cover helps the reduviid to graspand handle the prey organism.

BARTH (1952), in a very thorough histo-logical study, documented specialized setaeon the tibiae and femora of adult Zelusleucogrammus (PERTY) (Harpactorinae,Harpactorini) and glandular units that openon the integument. He assumed that the vis-cous secretions of these glands were held onthe leg by the unique structure of these spe-cialized hairs, which are smooth proximally

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Observations on the sticky trap predatorZelus luridus STÅL (Heteroptera,

Reduviidae, Harpactorinae), with thedescription of a novel gland associated

with the female genitalia1

C . W E I R A U C H

Abstract: The first instar nymph of Zelus luridus STÅL (Harpactorinae, Harpactorini) is shown here touse viscous substances, which are collected from the egg mass and applied to its appendages, to trap prey.In the adult of this species, glandular units in the legs are the source for a sticky cover on the fore tib-ia, which also assists in prey capture. Thus, a functional replacement of extrinsic sticky substances de-rived from the mother’s secretion on the egg mass with intrinsic secretions derived from the insect it-self takes place during the postembryonic development of Z. luridus. Structures on the legs, including aspecialized type of hair, the sundew seta, potential olfactory sensilla and glandular units including ex-ternal pores on the integument are examined and documented for the first instar nymph and the adultfemale of Z. luridus. The post-hatching behavior of the first instar nymph is documented, including thedescription of prey capture and behavior following the moult to the second instar. Two possible alter-native sources for the secretion on the egg mass of the female are described in addition to the subrectalgland present in many Harpactorinae: Apart from paired glandular areas on the syntergite 9/10, a com-posite tubular gland associated with the syntergite 9/10 in the female of Z. luridus is described here forthe first time.

Key words: Female genitalia, gland, Harpactorinae, prey capture, Reduviidae.

Denisia 19, zugleich Kataloge der OÖ. Landesmuseen

Neue Serie 50 (2006), 1169-11801This paper is dedicated to Ernst Heiss in celebration of his 70th birthday.

© Biologiezentrum Linz/Austria; download unter www.biologiezentrum.at

but possess numerous small projections intheir apical portion. EDWARDS (1966), alsoconcentrating on adults and apparently un-aware of the study by BARTH (1952), de-scribed the leg of Zelus luridus STÅL (asexsanguis; see HART (1986) for taxonomy ofthe genus Zelus in the United States). Henoticed specialized setae of the type studiedby BARTH (1952), named them sundew hairs(here referred to as “sundew setae“), andmentioned underlying glands, but assumedthat they opened through a second type ofsetae, and not directly on the integument.SWADENER & YONKE (1973) described im-mature stages of Zelus tetracanthus STÅL (asZ. socius, see HART (1986) for taxonomy),and stated the adult legs of this species to besticky, but made no observations on legstructure or preying behavior of the nymph.COBBEN & WYGODZINSKY (1975) noted spe-cialized setae on the legs of nymphs of Zelustetracanthus STÅL. More recently WOLF &REID (2001) added considerable knowledgeon adults as well as nymphs of a closely re-lated species, Zelus longipes (LINNAEUS).WOLF & REID (2001) documented sundewsetae in adults and recently hatchednymphs, and found that ring-like invagina-tions, which they interpreted as pores ofglandular units, were present in the adults,but not yet in the first instar nymphs. How-ever, also first instar nymphs were found topossess a sticky cover on their legs. As to theorigin of this sticky substance in the first in-star nymph, WOLF & REID (2001) formulat-ed the hypothesis that secretions depositedby the female on the egg mass might be itssource. Unlike many other Reduviidae,which lay their eggs separately (DISPONS

1955), Harpactorinae often deposit eggmasses of considerable size. Females areknown to secrete cement-like or sticky sub-stances on the egg mass during and afteroviposition (KERSHAW 1909; MILLER 1956),the sources of which are assumed to be thesubrectal glands known only in Harpactori-nae (KERSHAW 1909; DAVIS 1969; COBBEN

& WYGODZINSKY 1975).

As freshly hatched nymphs and adult fe-males (dried and alcohol) of Zelus luridus be-came available, several questions could beaddressed in the present study: Does thestructure of different setae on the legs in firstinstar nymphs and adults of Z. luridus corre-

spond to the situation described by WOLF &REID (2001) for Zelus longipes? Are the glan-dular units described in detail for adult Z.leucogrammus by BARTH (1952) and men-tioned by EDWARDS (1966) for adult Z.luridus also present in freshly hatchednymphs? Does observation of live first instarnymphs corroborate the hypothesis con-cerning the origin of their sticky cover fromthe secretions on the egg mass? Whichglands are associated with the female geni-talia that might be responsible for the stickycover on the egg mass?

Material and Methods

Material and life observations on firstinstar nymphs

One female of Zelus luridus was collectedin Cold Spring, New York, on June 21,2004, on a mulberry tree. The femaleoviposited a batch of 37 eggs during the fol-lowing night. Nymphs of this batch hatchedon July 1, 2004, during 1 pm and 8 pm.Nymphs were preserved in absolute alcohol,as was the female. Several first instarnymphs were killed in alcohol immediatelyafter hatching, i .e. before any sticky sub-stance could be applied to the legs. The re-maining nymphs were distributed into plas-tic containers with 3 to 6 specimens, andkept alive.

To observe preying behavior of first in-star nymphs, several specimens of Drosophilamelanogaster were introduced into the plas-tic containers that held several nymphs.The moult to the second instar was observedfor one nymph on July 13, 2004.

Hatching and post-hatching behavior ofthe first instar nymph of Zelus luridus wereobserved on a Nikon SMZ 1500. Pho-tographs were taken using a Microptics Sys-tem equipped with a Nikon D1X camera.

Scanning electron microscopy

First instar nymphs and the female werekilled and preserved in absolute alcohol.The fore legs were removed, dried, coatedwith a DENTON VACUUM DESKII andobserved on a HITACHI S-4700.

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Light microscopy of macerated specimens

The detached fore leg of the alcoholpreserved female and several first instarnymphs were macerated in KOH (approx.10 %), stained with Chlorazol black and ob-served using a NIKON Eclipse 80i. The ab-domen of one dried female (American Mu-

seum of Natural History: “L. Toxaway, N.C.;Collection of Mrs. A.T. Slosson, Ac. 26226)was removed, macerated in KOH, and thefemale external and internal genitalia weredissected, stained and set up for observationon a microscopic slide in glycerin. In speci-mens macerated with KOH, the epidermisincluding its gland cells is destroyed. How-

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Figs 1-6: The foretibia in thefemale of Zelusluridus. (1) Overview ofthe tibia showingsundew setae,peg-like setae,simple setae, andring-likeinvaginations; (2) Sundew setaeand ring-likeinvaginations; (3) Upper portionof a sundew setawith lateralprojections; (4) Close-up ofthe upper portionof a sundew setawith lateralprojections andtheir apical knobs; (5) integument ofthe fore tibiashowing ring-likeinvaginations andpores, which arepart of glandularunits in the tibia;(6) ring-likeinvaginationshowing verysmall pores on thesurface of theraised centralarea.Scales in µm.


ever, the sclerotized components of glandu-lar units, termed the ductule, which com-prises the receiving canal, the saccule, andthe conducting canal, is retained and thusobservable. Drawings were made using acamera lucida attachment on the NIKONmicroscope.

Terminology and Abbreviations

The terms for different types of setae –peg-like setae and simple setae – are used ac-cording to WOLF & REID (2001). As the finestructure of the “sundew hairs“, termed byEDWARDS (1966) and referred to as such byWOLF & REID (2001), suggests that thesehairs are also setae, I suggest calling them“sundew setae“. The term “ring-like invagi-nations“ coined by WOLF & REID (2001) isused here, even though hypotheses for apossible function of these structures differ(pores of glands [WOLF & REID 2001] vs.sensory structures [this paper]).

Abbreviations. bc, bursa copulatrix; cut,cuticle; gl synt 9/10, gland on syntergite9/10; gla synt 9/10, glandular area on synter-gite 9/10; lsp, lateral spermatheca (pseu-dospermatheca); mo, median oviduct; pls,peg-like seta; po, pore; pogl, pore of a glan-dular unit; rli, ring-like invagination; sac,saccule of a ductule; sus, sundew seta; spph,spermatophore; srgl, subrectal gland; ss, sim-ple seta; synt 9/10, syntergite 9/10; vf1, vf2,valvifer 1 and 2; vv1-3, valvula 1, 2, and 3.

Results

The fore tibia of the adult and firstinstar nymph of Zelus luridus

Sundew setae, peg-like setae, andsimple setae

The tibia of the fore leg in the adult ofZelus luridus is beset with large numbers ofsundew setae, and smaller numbers of bothpeg-like setae and simple setae (Fig. 1). Thesundew setae are ~ 150-200 µm long, have asmooth lower portion and an upper portionthat is equipped with laterally projectingspines. The spines taper toward the apexand end in a knob (Figs 2-4). Sundew setaeare also abundant on the fore tibia of thefirst instar nymph. The hairs are ~ 70 µmlong and thus much shorter than those inthe adult. Only the sundew setae in nymphskilled immediately after hatching wereclean (Fig. 10), those of older first instarsnymphs were covered with a sticky sub-stance that concentrated on the upper por-tion of the sundew setae that is covered withthe lateral projections (Fig. 9). Comparisonof the sundew setae of a first instar with theadult female shows that the lateral projec-tions in the adult are more abundant andlonger than those in the nymph (Figs 3, 12).Sundew setae in the first instar nymph arenot restricted to the fore tibia, but also oc-cur on the tibia of the other pairs of legs, onall femora, and in smaller numbers on thescapus and pedicellus of the antenna, andon head, thorax and abdominal dorsum.

Peg-like setae (Figs 1, 11, 13) are ratherrare in both the adult female and the first in-star nymph. The surface of the peg-like setais smooth and it does not possess any pores orother ornamentation as seen from the close-up examination of the first instar nymph(Fig. 13). Simple setae occur in small num-bers on the fore tibia of the adult female andon the first instar nymph (Figs 1, 8).

Ring-like invaginations

Ring-like invaginations (sensu WOLF &REID 2001) are abundant on the fore tibia ofthe adult of Z. luridus (Figs 1, 2). The in-vagination has a diameter of ~ 8µm, thecentral raised area a diameter of ~ 6µm. Ob-servation of the ring-like invagination athigh resolution reveals (Figs 5, 6) that the

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Fig. 7: Optical section as seenin the light microscope

through part of the fore tibiaof the female of Zelus luridus,

showing three types of hairs, aring-like invagination, and

numerous saccules ofglandular units inside the tibiaincluding one pore that openson the exterior surface of the

cuticle.


raised center consists of coarse cuticle withnumerous very small pores of less than 0.1µm in diameter. Ring-like invaginations al-so occur on the fore tibia of the first instarnymph of Z. luridus (Fig. 14), even thoughin fewer numbers than in the female. Theinvaginations in the nymph have the samedimensions and characteristics as those in

the adult. The observation of ring-like in-vaginations also in the first instar nymphcontradict WOLF & REID (2001), who statedthem to be present only in the adult.

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Figs 8-14: Thefore tibia in thefirst instar nymphof Zelus luridus. (8) Overview ofthe tibia showingsundew setae andone simple seta; (9) sundew setaewith sticky coverafter the nymphshas appliedsecretions fromthe eggs mass;(10) clean sundewseta of first instarnymph killedimmediately afterhatching; (11) peg-like seta; (12) close-up ofthe upper portionof a sundew setawith lateralprojectionsending in a knob;(13) close-up ofpeg-like seta; (14) ring-likeinvagination.


External pores and ductules ofinternal glandular units

In addition to the ring-like invagina-tions, a second type of pores is present onthe fore tibia of the adult, which was notmentioned by WOLF & REID (2001): Nu-merous simple circular pores of ~ 0.5 µm arespread over the cuticle (Fig. 5). Light mi-croscopical observation revealed that thesepores are connected through a cuticularcanal to glandular saccules that constitutepart of ductules and thus glandular unitsabundant in the tibia of the adult female(Fig. 7). Neither SEM nor light microscopi-cal observation revealed pores or glandularunits in the fore tibia of the first instarnymph.

Observations on the post-hatchingbehavior of the first instar nymph ofZelus luridus

The female deposited an egg mass com-prising 37 eggs during the night of the 21st ofJune 2004 (Fig. 15). Each egg was coatedwith a thin film of sticky substance. No ad-ditional copious layer of sticky substanceseems to have been applied to the entire eggmass. The nymphs hatched 11 days laterduring a period of approximately 7 hours.After hatching, which usually lasted 10 to20 minutes, the almost transparent nymphsmoved away from the egg mass. After ap-proximately 30 minutes, the color patternstarted to appear, which consists in the firstinstar nymph of dark annulations on thefemora and tibia and dark coloration of thetarsi as well as dark annulations on the an-tennae. As soon as the coloration started toappear, the nymphs moved back to the eggbatch, from which more nymphs were stillhatching (Fig. 16). The nymphs then start-ed to dip the tip of their right and left foretibiae alternatingly into the secretionaround the hatched or unhatched eggs atthe margin of the egg batch (Fig. 16). Thethreads of secretion between the egg massand the tip of the tibia were clearly observ-able (Fig. 17). Subsequently, the nymphsstarted to spread the sticky substance on theother appendages (Fig. 18). The sticky sub-stance derived from the egg mass was visibleas small translucent droplets that adhere tothe upper portion of the sundew setae ontibia and femur of the legs (Figs 16, 18). The

nymphs touched the tibia of the other legand the fore femora of the same leg with thesticky tip of the fore tibia. Besides, the tip ofthe fore tibia was stroked against the dorsalside of the middle femur, the middle tibiaagainst the hind femur and hind tibia, thehind tibia on the middle femur, and sporad-ically also the scapus of the antenna wastouched with the sticky fore legs. Occasion-ally, the tips of the fore femora were rubbedagainst each other. In a few instances,nymphs also touched the legs of anothernymph that was also engaged in applyingthe sticky substance to its appendages. Assoon as part of the appendages were coveredwith sticky material, some of the nymphsmoved away from the egg batch and contin-ued to spread the substances to as yet un-covered appendages, but occasionally re-turned to the batch to acquire new suppliesof viscous secretion.

Prey catching behavior of the firstinstar nymph

Specimens of Drosophila melanogasterwere introduced into cages with unfed firstinstar nymphs. Apparently aware of thepresence of the small flies, the reduviidnymphs raised their fore legs in such waythat the fore tibiae pointed out obliquely infront of the nymph. After a short while, a flybecame attached to the fore tibia of anymph and the tibia was moved towards thebody. The nymph then extended the labiumto get hold of the fly and presumably insert-ed the stylets, since movements of the flyceased immediately.

Observation on the post-moultingbehavior of one second instar nymph

After moulting to the second instar, thesingle nymph observed showed a behaviorthat resembled the behavior of recentlyhatched first instar nymphs: The freshlymoulted nymph started to touch the exuviawith the tip of the fore tibiae and then re-peated the process of applying sticky sub-stance to its appendages described above forthe first instar nymph.

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Female genitalia of Zelus luridus withdescription of a new composite glandand undescribed paired glandularareas on syntergite 9/10

Since the sticky cover of the egg mass inZelus luridus is apparently derived from thefemale, closer examination of the femalegenitalia including associated glandularstructures seemed desirable. The external fe-male genitalia comprise segments 8 and 9and their appendages, i.e. the valvifers ofthe 8th and 9th segment, and the valvulae 1to 3 (Fig. 19). Tergites 9 and 10 are fused inZelus luridus. This sclerite is here referred toas syntergite 9/10 and it forms a cover forthe genitalic appendages (Figs 19, 21). Theanus opens ventral to the syntergite 9/10.

The ectodermal portion of the internal fe-male genitalia consists of a large, membra-nous median bursa copulatrix that narrowsanteriorly into the median oviduct (Fig. 20).Unlike in many other Reduviidae, thepaired lateral spermathecae are not attachedto the median oviduct but insert in the an-terior portion of the bursa copulatrix (Fig.20). As in other Harpactorinae (DAVIS

1969), the median spermatheca or vermi-form gland is absent in Zelus luridus.

The subrectal gland, which was initiallydescribed by KERSHAW (1909) in theharpactorine Sycanus croceovittatus and ob-served by DAVIS (1969) in other Harpactori-nae, is present in Zelus luridus as a pair oflarge membranous lobes. The lateral lobes are

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Figs 15-18: Egg mass and post-hatchingbehavior of the first instar nymph of Z.luridus. (15) egg mass comprising 37 eggscovered with a translucent sticky secretion;(16) nymph touching the sticky cover ofthe egg mass with the tips of its foretibiae; (17) thread of sticky substance fromthe egg mass develops as the nymph foldsback the fore tibiae; (18) nymph appliesthe sticky secretion from the egg mass tothe fore femur of the opposite leg bytapping it with the apex of the sticky foretibia.


united medially, and they open through acommon opening ventral to the anus anddorsal to the valvulae 3. The surface of thesubrectal gland consists of a thin and heavilywrinkled membranous layer. UnpublishedSEM examination of the macerated subrectalgland in another Harpactorinae, Rhynocorisiracundus, failed to reveal pores in this mem-brane. The same result was obtained fromlight microscopical observation of the subrec-tal glands in Zelus luridus. Furthermore, noductules were observed during light micro-scopical observation of the subrectal glandsin Z. luridus. Thus, the subrectal gland unlikemany other glandular structures in Reduvi-idae, e.g. the metathoracic or abdominal

glands (STADDON 1979), seems to lack glan-dular units with sclerotized components.

However, two glandular structures com-prising ductules were found in this study tobe associated with the female genitalia ofZelus luridus. The ventrolateral parts of syn-tergite 9/10 show paired, well defined areas,which are beset with ductules. These duc-tules are characterized by saccules of ~ 30µm length (Fig. 22). The pores of theseglandular areas on the outside of syntergite9/10 could not be traced.

In addition, paired composite glandscomprising a tubular collecting canal andnumerous ductules with small saccules were

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Figs 19-22: Female genitalia andassociated glandular structures in thefemale of Z. luridus. (19) Female externalgenitalic structures in ventral view,showing valvifers, valvulae, and thesyntergite 9/10, indicated are also thepaired lateral glands associated withsyntergite 9/10; (20) ectodermal internalgenitalic structures, including bursacopulatrix with median oviduct and lateralspermathecae, subrectal gland and novelgland associated with syntergite 9/10, thesubstance inside the bursa is hereinterpreted as spermatophore; (21) syntergite 9/10 with paired lateralglandular areas, represented by numeroussaccules; (22) tubular gland associated withsyntergite 9/10, showing numerousglandular units as indicated by theirsaccules.


found in Zelus luridus beneath the syntergite9/10 (Figs 19, 20). The gland’s collectingcanal opens onto the anterior margin of thetergite 9/10, just dorsal to the articulation ofthe syntergite with the valvifers 2 (9th seg-ment). The wrinkled membranous surface ofthe collecting canal is beset with numerousductules. The saccules are ~ 10 µm long,and thus much smaller than the saccules ofthe syntergite 9/10 glandular areas describedabove.

Paired sternal glandular areas betweensternite 7 and valvifers 1 are absent in Zelusluridus. These glands were reported to bepresent in many Reduviidae, but absent inHarpactorinae (WEIRAUCH 2004).

Discussion

Sticky substances, which are used by thefirst instar nymph of Zelus luridus to trapprey, originate from the sticky cover of theegg batch. In the adults of Zelus leucogram-mus and Zelus luridus, on the contrary, glan-dular units in the fore tibia are the source forthe sticky cover of the fore tibia, which as-sists in trapping prey organisms (BARTH

1952; EDWARDS 1966). The secretion,which the female of Zelus luridus deposits onthe eggs and which the freshly hatched firstinstar nymph applies to its legs, is thus re-placed functionally with the secretion of theglandular units in the leg during a later de-velopmental stage.

WOLF & REID (2001) were unable tofind ring-like invaginations – abundant onthe fore tibia of the adult – on the legs of thefirst instar nymph of Zelus longipes. Sincethese authors interpreted ring-like invagina-tions as pores of glandular structures, they inconsequence claimed the absence of glandsin the first instar nymph of Z. longipes. Ac-cording to this observation and interpreta-tion, WOLF & REID (2001) then predictedthe use of sticky substances derived from theegg mass in prey capture by the first instarnymph of Zelus longipes, and they suggestedthe subrectal gland of the female as a poten-tial source for the viscous substances. Thepresent study, using Zelus luridus, not Zeluslongipes as a model, helps to clarify severalpoints, which remained unresolved in thestudy of WOLF & REID (2001) and which

were laid out as open questions above: Thetypes of setae and their fine structure in thefirst instar nymph and the female in Z.luridus correspond largely to the one pre-sented by WOLF & REID (2001) for Z.longipes. The observation of numerous glan-dular units – the potential source of thesticky cover of the legs – in the fore tibia ofadult Z. luridus by EDWARDS (1966) was cor-roborated in this study and their pores wereidentified on the surface of the tibia. Thefirst instar nymph of Z. luridus was found tolack glandular units of this type in the foretibia. First instar nymphs of Z. luridus wereobserved to gain a sticky substance from theegg mass, to spread it over their legs and touse this sticky cover during prey capture.

The subrectal gland of female Harpacto-rinae, which is also present in Zelus luridus, isnot the only gland associated with the femalegenitalia and thus not the only potentialsource for the sticky cover of the egg mass:Apart form paired glandular areas on synter-gite 9/10, a composite gland associated withsyntergite 9/10, which comprises numerousglandular units and a common collectingcanal, was discovered in the present study.

The leg of Zelus luridus compared tothe leg of Z. longipes

Sundew setae are numerous on the foretibia of first instar nymph and adult of Z.longipes (WOLF & REID 2001) and Zelusluridus and comprise a smooth lower portionand an upper portion that is beset with lat-eral projections. In both species, the sundewsetae are considerably longer in the adultthan in the first instar nymph. In Z. luridusthe projections are also more numerous andlonger in the adult than in the nymph.

In first instar nymphs that were allowedto cover themselves with viscous substancefrom the egg mass, the secretion seems toadhere mainly to the upper portion of thesundew seta that is equipped with lateralprojections (Fig. 9). This result agrees withobservations on Z. longipes (WOLF & REID

2001) and the hypothesis on the function ofthe sundew setae as holding devices for vis-cous substances as originally proposed byBARTH (1952).

At least in the first instar nymph of Z.luridus, sundew setae are not restricted to

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the fore tibia or to the legs, but also occur onother parts of the body, including the proxi-mal antennal segments, head and thorax.Additional observations are needed to tracethe fate of the sundew setae through the de-velopmental stages.

The peg-like setae observed in the first-instar nymph and adult of Z. luridus corre-spond to those described by WOLF & REID

(2001) in Z. longipes. The absence of poreson the shaft of these setae as indicated byWOLF & REID (2001) was corroborated in Z.luridus (Fig. 13). In contrast to what WOLF

& REID (2001) found in Z. longipes, simplesetae are present in Z. luridus also in the firstinstar nymph, even though only in smallnumbers.

The major discrepancy between the ob-servation and interpretation of leg struc-tures by WOLF & REID (2001) on Z. longipesand the results presented here lay in thering-like invaginations and the ductules ofglandular units and their associated pores.According to the results presented here, thelatter – the glandular units and their pores –are probably responsible for the viscous cov-er in the adult leg. The ring-like invagina-tions in Z. luridus seem structurally similarto those in Z. longipes, but they were hereshown to possess numerous very small poreson their raised central region. However, un-like in Z. longipes, where the ring-like in-vaginations were said to be restricted to theadult stage (WOLF & REID 2001), also thefirst instar nymph of Z. luridus possessesring-like invaginations. Furthermore, theproposed function of the ring-like invagina-tions as pores of glandular structures pro-posed by WOLF & REID (2001) appears un-likely for two reasons: Judging from externalappearance in the SEM, ring-like invagina-tions have properties of chemoreceptivesensilla, i. e. multiple very small pores onthe seta, and they may represent olfactorysensilla. Furthermore, the only type of glan-dular units observed inside the tibia of Z.luridus is connected to small pores that openon the integument (Figs 5, 7). The glandu-lar units, which are only present in theadult, are very likely responsible for the se-cretion of the viscous substance in the adult.A drawback of the interpretation of thering-like invaginations as sensilla and not as

gland pores is the fact that olfactory sensillaon an appendage that is covered with ex-trinsic viscous substance or intrinsic secre-tion are potentially useless. However, thering-like invaginations may have a functionwhile acquiring the sticky cover, not afterthe fact.

Viscous cover in the first and secondinstar and the adult

The post-hatching behavior of nymphsof Z. luridus documented in this study showsthat the sticky cover of the nymph is de-rived from the egg mass. Interestingly, thissticky cover seems to be transferred also tothe second instar, since a freshly hatchedsecond instar nymph was observed to collectand apply the viscous substance from itsown exuvia. A follow up study investigatinggland and hair morphology as well as behav-ior in the five nymphal instars may reveal atwhat stage the egg mass derived sticky cov-er is replaced by an intrinsic sticky cover,i.e. one that is secreted by the bug’s ownglandular units. Unclear also is, how longthe sticky egg mass cover preserves its vis-cous characteristic. The observed moult tothe second instar and use of the sticky cov-er of the exuvia took place more than 3weeks after oviposition. Thus it can be as-sumed that the stickiness lasts at least forthat period of time.

Observation of the prey capture ofDrosophila melanogaster by first instarnymphs in this study corroborates the hy-pothesis that the sticky cover of the legs as-sists during this behavior. However, otherpossible functions of the secretion cannot beruled out: Apart from fastening the egg massto the substrate, the viscous cover is alsothought to protect the eggs against para-sitoids (ALDRICH 1988) and it may preventthe eggs from desiccation. Transferred to thefirst instar nymph, the sticky cover mightthen also protect the nymph from predationthrough other insects. This hypothesiswould also account for the presence of sun-dew setae on parts of the body unlikely in-volved in prey capture, such as those on thedorsal surface of the head.

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Glandular structures associated withthe female genitalia as potentialsource for sticky egg mass cover

All Harpactorinae – and this includesZelus luridus – lack the median spermathecalgland (DAVIS 1969), which is responsible forthe production of egg cement in othergroups of Reduviidae, e.g. in Triatominae(LOCOCO & HUEBNER 1980a, 1980b). Thesubrectal gland has been proposed in thepast to be the source for the viscous coverfor the egg mass in many Harpactorinae(COBBEN & WYGODZINSKY 1975; WOLF &REID 2001). The subrectal gland occurs inmost Diaspidini, Harpactorini and Tegeiniamong Harpactorinae (DAVIS 1969; own un-published observation). The ultrastructureof the possibly aberrant eversible subrectalgland of Arilus carinatus (Harpactorini) wasdescribed by BARTH (1961). Light micro-scopical observation in this study failed toidentify ductules of glandular units on thesubrectal glands of Zelus luridus, and furtherinvestigation into the fine structure of thisgland and its potential function seems desir-able. In this study, I propose two alternativepossibilities for the source of the sticky eggcover in Harpactorinae: The female of Zelusluridus possesses paired glandular areas inthe posterio-ventral portion of the synter-gite 9/10, the secretions of which may play arole in mating or oviposition behavior. Thesame realm of possible functions also appliesto the composite tubular gland associatedwith syntergite 9/10, which was first de-scribed in this study. Further investigationinto the functional morphology of oviposi-tion in Harpactorinae as well as compara-tive morphological studies across Harpac-torinae will help to elucidate the functionsof the three glandular structures associatedwith the female genitalia in Zelus luridus.

Systematic perspective

The use of a sticky substance in preycapture of early instars of Z. luridus(Harpactorini, Harpactorinae) is paralleledin a striking way by the prey capture behav-ior of Apiomerus flaviventris (Apiomerini,Harpactorinae), where the first instarnymphs also collect sticky secretions fromthe egg mass to catch their prey (EISNER

1988). However, in contrast to the situationin Z. luridus, the sticky secretion on the egg

mass in A. flaviventris is not derived fromthe mother itself, but collected by her fromplants with viscous exudates and then ap-plied to the egg mass (EISNER 1988). Obser-vations on additional species of Apiomeriniand Harpactorini together with a phyloge-netic analysis of Harpactorinae are the nextsteps in order to gain an understanding ofthe evolution of sticky trap behaviour in theimmatures of this group of Reduviidae.

Acknowledgements

I wish to thank Randall T. Schuh for al-lowing me to examine dried specimens ofZelus luridus from the collection at theAmerican Museum of Natural History. Forcomments on the manuscript I am gratefulto Randall T. Schuh and Dimitri Forero.

Zusammenfassung

Im vorliegenden Beitrag wird gezeigt,dass Nymphen des ersten Nymphenstadiumsvon Zelus luridus STÅL (Harpactorinae, Har-pactorini) klebrige Substanzen vom Gelegeabsammeln und auf die Beine auftragen, umsie dann beim Beutefang einzusetzen. Schonbekannt war hingegen, dass Drüseneinheitenin den Beine der Imagines von Arten derGattung Zelus eine klebrige Substanz sezer-nieren, die ebenfalls beim Beutefang einge-setzt wird und dies wird hier fuer Z. luridusbestätigt. Während der Postembryonalent-wicklung von Z. luridus werden somit extrin-sische klebrige Substanzen, die von der Mut-ter stammen, funktionell durch eigen produ-zierte klebrige Drüsensekrete ersetzt. Bein-strukturen, darunter spezialisierte Haare,Sensillen und Drüseneinheiten, werden hierfür das erste Nymphenstadium und die Ima-go untersucht und dokumentiert. Das Ver-halten der Nymphen des ersten Nymphen-stadiums nach dem Schlüpfen, Beutefangund Häutung zum zweiten Stadium werdenbeobachtet. Zusätzlich zu den subrektalenDrüsen, die bei den Weibchen zahlreicherHarpactorinae bekannt sind, werden hierzwei bislang nich dokumentierte Drüsen be-schrieben, die als mögliche Quellen für dieSekretion auf dem Gelege in Frage kommen:Neben paarigen Drüsenbereichen auf denSyntergiten 9/10 wird hier für Z. luridus einetubuläre Drüse beschrieben, die mit demSyntergit 9/10 assoziiert ist.

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Address of the Author:

Dr. Christiane WEIRAUCH

American Museum of Natural HistoryDivision of Invertebrate ZoologyCentral Park West at 79th Street

New York, New York 10033, U.S.A.E-Mail: [email protected]

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