21

Properties of the hornet cuticle

CHAPTER 2

Properties of the hornet cuticle

Introduction

General information

The cuticle and in particular its propertiesplay an important role in the energy systemof the Oriental hornet (Vespa orientalis;HymenopteraVespinae) and so in the func-tion of the gravitational and directional or-gans.We started our research on the photoelectricproperties of the cuticle. Hornets tend to uti-lize light and heat energy of the sun by con-vert to electric energy. This energy is storedin a quasi-photo-voltaic cell, which in turn,is used, apparently, to create heat out of elec-tric current for thermoregulation of the indi-vidual insect and of the brood in the nest intimes of need (Gutmann and Lyons, 1981;Ishay and Barenholz-Paniry, 1995; Ishay etal, 1997).In the past we have studied various proper-ties of the cuticle of the Oriental hornet, suchas the photoelectric property (Croitoru et al,1978; Ishay and Croitoru, 1978; Ishay et al,1980a).Subsequently, we studied the effect thatxanthines and colchicine have on those prop-erties (Ishay et al, 1981; Rosenzweig et al,1985) and the temperature dependence of theelectric resistivity (Ishay et al, 1982). Addi-tionally, we studied the conditions which ef-fect the electric capacitance (Shimony andIshay, 1984) and furthermore the lumines-cence of the hornet cuticle (Ishay et al, 1988).

Electrical resistivity of the cuticle

The electrical resistivity to direct current ofthe brown and yellow strips of worker hornetcuticle was measured in the dark within a tem-perature range of 10- 32o C i.e. thermophotoe-lectric resistance (TPR). The effect of a num-ber of factors was assessed, such as: age (oneday old versus one week or more), pigment(brown or yellow) and diet (regular versusenriched with allopurinol or theophylline). Ineach of the possible 12 combinations of theabove factors, the cuticular resistivity wasmeasured in four continuous, but alternatingcycles of measurements of heating and cool-ing at temperatures ranging from 10-32o C.By use of a statistical model the followingfindings were obtained:(a) there is a clear temperature dependence ofthe electrical resistance.(b) under the influence of xanthines and struc-turally related substances (allopurinol) thecorrelation is practically linear, whereas intheir absence i.e when hornets are fed on sugarsolution only a marked hysteresis loop be-tween the warming and cooling lines is found.(c) the hysteresis loop obtained in the cuticleof Oriental hornets receiving sugar solutiononly, indicates the existence of cuticular“memory”.In summary, it means that drugs exert a sta-tistically significant effect, both on the levelof electric resistivity of the cuticle as well as

Parts of this chapter have appeared in :Physiol. Chem, Phys and Med. NMR 7 : 435-339 (1985)Physiol. Chem, Phys and Med. NMR 27 : 179-192 (1995)Physiol. Chem, Phys and Med. NMR 29 : 213-230 (1997)

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Properties of the hornet cuticleon the shape of the resistivity “line”. Neitherage or pigment excerts significant influence.On ordinary diet alone a hysteresis loop is ob-tained between the heating and cooling lines,which points to the effect of memory.

Thermoelectric and photoelectric currents inthe cuticle

Thermoelectric and photoelectric currents(TPC) were measured in the hornet cuticle.The spontaneous current, in the studied speci-mens, ranged between 30-40 nAmp underconditions- of darkness, whereas under illu-mination the current drops to near zero. Uponwarming up to 28- 29o C, the current rises to50-200 nAmp however subsequently, after awhile, it declines. This was obtained regard-less of whether the temperature is held steady,continued to rise or is lowered. In light, thecurrent values are close to zero. Factors caus-ing diminution of the current possibly arecooling, warming up beyond 29oC, illumina-tion and protracted measurement.

When the specimen is charged with an elec-trical current under fixed temperature, thecurrent attains several nAmp in darkness,however is usually less than that under illu-mination by an order of magnitude. The ca-pacitance values range between 1- 7 mFaradboth in light and in the dark. At a temperatureof 4o C or thereabout the cuticle undergoes“relaxation” and becomes charged. This oc-curs in the dark, for at least several hours,whereas transfer of the specimen to a highertemperature yields a discharge current of 30-40 nAmp, for two hours. Moreover, it is vitalfor the relative humidity to be high (about 90-99 %) thus enabling the episodes of chargeand discharge to be reversible, i.e., that thecuticle does not “tire” (Ishay et al, 1995).

The manner in which the cuticle undergoes“relaxation” during cooling in the dark andsubsequent rise in the discharge current upongradual warming in the dark qualifies it to be

named a material endowed with thermoelec-tric properties (Egli, 1960).From these studies it was pointed out that thecuticle acts like a photovoltaic system (Ishayet al, 1992) and that it exhibits a thermoelec-tric effect (Shimony and Ishay, 1981). Thecuticle behaves like an organic semiconduc-tor with traps (Ishay et al, 1980b).

Thermoregulation system of the extraretinalphotoreceptor apparatus

An important feature in the thermoregulationsystem is that of the extraretinal photorecep-tor apparatus. We have directed our study inparticular on the photoreceptor apparatus inthe yellow strips of the gaster of hornets. Thesystem is comprised of an air sac acting as abellows and of primary and secondary trachealducts which passes along a series of photore-ceptors and wind around the base of each pho-toreceptor to form individual tracheal loops.The respiratory rate changes in accordancewith the ambiant temperature and the physi-ological needs, so that within narrow limits,efficient thermoregulation is enabled by theconduction of air at the appropriate tempera-ture. The temperature of the conducted air isdetermined by an in situ process whereby ac-cumulating electric energy is converted to tothermal energy by a p-n junction system (theSeebeck effect). Additionally, the membranearound the trachea contains openings throughwhich a product of an olfactory gland isevaporated, that apparently serves as a ther-moregulatory pheromone.

The Oriental hornet breathes via trachea(Snodgrass, 1925; Duncan, 1939, Imms,1960). The gaster segments of hornets eachcontain a pair of spiracles on their tergitesplates. However, these openings of the tra-cheal system are clearly visible only on seg-ments II and III of the hornet, whereas on theother segments they can only be discernedwhen the abdominal segments are at fullstretch (Spradbery, 1973).

23

Properties of the hornet cuticle

As mentioned before, the spiracles admit airinto the tracheal ducts as well as into the dor-sal and ventral air sacs which extend trans-versely in the gaster. Wigglesworth (1963)lists a number of roles attributed to these airsacs, including the ventilation of muscles inthe course of flight of the insect.

For some time now we have been studyingwith the field emission scanning electron mi-croscopy and transmission electron micros-copy, the micromorphology (Goldstein andIshay, 1996) and the physiology (Goldsteinet al, 1996) of photoreceptors located in thegaster region of the hornet on the cuticularareas containing yellow pigment (Ishay et al.,1994; Kristianpoller et al., 1995; Ishay et al.,1997).

Consequently, we deemed it worthwhile toascertain the manner in which thermoregula-tion is effected in such parts of the cuticlewhich contain extraretinal photoreceptors –the so-called yellow stripes.Indeed, we identified morphologically in theseregions peripheral photoreceptors and thesewere encountered in all the species of socialwasps (Vespinae and Polistinae) which fly outof the nest in daytime for their daily needs(Goldstein and Ishay, 1996).

Details on the biology of social hornets andwasps have been published extensively in thelast 60 years (Duncan, 1939; Ishay et al., 1967;Wilson, 1971; Guiglia, 1972; Spradbery,1973; Edwards, 1980; Matsuura and Yamane,1990; Ugotini and Cannicci, 1996).In a previous study, one of the authors (JSIshay) investigated the respiratory rate dur-ing various daily activities of the hornet V.crabro which is prevalent throughout thenorthern hemisphere. Ishay found that whilestanding outside the nest this hornet breathes40-80 times a minute (compared to a muchslower respiratory rate inside the nest), whendrinking water it respires 100-135 times per

minute and while warming the pupal brood,between 160-210 times a minute.It stood to reason that such marked differencesin respiratory rate must be connected bothwith thermoregulation as well as with supply-ing oxygen to the tissues. As for the former,the question arose as to how the vespan res-piration acted to regulate the temperaturewithin the cuticle on the one hand and in itsambiance of the nest in general, on the other.The present chapter attempts to provide ananswer, albeit partial, to some of these que-ries.

Materials and Methods

Specimens of V. orientalis were collected fromnatural nests in the field in Israel, while speci-mens of V. crabro were collected in Frank-furt, Germany, as described elsewhere (Ishay,1964).Specimens to be tested were anesthetized bydiethyl ether and prepared, while narcotizedfor observation and photography through alight microscope and a Scanning ElectronMicroscope FE-SEM as well as by a methodpreviously described, using small strips offreshly collected homers (Ishay and Ganor,1992).It is worth pointing out that lightmicroscopyenables one to see the photoreceptors on theinner side of the cuticle and also a little of themorphology of surrounding structures, be theybeneath the basal membrane and under thehypocuticle, whereas FE-SEM does not en-able such viewing so long as the overlyinghypocuticle is intact.

Results

Figure 1 shows two hornets of V. orientalis:on the left one with an abdomen of ordinarylength and next to it, for comparison purposes,a queen with an expanded abdomen (gaster).In the latter, one notes that between the gas-tral segments with yellow pigment there ap-pears a cuticular stripe of a brown color.

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Properties of the hornet cuticleIn a hornet with an abdomen in the normal(contracted) state, such a brown stripe (and aconnection membrane) is masked by the yel-low stripe of the preceding segment becausethe distal stripes of yellow cuticle on the gasterusually protrude beyond the segment to over-lie the surface of the next segment.All the gastral segments are arranged like atelescope that can be artificially lengthened,either by pulling on the extremities as de-scribed in Figure 1 (left) or naturally duringthe process of respiration. This abdominal ex-tension is enabled owing to the fact that be-tween each two successive segments there isa connecting membrane which has beennamed the intersegmental conjunctive (ic)(Duncan, 1939). This IC is elastic so that dur-ing rest, in the relaxed state, the one segmentforms an eave which overlies the segmentbehind it. In this way we get “an extensivereduplication.” “This strengthens the tergurnand provides a sclerotized bearing to glideover the base of the succeeding segment”

(Duncan, 1939). This is also the area whereparasite insects of the strepsiptera stylopide(i.e., parasitize). As adults they ‘sit’ on thedorsal site of the abdomen between the scler-ites (Krombein, 1967). In our case, in gastralsegments 4 and 5, the part of the segment sche-matically demonstrated in Figure 2, that pro-duces the reduplication is invariably that con-taining the yellow pigment. In this ‘yellow’region both in the dorsal and ventral surfacesof the cuticle, there are two brown spots (I inFigure 2). If we evert the segment and inspectits interior surface (i.e., the surface facing thebody), we note that from the air sac whichextends across the brown region of the seg-ment (2 in Figure 2) emerge tracheal ductsthat proceed in the direction of the yellowstripe; these ducts emerge either individuallyfrom the central region of the air sac (3 in Fig-ure 2) or, from each lateral end of the air sac.These emerge as a braid of tracheae that ter-minates in the region of the brown spot onthe inner side of the yellow cuticle (4 in Fig

FIGURE 1. Two hornets are shown of which the one on the left has a contracted gaster as is customary during rest.The specimen on the right has been intentionally stretched in order to show firstly the difference in abdominal lengthin the two conditions (extension and relaxation) and secondly to reveal the two brown stripes which during relaxationare concealed underneath the other plates.

25

Properties of the hornet cuticle

ure 2). A similar pattern was displayed alsoon the inner, ventral side of the cuticle. Thetracheae comprising the ‘braid’ split into thin-ner ducts which proceed beneath the basalmembrane and the hypocuticle. Here each tra-cheal duct passes across a ‘line’ of photore-ceptors (usually 8-15), to each of which itsends a tracheal ‘loop’, which circumvents theventral side of the photoreceptors (6 in Fig-ure 2 represented as small circles). The yel-low stripe terminates in a tuft of setae (smallhairs). The membrane which interconnects thetwo brown spots (6 in Figure 2) represents amembrane, which links the segment to the onein front (and in the intact hornet this wouldbe the underside). This intersegmental con-junctive is the IC and it actually separates what

is attached to the interior of the body. Theupper part, above the membrane is protrud-ing outwards to the eave or reduplication. TheIC membrane thus bisects the yellow stripeapproximately at the middle.Figure 3 provides a schematic representationof the peripheral photoreceptor indicating themost prominent structures and their relativedistribution.The membranes of the air sacs and tracheaein V. orientalis are shown in Figures 47. Onesuch ramification leading into a tracheae (1),the structure of the outer membrane (2) andthe structure of the inner membrane (3) is dis-played in Figure 4. Figure 5 comprises a pho-tograph (in part) of the inner membrane in anair sac with longitudinal, hardened ribs (1) and

FIGURE 2. A scheme demonstrating structure of the cuticular yellow stripes. On top can be seen a homet with yellowstripes (-x I). In each of these stripes there are, both in the upper tergite and the bottom sternite, two brown dots (orspots) (1). When one inverts such a yellow stripe and views its interior surface under a microscope, one can see an airsac extending across the brown part of the segment (2) from which emerge braided clusters of tracheae that proceedto each of the two brown spots (4). From the same air sac arise also more attenuated tracheal ducts (3) and similarlythese ducts pass underneath the BM and the hypocuticle to reach the lower part of the photoreceptors and continue tothe end of the stripe (5); these also loop around each photoreceptor. The yellow stripe terminates with a line ofdispersed hairs (not seen here). Separating between the part protruding from the body and the part contained withinthe body is a membrane (6). A branch of a trachea which surrounds the base of a photoreceptor is shown (7) and thelower part of the photoreceptor enlarged to join the hypocuticular layers (8). YG-yellow ganules; BM-basal mem-brane; IC-intersegmental conjunctive; A-upper side of the cuticle (epicuticle); S-spiracle.

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Properties of the hornet cuticlethe ridges surrounding the passage to the tra-chea. Between the hardened ribs secondaryribs can be seen that apparently, respond totension. In Figure 6 we see a striped mem-brane in which some of the bands are pro-truded and hardened. The bands are comprised

of 2-3 secondary bands extending to a lengthof 10µm or more, originating from a regionat some remove from the connection with thetrachea. In all these three figures (4, 5 and 6),one can discern, at various magnifications,membranes of the air sac. In segment 4 of the

FIGURE 3. Schematic representation of the peripheral photoreceptor. 1) the epicuticle; 2) the reticular fibers at theperiphery of the bulge; 3) the exocuticle; 4) the axon that connects to the membrane of the bulge and the axonalmicrofibrils; 5) the gap junction of glial membranes; 6) the outer vitreous body; 7) the inner vitreous body; 8) theannular trachea; 9) the granular area at the center of the photoreceptor cell; 10) the microlamellae and their blackedges; 11) the nucleus of the PR cell; 12) a glial cell; 13) nucleus of glial cell; 14) basal membrane cell; 15) cuticleunder the basal membrane; 16) reticular tissue at the lower end of the bulge; 17) the pigment granules at the peripheryof the PR cell 18) the cuticular lamellae

27

Properties of the hornet cuticlegaster: these membranes bear contractions thatare typical for piezoelectric membranes.Those respond to tension and pressure (me-chanical stress) by producing electricity (orelectric polarity).Apparently, these portions of the membranesare capable of extending or contracting as theneed arises and according to the content ofair in the air sac. A typical trachea with annu-lar hardenings which protrude from the innersurface of the trachea the tenidia (1) is shownin Figure 7. Everywhere, small masses arepresent which apparently contain aggregatesof lipid cells. Presented in Figure 8 is a viewof the inner side of segment 4 in the gaster ofthe homet. Here “I” indicates the surface ofthe region containing yellow pigment. Next,“T’ indicates the attachment point of the‘braid’ of the tracheal ducts to the ‘spot’ ofbrown cuticle. The interconnecting membranewhich encircles this region belongs (and con-

nects) further on to the IC is situated moreanteriorly to it. On the right side of the figure,there is also the same ‘braid’ of tracheae, iscovered and therefore invisible. Yet the ‘braid’connects with the air sac through thicker ducts(4) that link it to the sac (5). Behind the ICone can discern an amorphic mass (6) whichis possibly a glandular excretion (see below).A somewhat more enlarged view, of the tra-cheal ‘braid’ is shown in Figure 9, in whichthe following landmarks are discernible: I )the inner membrane of the brown stripe of thesegment (No. 4): 2) the membrane of the airsac with the annular bracings on the inner side:3) a ‘braid’ of tracheal ducts that interlink tothe brown ‘spot’ on the cuticle containingyellow pigment: 4) the epithelium of the IC:and 5) the inner region of the yellow stripe.Further details of the air sac are shown in Fig-ure 10. Here, we can see: annular bracings (I)tracheal ducts interlinking one sac to the other

FIGURE 4. The microstructure of the air-sac membranes at their juncture with a tracheal duct. Noticeable are struc-ture of the membrane at the junction point (1). structure of the external membrane (2) and of the internal one (3).Even from this viewing. it is reasonable to suppose that these membranes are stretchable when the air sac fills up andcontractible when the air sac empties, whether externally through the spiracles (S) in Figure 2) or internally into thetracheae and tracheoles that carry air to the tissues and cells.

1

2

3

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Properties of the hornet cuticle

FIGURE 5. Girding the passage to the tracheal duct are a series of ridge I ) on the epithelium, with secondary ridgesbetween them. resembling the slats of a sunshade. These configurations are apparently typical for membranes thatundergo stretching and relaxation

FIGURE 6. Prominent ridges are visible which are comprised of 2 or more stripes (1). More prominent are thosewhich are braided together. Each of the secondary stripes that comprise the primary ridged stripe is shorter than thetotal stripe. The membrane was photographed from a distance at some remove from the opening of the tracheal duct.What we see are the ridges appearing intermittently.

1

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Properties of the hornet cuticle

FIGURE 7. Typical tracheal outlet with annular ridges on the side facing the tenidium (1). Everywhere nearby and onthe exterior of the trachea can be seen small protuberances (2) containing, apparently, aggregates of fat cells

FIGURE 8. A SEM micrograph showing the inner surface of the yellow stripe (1), the entry point of the tracheal‘braid’ into the brown spot (2). The membrane designated as intersegmental conjunctive (IQ (3). tracheal ductsentering the air sac (4) and finally portions of the air sac (5). Note that a picture taken by SEM does not enableviewing beneath the BM and hypocuticle and therefore one cannot see the lower site of the photoreceptors, nor thetracheal loops which envelop them. These structures can be seen through a light microscope. Behind the IC aremasses of an amorphic material (6). These may possibly represent a gland that produces pheromones

1

2

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Properties of the hornet cuticle(2): bracing bands extending down the innerlength of the sac (3); and finally the support-ing membrane across the tracheal ducts whichproceed into the ‘braid’ (4). The tracheaewhich emerge from the air sac gradually nar-row into more delicate ducts (1 in Figure II)which receive support from transverse mem-brane inside (2 in Figure II). This supportingmembrane is increased in size upto to thelower portion of the tracheal duct ‘braid’which is fully replete with transverse support-ive membrane. This membrane appears like amesh or rete fastened on the exterior of allducts in the braid (see 1 in Figure 1). Under-neath the membrane (BM + hypoticle) are thetracheal ducts.At the point of entry to the brown spot area (itlooks like a basket), the “network ” individu-ally enveloping each duct comes to an end. Inthe same region (Figure 13), the cuticle ofbrown spot within the “yellow “area forms

an elevation reminiscent of a stunted gobletand this at the juncture point between the tra-cheae and the cuticle (which is not seen in thepicture); around and quaquaversal (that radi-ate from a central mass) from the goblet(which height from the distal side of the cu-ticle is about 0.5 mm) extends the membranecalled IC.This membrane which covers the trachealbraid on the distal side is comprised of bands.Hence in Figure 13, we discern the ICmade of bands (10, the bottom of the trachealduct braid (2), and the tracheal ducts envel-oped in a supportive membrane (3).An enlargement of the membrane at the baseof the tracheal duct braid is shown in Figure14. One clearly sees that the area is repletewith bands, whose width is usually less than10 µm and sometimes even as little as 5 µm,while on the underside of the bands (2) thereare “fringes” (like the fringes of a dress) of

FIGURE 9. View of a tracheal ‘braid’. One can see theinner membrane in the brown stripe of segmemt IV (I).the membrane of an air sac with lacework bracings onthe inner side (2). a , “braid’ of tracheal ducts (3) whichconnect with the brown spot in the yellow stripe of’ thesegment. the IC membrane (4) and the inner region ofthe yellow stripe (5).

FIGURE 10. Further enlargement of the previous pic-ture, showing the latticed bracings ( I ). tracheal ductspassing between two air sacs (2). supportive ridges along,the inside of the sac (3). supportive membrane across thetracheal ducts that form the braid (4) and finally the ICmembrane (5).

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3

4

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Properties of the hornet cuticle

FIGURE 11. The tracheal ducts which form the *braid*. In the top part of the picture the ducts are thicker (I) but thesegradually “split” into more delicate ducts (2) At bottom can be seen a supportive netting that enwraps each cluster ofducts

FIGURE 12. This micrograph shows the supportive network ( I ) that envelops the tracheal ducts that pas, from the airsac into the brown spot of the yellow stripe.

1

32

Properties of the hornet cuticle

FIGURE 13. On the right can be seen a basket-like structure. This is a structure that is part of the IC enveloping thetracheal ‘braid’ as it connects with the brown spot on the yellow stripe ( I ). Above the basket-like structure one cansee the ends of’ the tracheae (2) which are surrounded by a supportive network (3), This network seems to be a partof and interlinked with the membrane of the air sac (4). On close inspection. one notes that the IC membrane whichmakes up the basket-like structure is composed of multiple bands (see below).

FIGURE 14. Enlargement of the membrane in the basket which is at the base of a tracheal ‘braid’. One notes that themembrane is multiple banded ( 1). each hand 5-10 µm in width. there are gaps between each two bands (stripes) and“fringes” are suspended from these bands to touch the underlying band. The “fringes” are of variable length andbetween every group of “fringes” there are gaps (3).

1

2

3

4

1

2

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33

Properties of the hornet cuticle

FIGURE 15. Enlargement of a number of bands and “fringes’ that were shown in the previous Figure. One candiscern groups or fringes (4-12) of variable length (arrow), all of which are touching the band underneath them.Between one group of “fringes” and another, there is a gap (1) and there are slits of about I µm between every twobands at the bases of the “fringe”( 2). The bands are concentric, with intercalation, of other bands, so that there is nodirect parallel continuity of long bands.

FIGURE 16. Micrograph taken via light microscope of the tracheal duct(s)) passing between the photoreceptorsunderneath the BM and hypocuticle. The light-colored circles are the photoreceptors and the light reflecting throughthem from the other (upper) end of the cuticle(1) Also visible is the tracheal ducts passing along a row of photorecep-tor (3). The magnification is x150. Bar=100µm

1

2

1

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34

Properties of the hornet cuticle

FIGURE 17. Enlargement of a portion of the previous figure. Hcrc, can be seen a tracheal branch passing between thephotoreceptors ( I ) and looping around each one of them (2). Also visible are what appear to be nerve branches ( 3).Magnification x400. ar 100 µm

FIGURE 18. After peeling off the BM and hypocuticle in the yellow stripe one sees the base of the photoreceptor (1).the concentric rings of cuticle enveloping the entire photoreceptor (2). the granules of yellow pigment (3) and thetracheal loop that envelops the base of the photoreceptor (4). Note that the external diameter ot the tracheal loop hereis about 3 mm. We need to point out that the specimen from which this picture was taken was the hornet V. crabro.

1

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Properties of the hornet cuticlenonuniform length which impinge upon theband underneath them. The bands are attenu-ated only in the region abutting the tracheal‘braid’ whereas outside this region, they arebroader (3).A larger magnification of these bands is givenin Figure 15, where one can see: (1) areas inwhich the bands are not separate throughoutbut only intermittently so (1). (2) the numberof “fringes” in each bundle is between 4-12.(3) the gap between any two bands issmallabout 12 µm (2). (4) the “fringes” are ofnonuniform length usually between 110 µm.(5) in the same bundle there are no two con-secutive bands of the same length. All the“fringes” touch at various places the lowerband (arrow).Tracheae pass through the tracheal ‘braid’ andthen beneath the BM and the hypocuticle fol-lowing which they traverse the yellow cuticleand contribute a tracheal loop around the bot-tom of each photoreceptor. All this is shownin Figure 16, which was taken via LM. Herewe see clear circles through which light re-flects from the other (upper) side of the cu-ticle; these are the photoreceptors. See alsoFigure 2 (6) and Figure 3 (4), this is the lowerend (the inner cuticle) (arrow at 1) with a tra-cheal loop around the circle (arrow at 2) andalso tracheal ducts that pass longitudinally,arrow at (3).In Figure 17, taken through a LM at highermagnification (x 1000), one can see a branch-ing of the tracheae ( 1) which sends a trachealloop to the base of a photoreceptor (arrow at2). In the region is discernible also a neuralfibre (3).Removal of the BM together with the hypo-cuticular layers reveals the lower side of thephotoreceptor, Figure 18 (1), with concentricrings of cuticle sealing it there (2), granulesof yellow pigment (3) and also part of theannular trachea which girdles the base of thephotoreceptor (4). This picture was taken fromthe cuticle of V. crabro, whereas all the othersare based on the cuticle of V. orientalis.

Discussion

The system described in the present studycontains: (a) A mechanism that enables thepassage of air in pipes, as in a thermoregula-tory apparatus, and is comprised in the caseof hornets of air sacs of variable volume thatact as a bellows and of hardened tracheaewhich transport the air and split into moreslender and elastic branches that pass amongthe photoreceptors. Except in the region ofthe spiracles, where the air can be admittedor expelled by the opening or closure of theserespiratory structures, the rest of the trachealsystem is a closed one. (b) The outlets ofglands that release their evaporant contentsbetween the segments of the gaster. We con-jecture that the described system is gearedprimarily to push and compress air into thebases of the photoreceptors so as to regulatethe temperature there and if so, the rate ofrespi-ratory movement is coordinated withphysiologic activities of the hornet. The cool-ing or warming accomplished via the trans-port of air within pipes is well known andwidely used in human technology.The process requires, of course: (1) thermo-sensors that are sensitive to temperature sothat any deviation from the desired tempera-ture will activate a corrective system. (2) thedetermination of a set point, that is, the opti-mal temperature which needs to be an equi-librium between heat creation and heat lossand (3) the ability to raise or lower, ad libi-tum, the temperature of the air conductedthrough the pipes especially through thoseloops of tracheae around the base of the pho-toreceptors. As for the latter set-point, thereis solid evidence that the temperature in thenest of hornets (Vespinae) is constantly set to29oC (Ishay et al., 1967; Ishay and Runner,1971; Ishay, 1972; Ishay, 1973; Heinrich,1981; Ishay and Barenholz-Paniry, 1995).It may also be presumed that thermoregula-tory sensors are distributed throughout thehornet cuticle. Additionally, there are tracheaethroughout its length, while air sacs are ar

36

Properties of the hornet cuticleranged transversely in the abdominal seg-ments as well as elsewhere. The air sacs areactively contractible and can thus compressthe air in the system through the use ofmuscles, following which they relax passively.The air sacs which are arranged crosswise inthe abdominal segments underneath the cu-ticle can also act as cushions that glide be-tween the partly superposed segments andthus prevent excess friction between themduring respiration. Judging strictly by their morphological ap-pearance, the membranes of the air sac ap-pear to be piezoelectric, responding to pres-sure by creating an electric voltage whichhelps to signal the physical properties of theair. The air that flows in the regions of theyellow cuticle provides oxygen to the localcells, as is to be expected, but the density ofthe tracheae in these regions points to another,very important role, namely, thermoregula-tion. Such thermoregulation results from theuptake of light in the visible wavelengths andof heat and their conversion to electric energy,whether as voltage (under illumination) or ascurrent (in darkened, inner portion of the cu-ticle or under conditions of darkness) and byanalogy the cuticular region resembles a so-lar cell (Maycock and Stirewalt, 1981; Ishayet al.,1982; Ishay and Litinetsky, 1996; Ishayet al., 1997).

In view of the fact that the cuticular stripespossessing yellow pigment protrude to theexterior of the body, it seems that maintenanceof their proper function is crucial, and this forthe following reasons: (a) intensive or ordi-nary respiratory activity persists even whenthe hornet is anesthetized (Ishay et al., 1994)as well as when the abdomen (of any insect)is detached from the body (Huber, 1960;Miller, 1965; Farley et al., 1967), being regu-lated by the ventral ganglia, both the thoracicand abdominal ones (Grass, 1976). (b) theconcentration of tracheae in a ‘braid’ whicheffectively provides tracheal rami to the yel-low stripes suggests an effort to ensure maxi-

mal yellow stripe surfaces that are free fromany interference by other tissues (and actingas thermoradiators). (c) the creation of a tra-cheal loop around the base of each photore-ceptor. We still need to address the questionas to how the air flowing through the trachealducts acquires the desired temperature.At temperatures below optimal the adult hor-nets (and wasps) commence to blow hot airaround the developing brood (pupae) andthereby warm it to the desired temperature(Ishay and Rutter, 1971), whereas when thetemperature is above optimal, the adult hor-nets within the nest commence to ventilate thebrood or the entire nest (Ishay et al., 1967;Sadeh et al., 1977).Thus, while the air sacs do contain the greatershare of the air supply, the temperature of theair is determined according to need by its pas-sage through the tracheal loops, which girdthe envelope of the photoreceptor. Here, eachtracheal loop comes in contact with 1) the cu-ticular envelopes of the photoreceptor, theyare electrically of n type (i.e., electron accep-tors); and. 2) the yellow granules, which areof p type (i.e., electron donors).Flow of air passing through the tracheal loopis thus exposed to this p-n junction where elec-tric energy is stored, or caused to flow, andthis electric energy is transformed, accordingto need, into thermal energy, which is utilizedin thermoelectric circuits such as have beendescribed originally by Seebeck and Peltier.This process is geared primarily to providethermoregulation of areas of the abdominalcuticle or other areas that contain extraretinalphotoreceptors, since it is crucial to keep thephotoreceptors from overheating. The ther-moregulatory activity is important also for theentire nest, just as in vertebrates the excessheat produced in the striated muscles and theliver is transported via the circulation to allparts of the body.It is known that in humans, the metabolic pro-cess which takes place in the retina, needs highsupply of oxygen and creates energy excess.The high rate of blood flow through the uvea

37

Properties of the hornet cuticleis very important by giving high pO

2 in the

uvea, which facilitates the diffusion of oxy-gen into the retina and it also helps to protectthe eye from thermal damage even underrather extreme conditions, such as arcticsnowstorms, Finish sauna bath, and observa-tion of very bright objects (Moses, 1975).We see in the air supply system in the Orien-tal homet the exact analogue of the choroidalsystem in humans fulfilling the same physi-ological functions and complementary to it asource of energy for its physical activity.As for vaporization of volatile substancesthrough the openings in the membrane desig-nated as IC near the brown spots in the yel-low stripes, we note that Spradbery (1973)points out that in sternite VI of hornet abdo-men a gland is located, which Vecht (1968)described as one that releases pheromones.What we have described herein is probably adifferent gland, because its numerous open-ings are located on gastral segment IV, andthe gland itself is situated on a tergite (a dor-sal plate). So far as we know, no glands havebeen described in the specified segment andours is thus a first description.We are in the dark yet as to the chemical na-ture of the substance evaporated from the de-scribed gland, but the mere fact of being con-

nected to the bellows system of the air sacswith tracheae must enhance its efficiency. Wehave observed that when the bellows increa-ses its activity the gland releases its volatilecontents at a comparable rate and accordingto the intensity of respiration. It stands to rea-son that the gland is associated with ther-moregulation, perhaps also signaling, alteringand recruiting in emergencies, to the urgentneed of cooling or heating the brood. If thisbe true, then this is the venue of thermoregu-latory pheromone which has been speculatedabout years ago (Ishay, 1972; Königer, 1977).Interestingly, a structure comprised of fringeshas previously been reported in termites(Caloterme fiavicollis) by Lebrun (1971) inconjunction with a tergal gland in the 9th ab-dominal segment. The fringes in his and alsoin our case are of variable length and all pro-trude downwards where they encounter theunderlying epithelial layer and act as mecha-noreceptors, being responsible, apparently forsensing the distance between two adjacentepithelial layers and accordingly regulatingthe local air pressure. It seems that the vari-able length of the fringes enables to differen-tially determine the distance from one epithe-lial layer to the next and also gauges air pres-sure extent in the system.

38

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