We started our research on the photoelectric properties of the cuticle. Hornets tend to utilize light and heat energy of the sun by convert to electric energy. This energy is stored in a quasiphotovoltaic cell, which in turn, is used, apparently, to create heat out of electric current for thermoregulation of the individual insect and of the brood in the nest in times of need (Gutmann and Lyons, 1981; Ishay and Barenholz-Paniry, 1995; Ishay et al, 1997). In the past we have studied various properties of the cuticle of the Oriental hornet, such as the photoelectric property (Croitoru et al, 1978; Ishay and Croitoru, 1978; Ishay et al,1980a). Subsequently, we studied the effect that xanthines and colchicine have on those properties (Ishay et al, 1981; Rosenzweig et al, 1985) and the temperature dependence of the electric resistivity (Ishay et al, 1982). Additionally, we studied the conditions which effect the electric capacitance (Shimony and Ishay, 1984) and furthermore the luminescence of the hornet cuticle (Ishay et al, 1988).

Electrical resistivity of the cuticle
The electrical resistivity to direct current of the brown and yellow strips of worker hornet cuticle was measured in the dark within a temperature range of 10-32 °C i. e. thermophotoelectric resistance (TPR). The effect of a number of factors was assessed, such as: age (one day old versus one week or more), pigment (brown or yellow) and diet (regular versus enriched with allopurinol or theophylline). In each of the possible 12 combinations of the above factors, the cuticular resistivity was measured in four continuous, but alternating cycles of measurements of heating and cooling at temperatures ranging from 10-32 °C. By use of a statistical model the following findings were obtained: (a) there is a clear temperature dependence of the electrical resistance. (b) under the influence of xanthines and structurally related substances (allopurinol) the correlation is practically linear, whereas in their absence i. e when hornets are fed on sugar solution only a marked hysteresis loop between the warming and cooling lines is found. (c) the hysteresis loop obtained in the cuticle of Oriental hornets receiving sugar solution only, indicates the existence of cuticular "memory". In summary, it means that drugs exert a statistically significant effect, both on the level of electric resistivity of the cuticle as well as on the shape of the resistivity "line". Neither age or pigment excerts significant influence. On ordinary diet alone a hysteresis loop is obtained between the heating and cooling lines, which points to the effect of memory.

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)

When the specimen is charged with an electrical current under fixed temperature, the current attains several nAmp in darkness, however is usually less than that under illumination by an order of magnitude. The capacitance values range between 1-7 mFarad both in light and in the dark. At a temperature of 4 °C or thereabout the cuticle undergoes "relaxation" and becomes charged. This occurs in the dark, for at least several hours, whereas transfer of the specimen to a higher temperature yields a discharge current of 30- 40 nAmp, for two hours. Moreover, it is vital for the relative humidity to be high (about 90- 99 %) thus enabling the episodes of charge and discharge to be reversible, i. e., that the cuticle does not "tire" (Ishay et al, 1995).

The manner in which the cuticle undergoes "relaxation" during cooling in the dark and subsequent rise in the discharge current upon gradual warming in the dark qualifies it to be named a material endowed with thermoelectric properties (Egli, 1960). From these studies it was pointed out that the cuticle acts like a photovoltaic system (Ishay et al, 1992) and that it exhibits a thermoelectric effect (Shimony and Ishay, 1981). The cuticle behaves like an organic semiconductor with traps (Ishay et al, 1980b).

Thermoregulation system of the extraretinal photoreceptor apparatus
An important feature in the thermoregulation system is that of the extraretinal photoreceptor apparatus. We have directed our study in particular on the photoreceptor apparatus in the yellow strips of the gaster of hornets. The system is comprised of an air sac acting as a bellows and of primary and secondary tracheal ducts which passes along a series of photoreceptors and wind around the base of each photoreceptor to form individual tracheal loops. The respiratory rate changes in accordance with the ambiant temperature and the physiological needs, so that within narrow limits, efficient thermoregulation is enabled by the conduction of air at the appropriate temperature. The temperature of the conducted air is determined by an in situ process whereby accumulating electric energy is converted to to thermal energy by a p-n junction system (the Seebeck effect). Additionally, the membrane around the trachea contains openings through which a product of an olfactory gland is evaporated, that apparently serves as a thermoregulatory pheromone.

The Oriental hornet breathes via trachea (Snodgrass, 1925; Duncan, 1939, Imms, 1960). The gaster segments of hornets each contain a pair of spiracles on their tergites plates. However, these openings of the tracheal system are clearly visible only on segments II and III of the hornet, whereas on the other segments they can only be discerned when the abdominal segments are at full stretch (Spradbery, 1973). As mentioned before, the spiracles admit air into the tracheal ducts as well as into the dorsal and ventral air sacs which extend transversely in the gaster. Wigglesworth (1963) lists a number of roles attributed to these air sacs, including the ventilation of muscles in the course of flight of the insect.

For some time now we have been studying with the field emission scanning electron microscopy and transmission electron microscopy, the micromorphology (Goldstein and Ishay, 1996) and the physiology (Goldstein et al, 1996) of photoreceptors located in the gaster region of the hornet on the cuticular areas containing yellow pigment (Ishay et al., 1994; Kristianpoller et al., 1995; Ishay et al., 1997).

Consequently, we deemed it worthwhile to ascertain the manner in which thermoregulation is effected in such parts of the cuticle which contain extraretinal photoreceptors Ñ the socalled yellow stripes. Indeed, we identified morphologically in these regions peripheral photoreceptors and these were encountered in all the species of social wasps (Vespinae and Polistinae) which fly out of the nest in daytime for their daily needs (Goldstein and Ishay, 1996).

Details on the biology of social hornets and wasps have been published extensively in the last 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 (JS Ishay) investigated the respiratory rate during various daily activities of the hornet V. crabro which is prevalent throughout the northern hemisphere. Ishay found that while standing outside the nest this hornet breathes 40-80 times a minute (compared to a much slower respiratory rate inside the nest), when drinking 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 differences in respiratory rate must be connected both with thermoregulation as well as with supplying oxygen to the tissues. As for the former, the question arose as to how the vespan respiration acted to regulate the temperature within the cuticle on the one hand and in its ambiance of the nest in general, on the other. The present chapter attempts to provide an answer, albeit partial, to some of these queries.

Materials and Methods
Specimens of V. orientalis were collected from natural nests in the field in Israel, while specimens of V. crabro were collected in Frankfurt, Germany, as described elsewhere (Ishay, 1964). Specimens to be tested were anesthetized by diethyl ether and prepared, while narcotized for observation and photography through a light microscope and a Scanning Electron Microscope FE-SEM as well as by a method previously described, using small strips of freshly collected homers (Ishay and Ganor, 1992). It is worth pointing out that lightmicroscopy enables one to see the photoreceptors on the inner side of the cuticle and also a little of the morphology of surrounding structures, be they beneath the basal membrane and under the hypocuticle, whereas FE-SEM does not enable such viewing so long as the overlying hypocuticle is intact.

Results
Figure 1 shows two hornets of V. orientalis: on the left one with an abdomen of ordinary length and next to it, for comparison purposes, a queen with an expanded abdomen (gaster). In the latter, one notes that between the gastral segments with yellow pigment there appears a cuticular stripe of a brown color. In a hornet with an abdomen in the normal (contracted) state, such a brown stripe (and a connection membrane) is masked by the yellow stripe of the preceding segment because the distal stripes of yellow cuticle on the gaster usually protrude beyond the segment to overlie the surface of the next segment. All the gastral segments are arranged like a telescope that can be artificially lengthened, either by pulling on the extremities as described in Figure 1 (left) or naturally during the process of respiration. This abdominal extension is enabled owing to the fact that between each two successive segments there is a connecting membrane which has been named the intersegmental conjunctive (ic) (Duncan, 1939). This IC is elastic so that during rest, in the relaxed state, the one segment forms an eave which overlies the segment behind it. In this way we get "an extensive reduplication." "This strengthens the tergurn and provides a sclerotized bearing to glide over the base of the succeeding segment" (Duncan, 1939). This is also the area where parasite insects of the strepsiptera stylopide (i. e., parasitize). As adults they 'sit' on the dorsal site of the abdomen between the sclerites (Krombein, 1967). In our case, in gastral segments 4 and 5, the part of the segment schematically demonstrated in Figure 2, that produces the reduplication is invariably that containing the yellow pigment. In this 'yellow' region both in the dorsal and ventral surfaces of the cuticle, there are two brown spots (I in Figure 2). If we evert the segment and inspect its interior surface (i. e., the surface facing the body), we note that from the air sac which extends across the brown region of the segment (2 in Figure 2) emerge tracheal ducts that proceed in the direction of the yellow stripe; these ducts emerge either individually from the central region of the air sac (3 in Figure 2) or, from each lateral end of the air sac. These emerge as a braid of tracheae that terminates in the region of the brown spot on the inner side of the yellow cuticle (4 in Fig ure 2). A similar pattern was displayed also on the inner, ventral side of the cuticle. The tracheae comprising the 'braid' split into thinner ducts which proceed beneath the basal membrane and the hypocuticle. Here each tracheal duct passes across a 'line' of photoreceptors (usually 8-15), to each of which it sends a tracheal 'loop', which circumvents the ventral side of the photoreceptors (6 in Figure 2 represented as small circles). The yellow stripe terminates in a tuft of setae (small hairs). The membrane which interconnects the two brown spots (6 in Figure 2) represents a membrane, which links the segment to the one in front (and in the intact hornet this would be the underside). This intersegmental conjunctive is the IC and it actually separates what is attached to the interior of the body. The upper part, above the membrane is protruding outwards to the eave or reduplication. The IC membrane thus bisects the yellow stripe approximately at the middle. Figure 3 provides a schematic representation of the peripheral photoreceptor indicating the most prominent structures and their relative distribution. The membranes of the air sacs and tracheae in V. orientalis are shown in Figures 47. One such ramification leading into a tracheae (1), the structure of the outer membrane (2) and the structure of the inner membrane (3) is displayed in Figure 4. Figure 5 comprises a photograph (in part) of the inner membrane in an air sac with longitudinal, hardened ribs (1) and the ridges surrounding the passage to the trachea. Between the hardened ribs secondary ribs can be seen that apparently, respond to tension. In Figure 6 we see a striped membrane in which some of the bands are protruded and hardened. The bands are comprised of 2-3 secondary bands extending to a length of 10mm or more, originating from a region at some remove from the connection with the trachea. 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 gaster: these membranes bear contractions that are typical for piezoelectric membranes. Those respond to tension and pressure (mechanical stress) by producing electricity (or electric polarity).

Apparently, these portions of the membranes are capable of extending or contracting as the need arises and according to the content of air in the air sac. A typical trachea with annular hardenings which protrude from the inner surface of the trachea the tenidia (1) is shown in Figure 7. Everywhere, small masses are present which apparently contain aggregates of lipid cells. Presented in Figure 8 is a view of the inner side of segment 4 in the gaster of the homet. Here "I" indicates the surface of the region containing yellow pigment. Next, "T' indicates the attachment point of the 'braid' of the tracheal ducts to the 'spot' of brown cuticle. The interconnecting membrane which encircles this region belongs (and connects) further on to the IC is situated more anteriorly to it. On the right side of the figure, there is also the same 'braid' of tracheae, is covered and therefore invisible. Yet the 'braid' connects with the air sac through thicker ducts (4) that link it to the sac (5). Behind the IC one can discern an amorphic mass (6) which is possibly a glandular excretion (see below). A somewhat more enlarged view, of the tracheal 'braid' is shown in Figure 9, in which the following landmarks are discernible: I ) the inner membrane of the brown stripe of the segment (No. 4): 2) the membrane of the air sac with the annular bracings on the inner side: 3) a 'braid' of tracheal ducts that interlink to the brown 'spot' on the cuticle containing yellow 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 Figure 10. Here, we can see: annular bracings (I) tracheal ducts interlinking one sac to the other

(2): bracing bands extending down the inner length of the sac (3); and finally the supporting membrane across the tracheal ducts which proceed into the 'braid' (4). The tracheae which emerge from the air sac gradually narrow into more delicate ducts (1 in Figure 11) which receive support from transverse membrane inside (2 in Figure 11). This supporting membrane is increased in size upto to the lower portion of the tracheal duct 'braid' which is fully replete with transverse supportive membrane. This membrane appears like a mesh or rete fastened on the exterior of all ducts in the braid (see 1 in Figure 12). Underneath the membrane (BM + hypoticle) are the tracheal ducts. At the point of entry to the brown spot area (it looks like a basket), the "network " individually enveloping each duct comes to an end. In the same region (Figure 13), the cuticle of brown spot within the "yellow "area forms an elevation reminiscent of a stunted goblet and this at the juncture point between the tracheae and the cuticle (which is not seen in the picture); around and quaquaversal (that radiate from a central mass) from the goblet (which height from the distal side of the cuticle is about 0.5 µm) extends the membrane called IC. This membrane which covers the tracheal braid on the distal side is comprised of bands. Hence in Figure 13, we discern the IC made of bands (10, the bottom of the tracheal duct braid (2), and the tracheal ducts enveloped in a supportive membrane (3). An enlargement of the membrane at the base of the tracheal duct braid is shown in Figure 14. One clearly sees that the area is replete with bands, whose width is usually less than 10 µm and sometimes even as little as 5 µm, while on the underside of the bands (2) there are "fringes" (like the fringes of a dress) of nonuniform length which impinge upon the band underneath them. The bands are attenuated only in the region abutting the tracheal 'braid' whereas outside this region, they are broader (3).

A larger magnification of these bands is given in Figure 15, where one can see: (1) areas in which the bands are not separate throughout but only intermittently so (1). (2) the number of "fringes" in each bundle is between 4-12. (3) the gap between any two bands is smallabout 12 µm (2). (4) the "fringes" are of nonuniform length usually between 1-10 µm. (5) in the same bundle there are no two consecutive bands of the same length. All the "fringes" touch at various places the lower band (arrow). Tracheae pass through the tracheal 'braid' and then beneath the BM and the hypocuticle following which they traverse the yellow cuticle and contribute a tracheal loop around the bottom of each photoreceptor. All this is shown in Figure 16, which was taken via LM. Here we see clear circles through which light reflects from the other (upper) side of the cuticle; these are the photoreceptors. See also Figure 2 (6) and Figure 3 (4), this is the lower end (the inner cuticle) (arrow at 1) with a tracheal loop around the circle (arrow at 2) and also tracheal ducts that pass longitudinally, arrow at (3). In Figure 17, taken through a LM at higher magnification (x 1000), one can see a branching of the tracheae ( 1) which sends a tracheal loop to the base of a photoreceptor (arrow at 2). In the region is discernible also a neural fibre (3). Removal of the BM together with the hypocuticular layers reveals the lower side of the photoreceptor, Figure 18 (1), with concentric rings of cuticle sealing it there (2), granules of yellow pigment (3) and also part of the annular trachea which girdles the base of the photoreceptor (4). This picture was taken from the cuticle of V. crabro, whereas all the others are based on the cuticle of V. orientalis.

Discussion
The system described in the present study contains: (a) A mechanism that enables the passage of air in pipes, as in a thermoregulatory apparatus, and is comprised in the case of hornets of air sacs of variable volume that act as a bellows and of hardened tracheae which transport the air and split into more slender and elastic branches that pass among the photoreceptors. Except in the region of the spiracles, where the air can be admitted or expelled by the opening or closure of these respiratory structures, the rest of the tracheal system is a closed one. (b) The outlets of glands that release their evaporant contents between the segments of the gaster. We conjecture that the described system is geared primarily to push and compress air into the bases of the photoreceptors so as to regulate the temperature there and if so, the rate of respiratory movement is coordinated with physiologic activities of the hornet. The cooling or warming accomplished via the transport of air within pipes is well known and widely used in human technology. The process requires, of course: (1) thermosensors that are sensitive to temperature so that any deviation from the desired temperature will activate a corrective system. (2) the determination of a set point, that is, the optimal temperature which needs to be an equilibrium between heat creation and heat loss and (3) the ability to raise or lower, ad libitum, the temperature of the air conducted through the pipes especially through those loops of tracheae around the base of the photoreceptors. As for the latter setpoint, there is solid evidence that the temperature in the nest of hornets (Vespinae) is constantly set to 29 °C (Ishay et al., 1967; Ishay and Runner, 1971; Ishay, 1972; Ishay, 1973; Heinrich, 1981; Ishay and BarenholzPaniry, 1995). It may also be presumed that thermoregulatory sensors are distributed throughout the hornet cuticle. Additionally, there are tracheae throughout its length, while air sacs are arranged transversely in the abdominal segments as well as elsewhere. The air sacs are actively contractible and can thus compress the air in the system through the use of muscles, following which they relax passively. The air sacs which are arranged crosswise in the abdominal segments underneath the cuticle can also act as cushions that glide between the partly superposed segments and thus prevent excess friction between them during respiration. Judging strictly by their morphological appearance, the membranes of the air sac appear to be piezoelectric, responding to pressure by creating an electric voltage which helps to signal the physical properties of the air. The air that flows in the regions of the yellow cuticle provides oxygen to the local cells, as is to be expected, but the density of the tracheae in these regions points to another, very important role, namely, thermoregulation. Such thermoregulation results from the uptake of light in the visible wavelengths and of heat and their conversion to electric energy, whether as voltage (under illumination) or as current (in darkened, inner portion of the cuticle or under conditions of darkness) and by analogy the cuticular region resembles a solar cell (Maycock and Stirewalt, 1981; Ishay et al., 1982; Ishay and Litinetsky, 1996; Ishay et al., 1997).

In view of the fact that the cuticular stripes possessing yellow pigment protrude to the exterior of the body, it seems that maintenance of their proper function is crucial, and this for the following reasons: (a) intensive or ordinary respiratory activity persists even when the 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 regulated by the ventral ganglia, both the thoracic and abdominal ones (Grass, 1976). (b) the concentration of tracheae in a 'braid' which effectively provides tracheal rami to the yellow stripes suggests an effort to ensure maximal yellow stripe surfaces that are free from any interference by other tissues (and acting as thermoradiators). (c) the creation of a tracheal loop around the base of each photoreceptor. We still need to address the question as to how the air flowing through the tracheal ducts acquires the desired temperature. At temperatures below optimal the adult hornets (and wasps) commence to blow hot air around the developing brood (pupae) and thereby warm it to the desired temperature (Ishay and Rutter, 1971), whereas when the temperature is above optimal, the adult hornets within the nest commence to ventilate the brood or the entire nest (Ishay et al., 1967; Sadeh et al., 1977). Thus, while the air sacs do contain the greater share of the air supply, the temperature of the air is determined according to need by its passage through the tracheal loops, which gird the envelope of the photoreceptor. Here, each tracheal loop comes in contact with 1) the cuticular envelopes of the photoreceptor, they are electrically of n type (i. e., electron acceptors); and. 2) the yellow granules, which are of p type (i. e., electron donors). Flow of air passing through the tracheal loop is thus exposed to this p-n junction where electric energy is stored, or caused to flow, and this electric energy is transformed, according to need, into thermal energy, which is utilized in thermoelectric circuits such as have been described originally by Seebeck and Peltier. This process is geared primarily to provide thermoregulation of areas of the abdominal cuticle or other areas that contain extraretinal photoreceptors, since it is crucial to keep the photoreceptors from overheating. The thermoregulatory activity is important also for the entire nest, just as in vertebrates the excess heat produced in the striated muscles and the liver is transported via the circulation to all parts of the body. It is known that in humans, the metabolic process which takes place in the retina, needs high supply of oxygen and creates energy excess. The high rate of blood flow through the uvea is very important by giving high pO 2 in the uvea, which facilitates the diffusion of oxygen into the retina and it also helps to protect the eye from thermal damage even under rather extreme conditions, such as arctic snowstorms, Finish sauna bath, and observation of very bright objects (Moses, 1975). We see in the air supply system in the Oriental homet the exact analogue of the choroidal system in humans fulfilling the same physiological functions and complementary to it a source of energy for its physical activity. As for vaporization of volatile substances through the openings in the membrane designated as IC near the brown spots in the yellow stripes, we note that Spradbery (1973) points out that in sternite VI of hornet abdomen a gland is located, which Vecht (1968) described as one that releases pheromones. What we have described herein is probably a different gland, because its numerous openings are located on gastral segment IV, and the gland itself is situated on a tergite (a dorsal plate). So far as we know, no glands have been described in the specified segment and ours is thus a first description. We are in the dark yet as to the chemical nature of the substance evaporated from the described gland, but the mere fact of being connected to the bellows system of the air sacs with tracheae must enhance its efficiency. We have observed that when the bellows increases its activity the gland releases its volatile contents at a comparable rate and according to the intensity of respiration. It stands to reason that the gland is associated with thermoregulation, perhaps also signaling, altering and recruiting in emergencies, to the urgent need of cooling or heating the brood. If this be true, then this is the venue of thermoregulatory pheromone which has been speculated about years ago (Ishay, 1972; K"niger, 1977). Interestingly, a structure comprised of fringes has previously been reported in termites (Caloterme fiavicollis) by Lebrun (1971) in conjunction with a tergal gland in the 9th abdominal segment. The fringes in his and also in our case are of variable length and all protrude downwards where they encounter the underlying epithelial layer and act as mechanoreceptors, being responsible, apparently for sensing the distance between two adjacent epithelial layers and accordingly regulating the local air pressure. It seems that the variable length of the fringes enables to differentially determine the distance from one epithelial layer to the next and also gauges air pressure extent in the system.

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