This thesis deals with the structure and the role of the orientation and navigation organs in the Oriental hornet Vespa orientalis L. 1771 (Hymenoptera, Vespinae), with special reference to the ocelli.

The role of the cuticle, acting as a voltaic system providing the energy for the hornet to fly and to navigate has been discussed in chapter 2. The presence of photoreceptors in the cuticular wall with their 3-dimensional appearance and their role in navigation and orientation, have been discussed in chapter 3. The presence of three different types of hair cell structures existing of groups of cilia, around the area of the photoreceptors in the head of the hornet, and their possible role and function in navigation and orientation, as well as their fine structure as far that could be visualized by FE-SEM are discussed in chapter 4. The main attention in this thesis is given to the role of the ocelli, chapter 5.

The hornet possesses three ocelli, which are situated on the vertex plate. One of those is median and anterior, while the remaining two are lateral and posterior. Each ocellus is covered with a uniform, transparent cornea, which is formed by the corneogenic cells. Underneath the cornea, each ocellus has a single lens composed of an acellular matter. Further down, below the lens, there are the retinular cells. The arrangement of the latter is such that every two retinular cells create between them, at their deeper parts, the structure known as the rhabdom. This rhabdom is actually formed by the fusion of the membranes of the two adjacent cells. At their upper parts, the two retinular cells are separated by an extension of the superposed corneogenic cell. This separatory extension or process is translucent and serves as a lens that directs and concentrates light toward the deeper lying rhabdom. The retinular cells contain a black pigment which prevents any lateral dispersion of light.

Each rhabdom, with the two retinular cells comprising it, and the corneogenic cell with its extension intruding between the retinular cells, together form an entity analogous to the ommatidium in a compound eye, for which I herein propose the name ocellon. From each ocellon arises a duplex neural fiber and all the nerve fibers arising from the complete ocellus together comprise the ocellar nerve. As for the ocellon, its structure is geared for picking up crepuscular light, as does a scotopic eye, and also polarized light. These properties of the ocellon suggest that the ocelli proper have a role in vespan orientation. The macroscopic structure of the ocelli and their spatial orientation enable the hornet, on the one hand, to determine the sun's direction during flight, and on the other hand they serve as a clock that lends hornets a temporal orientation.

Returning now to the retinular cell, we note that it is composed of two functional parts, namely, an external sensory portion and an internal neural portion. Between these two parts is located the "neck" of the cell, which passes through the socalled areolar area. The sensory unit creates at its base a rhabdom comprised of the two retinular cells, which are interlinked by a full-fledged desmosome. Microtubuli and microfilaments traverse this region from cell to cell. As already mentioned before, in this upper parts the retinular cells separate to enable intrusion of the process of the corneogenic cell. Each ocellus is entirely encased in a membrane containing pigmented cells and glia cells. Three ocelli are situated in the vertex plate which is shaped like a three-sided pyramid with a truncated tip. Each side of this pyramid bears one ocellus and of the three sides the anterior one is the longest. Such a configuration of 3 ocelli in fact enables coverage of the entire 360°field of vision, affording a panoramic view.

From the median ocellus, a line descends to the frons plate, forming the coronal suture. In the deepest part of the coronal suture, one finds the conus, which is orientated at 90° inwards to the frons plate. In its interior, the conus shows a complex structure comprised of an envelope of ciliary cells which enclose a system of cords bearing weights. In fact, three types of ciliary cells were encountered in association with the conus. The three types differed morphologically from one another, but whether they also differ functionally remains to be seen.

As for the conus proper it has its inner array of cords which we now recognize as a specialized organ functionally associated with the sensing of gravitation, and we have named this graviceptor the Ishay organ. We have established that all of the frons plate and part of the vertex place are covered with ciliary cells capable of sensing radial acceleration. In the vicinity of each lateral ocellus is located a structurally unique organ, the so-called paraocellar organ. The organ has at its base ciliary cells, contains a high concentration of Ca-ions and around its middlesetae arranged in rings or circles; structurally it is adapted to sense linear acceleration.

Together the conus with its "Ishay organ", and the two paraocellar organs have the joint function of determining the angle of each ocellus vis-a-vis the direction of the gravitational force and this at every given moment during hornet flight. This capability figures importantly in vespan determination of the sun's angle in relation to the zenith. In other words, the hornet flight orientation is achieved by a synthesis of data deriving from the ocelli regarding the direction of the gravitational force. In actuality the vespan flight orientation is accomplished through a stereotypically circular movement of the flying hornet. In the course of this circuitous trajectory, the flying hornet can ascertain the angle of the sun as well as its direction and distance from the nest. Also during such orientation flight, the sun impinges on the hornet's ocelli, forming a sort of bow with a single string in which the angle between the string and a perpendicular bisecting the ocellus corresponds to the angle of the sun in relation to the zenith, bearing in mind that the zenith is antipodal to the gravitation. Roughly speaking, this circular flight movement of the hornet and the ancillary string and bow configuration are reminiscent of the waggle dance of the bee, with its orientation towards flowers in the field and the angle between the sun and the field, whose apex indicated the site of the hive.

During flight, the hornet maintains its balance by the use of its equilibrium organs. Specifically, the roll motion is gauged by the paraocellar organs and the conus; the pitch motion by the ocelli, the paraocellar organs and the paraocellar crests (see below) and yaw motion Ñ by the ocelli. As for the paraocellar crests, these actually function as gyroscopes and consist of a fluidfilled canal encased by a ciliary cells, the whole of which is rather analogous to the semicircular canal in the ear of vertebrates. Vespan sensing of gravitation (graviception) and sensing of flight (photoreception) are dependent on a suitable temperature level. The maintenance of a steady temperature is relegated in hornets to the cuticle, which acts as an organic semiconductor. At an optimal temperature of 29 °C the condition is maximal and the resistance is very low. Under these conditions, the exposure of a hornet to light leads to the appearance of a photovoltaic effect, that is, the accumulation of voltage in the cuticle.

This voltage in the cuticle, coupled probably due to a Seebeck effect, results in cooling or warming of the air in the tracheal loops surrounding in the peripheral photoreceptors. This tracheal air is sucked into the abdominal air sacs, to be blown out, as needed, upon the brood inside the nest. In this fashion, thermoregulation is affected both in the body of the hornet as well as in the entire nest.