AFM study of microstructures on the cornea of the compound eye and ocelli of the hornet Vespa orientalis (Insecta, Hymenoptera).

The present study describes a novel structure occurring on the cornea in the compound eye and the ocellus of the Oriental hornet. This description is based on observations carried out via scanning electron microscopy (SEM) and primarily via atomic force microscopy (AFM). We report herein that the vespan cornea is densely covered with cupola-shaped protrusions, which in the compound eye have bases about 0.2 microm in diameter and an average height of 0.03 microm while upon the ocelli their diameter is rather variable, with some measuring 0.2-0.3 microm, and a length of submicrons to microns. Interspersed among the brief, cupola-shaped structures there are winding, longer structures whose height appears uniform at around 0.03 microm. It appears that the structures are formed from the epicuticular layer of the cornea and we conjecture that in both the compound eye and in the ocelli they serve to enhance the absorption of light, as well as to reduce light reflection.

Sub-micromorphology of the epicuticle of hornets: AFM studies.

The upper part of the hornet cuticle in the abdominal region reveals several structures. First, situated at intervals of 10 microm or more apart, there are depressions housing a peripheral photoreceptor (PP) and between every two of these, there are horizontal flats resembling an irregular polygon which are mostly elongated and about 100 microm2 in area. Upon each such terrace-like flat there are tile-shaped protuberances up to several micrometres in length and mostly running parallel to one another, covering the entire surface. Between every two such terraces there is a 'step' of up to 1.4 microm in height. Both the 'tiles' and 'steps' terminate in front of the distal part of each PP, which is a recessed and smooth region. Secondly, in other regions of the cuticle one can note other surface configurations such as: (a) a paucity of 'tiles'; (b) no 'tiles' at all but a chessboard-like pattern, the 'squares' of which contain rectangles recessed to various depths, interspersed among 'plaques' that are not recessed and may even protrude; and (c) anfractuous (characterized by windings and turnings) flats that are intermittently criss-crossed by grooves. In addition to the above, most of the cuticle also displays numerous setae (i.e. small hairs). We suppose that all the described structures and configurations either contribute to enlargement of the cuticular surface, or act as an optical grid, thereby enhancing the absorption of light. We further speculate that for hornets in flight, situations may arise where part of the above mentioned structures orientates perpendicular to the sunlight and this entire orientation acts to direct the solar energy toward and into the PP so as to enable greater utilization of light as an energy source.