The cuticle is the largest active organ in the insect. It consists of many parallel lamellae with vertical interlamellar support between them and of interlamellar tubular membranes. The upper part of the cuticle in the abdominal region, the epicuticle, reveals several structures: 1) at intervals of 100μm or more apart, there are depressions housing a peripheral electromagnetic photoreceptor (PER), and 2) between every two of these, there are horizontal flats resembling an irregular polygon which are mostly elongated and about 104μm2 in area. 3) Upon each such terrace-like flat there are tile-shaped protuberances up to several micrometers in length and mostly running parallel to one another, covering the entire surface. In addition, most of the cuticle also displays numerous setae (small hairs). We suppose that the described structures and configurations contribute to enlargement of the cuticular surface and act as an optical grid enhancing the absorption of light. The cuticle of hornets is composed of mostly brown stripes and of some yellow stripes. In the brown cuticle the colour stems from incrustation of the pigment melanin, while in the yellow stripes the yellow pigment is concentrated in granular pockets between the lower layers of the endocuticle and the yellow colour shows through the transparent cuticle. Both brown and yellow cuticles are composed of more than 30 layers. The upper layers are about 1- 5μm thick and as we proceed inwards, they become thinner so that the deepest ones are thinner than the uppermost layer by about two orders of magnitude. Hornets are flying outside their nest, only in daytime, mostly at noon hours, when the UVB sun irradiation is at maximum. Vespan cuticle is photovoltaic, so that upon exposure to light one can record levels of spontaneous voltage of 30-180mV. The cuticular response to light is a two-phased affair, to wit: 1) immediately upon exposure to light there is a rise of the current from zero to nearly 5μA within the first minute, and in the next minute a further rise to about 6μA. During this phase of the response, the cuticle behaves as a positive photoelectric material. Later on current generation decreases along with a morphological reorganization of the cuticle to protect it from the damage of exposure to UV light.
Department of Physiology and Pharmacology, Sacker Faculty of Medicine, Tel Aviv University, Ramat Aviv 69978, Israel.
Publication date: June 1, 2005
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