Lqe

390 ventral regions. Occasionally, these regions are physically separate, as in the male mayfly,

Cloeon, and water beetle, Gyrinus.

A compound eye comprises a varied number of photosensilla, the ommatidia. The number ranges from 1 in the ant Ponera punctatissima to as many as 28,000 in some dragonflies. Each ommatidium (Figure 12.11) includes alight-gathering component (corneal lens plus crystalline cone); the primary sense cells (retinular cells), which collect and transduce light energy; and various enveloping (pigment) cells. The lens, a region of transparent cuticle, is produced by the primary pigment cells. Its surface is usually smooth, but in some nocturnal moths it is covered with numerous minute pimples, about 0.2 |am high, which by reducing glare improve the amount oflight transmitted (equivalent to "blooming" a camera lens with an antireflection coat of magnesium fluoride). Another likely function of this antireflection layer is to improve camouflage during daylight hours, the dull surface of the moth's eye being less visible to would-be predators. In some species, for example, tabanid flies and lacewings the eyes are colored metallic gold-green due to the multilayered nature of the lens (Stavenga, 2002) (see Chapter 11, Section 4.3). The crystalline cone is a clear, hard material produced by four cells (Semper's cells). The material is typically intracellular, and the nuclei are situated around it (eucone type). In some species the material is extracellular (pseudocone type); in others there is no crystalline cone and the cells, which are transparent, occupy the area (acone type). Primitively, eight retinular cells occur beneath the crystalline cone, though one of these is usually degenerate or eccentrically located. The seven remaining cells are arranged around a central axis in most species; occasionally, they exist in two tiers of three and four cells, respectively. The mature sensory cells are unipolar, that is, lack dendrites. Instead, their inner surface is modified to form a receptive area, the rhab-domere. Collectively, the rhabdomeres form a rhabdom. By means of electron microscopy, the rhabdomeres can be seen to comprise closely packed microvilli, of diameter about 500 A, which extend from the cell surface. In cross section the microvilli are hexagonal. The details of rhabdom construction are varied though, in general, "open" and "closed" types can be distinguished (Figure 12.11C,D). In the open type, found in Diptera and Hemiptera, individual rhabdomeres are physically separated and each serves as an optical waveguide; in the closed type, common to most insects, the rhabdomeres are wedge-shaped and closely packed around the central axis. In a closed rhabdom the rhabdomeres function collectively as a single waveguide. However, even in the most compact rhabdoms, there is extracellular space between microvilli to permit the ionic movements that are the basis of impulse transmission (Goldsmith and Bernard, 1974). The rhabdom contains visual pigments that resemble those of the vertebrate eye; that is, they are conjugated proteins called rhodopsins. The photosensitive component of the molecule, the chromophore, is retinaldehyde (retinal), the aldehyde of vitamin A, or closely related derivatives (Section 7.1.3). Surrounding the photosensitive cells are the secondary pigment cells which, like the primary pigment cells, contain granules of red, yellow, and brown/black pigments (mainly ommochromes). These pigments, especially the browns and blacks are vitally important as they strongly absorb light that enters the eye at oblique angles. Thus, the rhodopsins are activated only by light that enters an ommatidium almost parallel to its longitudinal axis (but see below). Proxi-mally, the retinular cells narrow to form discrete axons that enter the optic lobe. Between the axons, outside the basal lamina, tracheae may occur which, in addition to their respiratory function, may serve to reflect light back along the rhabdom.

Two ommatidial types can be distinguished according to the arrangement of retinular and pigment cells (Figure 12.11A, B). In photopic (apposition) ommatidia characteristic of diurnal insects, the retinular cells span the distance between the crystalline cone and

FIGURE 12.10. (A) Photopic ommatidium; (B) scotopic ommatidium; (C) open rhabdom; and (D) closed rhabdom. [A, B, after V. B. Wigglesworth, 1965, The Principles of Insect Physiology, 6th ed., Methuen and Co. By permission of the author. C, D, after T. H. Goldsmith and G. D. Bernard, 1974, The visual systems of insects, in: The Physiology ofInsecta, Vol. II, 2nd ed. (M. Rockstein, ed.). By permission of Academic Press, Inc., and the authors.]

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook


Post a comment