Simple Eyes

Many adult insects and juvenile exopterygotes possess in addition to compound eyes, three simple eyes, dorsofrontal in position, known as ocelli. The larvae of endopterygotes have, as their sole photosensory structure, stemmata (lateral ocelli).

Ocelli. The structure of an ocellus is shown in Figure 12.16A. It comprises usually about 500-1000 photosensitive cells beneath a common cuticular lens (Goodman, 1970). The cells are arranged in groups of two to five cells, and, distally, each differentiates into a rhabdomere to form a central rhabdom. In contrast to the retinular cells of ommatidia, the photosensitive cells of dorsal ocelli are second-order sense cells; that is, their axons do not themselves conduct information to the central nervous system, but synapse within the ocellar nerve with axons of cells originating in the brain.

Ocellar function is poorly understood. Though an ocellus is able to form an image, it does so below the level of the rhabdom and, therefore, the image has no physiological significance. The modern view of their main function is that they serve to detect the horizon (hence, being out of focus ensures that extraneous details do not impede this function) and are thus important in maintaining stability during level flight. However, in some insects, other functions occur. For example, ocelli respond to the same wavelengths as compound eyes but are much more sensitive than compound eyes; that is, they are stimulated by very low light intensities. Thus, they may measure light intensity, and the information derived from them may be used to modify an insect's response to stimuli received by the compound eye. Painting ocelli may cause temporary reversal or inhibition oflight-directed behavior, or reduce the rapidity with which an insect responds to light stimuli. Such observations suggest that ocelli act as "stimulators" of the nervous system, so that an insect detects and responds more rapidly to light entering the compound eyes. In addition, in some species ocelli appear essential for the maintenance of diurnal locomotor rhythms. Rarely, an apparent duplication of functions associated with compound eyes has been described. For example, in workers of the desert ant Cataglyphis bicolor the ocelli (as well as the compound eyes) determine the plane of polarized light, enabling ants with their compound eyes experimentally occluded to

FIGURE 12.15. (A) Ocellus of Aphrophora (Cercopidae: Hemiptera); and (B) stemma of Gastropacha (Lep-idoptera). [After V. B. Wigglesworth, 1965, The Principles of Insect Physiology, 6th ed., Methuen and Co. By permission of the author.]

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.

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