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392 basal lamina. The secondary pigment cells lie alongside the retinular cells, and the pigment within the former does not migrate longitudinally. Scotopic (superposition) ommatidia, found in nocturnal or crepuscular species, have short retinular cells whose rhabdom is often connected to the crystalline cone by a translucent filament that serves to conduct light to the rhabdom. The secondary pigment cells do not envelop the retinular cells and their pigment granules are capable of marked longitudinal migration, allowing light from adjacent ommatidia to reach each rhabdom, enhancing rhodopsin activation.

7.1.1. Form and Movement Perception

It seems likely that early students of insect vision would assume that insects "see" in the same manner as humans; that is, a reasonably sharply defined image would form in the compound eye. Thus, early ideas on compound eye function were attempts to reconcile the observable structure of the eye with this assumed function. The classic "mosaic theory" of insect vision, introduced by Muller (1829) and expanded by Exner (1891) (cited from Goldsmith and Bernard, 1974), proposed that each ommatidium is sensitive only to light that enters at a small angle to its longitudinal axis. More oblique light rays are absorbed by pigment in the cells surrounding an ommatidium. It was assumed that little overlap existed between the visual fields of adjacent ommatidia, and thus, it was suggested, there formed in an eye an erect, mosaic image, being a composite of a large number of point source images, each formed in a separate ommatidium. The image would focus at the level of the rhabdom and its "sharpness," that is, visual acuity, would depend on the number of ommatidia per unit surface area of the eye.

Exner noted that the corneal lens and crystalline cone (which together function as a lens cylinder) appeared laminated (Figure 12.12) and suggested that this was the result of

FIGURE 12.11. Lens cylinder, comprising a series of concentric lamellae of different refractive index. Refractive index is greatest along the axis xy. [After V. G. Dethier, 1963, The Physiology of Insect Senses, John Wiley and Sons, Inc. 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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