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An ability to judge distance is especially important for quick-moving and/or predaceous sensory systems insects. This ability may be dependent on binocular vision and/or motion parallax. In insects binocular vision is achieved when ommatidia in each eye are stimulated by the same light source, that is, when the ommatidial axes cross. For this to occur, the surface of the eyes is highly curved, which ensures a considerable overlap of their visual fields. Many predaceous species will not attack prey unless it is within catchable distance. The ability to differentiate distance appears to derive from the fact that only when certain ommatidia are stimulated is the catching reflex induced (Figure 12.14). The axes of these ommatidia cross at a point within the range of the capturing device, for example, the labium in odonate larvae, the forelegs of mantids, and the mandibles of tiger beetles. The system is not foolproof as errors in distance estimation are proportional to (a) the interommatidial angle and (b) the distance of the object in relation to the width between the eyes. To reduce possible error, the best predators have a fovea where the interommatidial angle is very small and widely set eyes, respectively. In the examples just given, the eyes are of the apposition type, with little overlap in the visual field between adjacent ommatidia and, furthermore, a fovea. By contrast, mantispids (Neuroptera) have superposition eyes where significant overlap among the visual fields of nearby ommatidia occurs. Nevertheless, when these insects feed (on bright sunny days), they appear to use a similar triangulation mechanism to that of mantids. That is, their forelegs strike when a specific set of ommatidia in each eye is stimulated by the sight of the prey. An important difference, however, between the two systems is that, in the absence of a fovea, mantispids are unable to perceive and catch prey that moves as quickly as that caught by mantids (Kral et al., 2000).

FIGURE 12.13. Distance perception in Aeshna larva. The insect can perceive the distance of any point that simultaneously stimulates ommatidia in both compound eyes (e.g., points A, B, and C). However, the insect extends its labium only when omma-tidia whose visual axes fall between A and B are stimulated. [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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