Wpc

394 bending, which is especially critical to the formation of a superposition image. In diurnal species there is little evidence for a gradient of refractive index in the lens and crystalline cone. By contrast, since the early 1970s, evidence has accumulated that superposition eyes invariably utilize a lens cylinder arrangement, as a result of which a single erect image is formed deep within the eye.

Although an image of sorts forms in the eye, it is now accepted that for the majority of insects an image per se has no physiological significance. The real function of the compound eye appears to be that of movement perception. The eye's structure is ideally suited for this function, with each ommatidium sensitive only to light shining parallel with or at very small angles to the longitudinal axis of the ommatidium. Clearly, the eye's resolution (i.e., ability to detect motion) will be related to the number of ommatidia and hence the interommatidial angle (the angle between the longitudinal axes of adjacent ommatidia). Values for ommatidial angles vary from tens of degrees in some apterygotes to 0.24° in the dragonfly Anax junius (Land, 1997a,b), though in many common flying insects they are in the order of 1° to 3°. Typically, eyes with the best resolution are found in predators such as Odonata, mantids, and hunting wasps, though some male Diptera which use visual cues to detect and capture mates also have high-resolution eyes. It should also be noted that resolution is often non-uniform over the whole eye. Rather, there are regions with high resolution (acute zones or "foveas") as well as regions with lower resolution, the size ratio of the two regions and the shape and position(s) of the foveas being determined by the "visual ecology" of the insect (Land 1997a,b). For example, in many predaceous species and in males that visually detect females the fovea is forward- or upward-pointing so that prey or a potential mate can be detected against the sky. Likewise, in insects that live on flat surfaces, notably water striders (Gerridae), the eyes have a narrow horizontal band of high resolution that enables the water strider to hunt for prey caught on the water surface.

Thus, a key element in insect vision is movement, more specifically, the change in relative position of the object and the insect's eyes. Many behavioral observations indicate a preference by insects for moving objects or objects with complex shapes. Bees prefer moving to stationary flowers, and they are attracted more by multistriped than by solid patterns. Dragonfly larvae will attempt to capture prey only when it is moving. Before locusts jump, they undertake "peering movements," side-to-side movements of the head that enable them to judge distance (Section 7.1.2). Stimulation of the eye by a series of changes of light intensity is known as the flicker effect. The number of stimuli per unit time to which the eye is sensitive (the flicker fusion frequency) depends on the rate at which the eye recovers from a previous stimulus and provides the basis for grouping eyes into "slow" and "fast" categories. Slow eyes have low flicker fusion frequencies and are found in more slowly moving, nocturnal insects, whereas fast eyes, with very high flicker fusion frequencies, are characteristic of fast-flying, diurnal species. As the position of the insect changes in relation to an object, the eye will receive a succession of light stimuli. Provided that the rate of change of position does not lead to a rate of stimulation that exceeds the flicker fusion frequency, the insect will, in effect, scan the object and obtain a sense of its shape. In other words, the eye translates form in space into a sequence of events in time. It follows that insects whose eyes have a high flicker fusion frequency have the best form perception. Bees, for example, are readily trained to distinguish solid shapes from striped patterns, though they cannot distinguish between two solid shapes or between two patterns of stripes. Many insects that hunt on the wing, such as dragonflies and some wasps, have excellent form perception especially species that are prey-specific. Some solitary wasps find their nest by recognizing landmarks adjacent to the entrance.

7.1.2. Distance Perception

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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