Twc

462 sky at the horizon. Goodman's (1965) study of the importance of the horizon in orientation of the desert locust showed that the insect rotates its head so as to maintain the horizon horizontally across the visual field of the compound eyes. Her study also demonstrated that the dorsal light reaction overrides the "horizon response" at higher light intensity, and vice versa. Alignment of the thorax and abdomen with the head is achieved by means of proprioceptive hairs on each side of the neck. When the thorax is out of line with the head, the hairs on each side are differentially stimulated. This information induces appropriate wing twisting to compensate.

Wing movements are initiated in many insects by means of the "tarsal reflex," that is, loss of contact between the substrate and tarsi, as well as by a variety of non-specific stimuli. It seems probable that sensory input to inhibit the tarsal reflex is fed in via campaniform sensilla on the femur and trochanter which are sensitive to stresses in the cuticle in the standing insect (Chapter 12, Section 2.2). The reflex stimulates unfolding of the wings (in some species this involves the same muscles as are used in flapping the wings), stiffening of the pleural wall (contraction of the pleurosternal muscles), and raising the legs to the flying position. The tarsal reflex is not found in insects that capture prey during flight, in insects that vibrate their wings during "warm-up" prior to takeoff, in bees that employ wing movements in hive ventilation, or in beetles that must raise the elytra and unfold the wings prior to flight.

Some insects, for example, Drosophila, will fly until exhausted, provided their tarsi no longer make contact with the substrate. For most species, however, additional stimuli are necessary to maintain wing movements, especially airflow, which stimulates hairs on the upper part of the head, Johnston's organs in the antennae, and proprioceptors at the base of the wings. In the honey bee, flight is maintained as a result of visual stimuli received as the insect moves forward over the ground.

Visual signals experienced by the compound eyes just prior to landing stimulate leg extension. Goodman (1960), working on the fly Lucilia sericata, concluded that the response was based on three variables acting singly or in combination: (1) decrease in light intensity experienced by successive ommatidia as the fly approaches the substrate; (2) number of ommatidia stimulated; and (3) rate of successive stimulation of ommatidia (based on the increase in angular velocity which occurs as the fly comes closer to the substrate).

Halteres. Deserving special mention are the much modified hind wings (halteres) of Diptera (Figure 14.16), which function as gyroscopic organs of balance. The halteres vibrate in the vertical plane only, at the same frequency as the wings with which they are in antiphase. These vertical vibrations, combined with the movement of the insect through the air, cause the development of torque at the base of the halteres, which is monitored by various groups of campaniform sensilla oriented in different planes. Any change in the orientation of the insect will result in modification of the torques generated at the base of each haltere. Yawing can be detected by differential stimulation of the groups of sensilla in each haltere, whereas the detection of pitching and rolling requires the combined input of both halteres, which is assessed in the thoracic ganglion. To correct for instability in any of the planes of motion, the insect alters the extent to which each wing is twisted during beating. For a detailed discussion of the gyroscopic nature and function of halteres, see Pringle (1957) and Nalbach (1993).

3.3.5. Flight Metabolism

Insect flight is an energetically costly process, in which large quantities of substrate are oxidized in the generation of ATP. The insect body is eminently adapted to facilitate

FIGURE 14.16. Haltere of Lucilia (Diptera). The orientation of the various groups of campaniform sensilla is indicated by double-ended arrows. [After J. W. S. Pringle, 1948, The gyroscopic mechanism of the halteres of Diptera, Philos. Trans. R. Soc. Lond. Ser. B 233:347-384. By permission of The Royal Society, London, and 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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