Sensory Systems

chortlotonal ligament

FIGURE 12.4. (A) Single chordotonal sensillum; and (B) chordotonal organ. [After V. G. Dethier, 1953, Mechanoreception, in: Insect Physiology (K. D. Roeder, ed.). Copyright © 1953 John Wiley and Sons, Inc. Reprinted by permission of John Wiley and Sons, Inc.]

tissue that stretch between two points. The proximal end of the sensory neuron is attached to one point by means of a ligament and the distal end is covered by a cap cell, which is attached to the second point (Figure 12.4B). Chordotonal sensilla are highly sensitive. Thus, a change in the relative position of the points that causes the strand's length to be altered by as little as 1 nm will produce bending or stretching of the dendritic membrane, hence stimulation of the sense cell. Frequently, alteration of the positions of the points is brought about as a result of pressure changes, for example, in the air within the tracheal system, in the hemolymph within the body cavity, or in aquatic insects in the water in which they are swimming. In relatively few insects chordotonal sensilla are aggregated in large numbers and capable of being stimulated by changes in external air pressure, that is, sound waves (Section 3).

Stretch receptors (Figure 12.5) comprise a multipolar neuron (Type II) whose dendrites terminate in a strand of connective tissue or a modified muscle cell, the ends of which are attached to the body wall, intersegmental membranes, and/or muscles. As the points to which the ends are attached move with respect to each other, the receptor is stimulated. Stretch receptors are probably most important in providing information to the central nervous system on rhythmically occurring events within the insect, for example, breathing movements, waves of peristalsis along the gut, and locomotion.

A peripheral system of multipolar sensory neurons is located beneath the body wall in many larvae whose cuticle is thin and flexible, or beneath the intersegmental and arthrodial membranes of insects with a rigid integument. The nerve endings are presumably stimulated by tension in the body wall or movements of joints. A similar arrangement is present in the wall of the bursa copulatrix of Pieris rapae, measuring the degree of stretching of this structure when a spermatophore is present (Chapter 19, Section 3.1.3), and in the

FIGURE 12.5. Stretch receptor of male mosquito. [After R. F. Chapman, 1971, The Insects: Structure and Function. By permission of Elsevier/North-Holland, Inc., and the author.]

alimentary canal, though the sensory neurons in this case pass their information on to the visceral nervous system (Chapter 13, Section 2.2).

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Beekeeping for Beginners

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