Mechanical Stimuli

The stimuli grouped here are those associated with distortion caused by mechanical movement as a result of the environment itself, the insect in relation to the environment, or internal forces derived from the muscles. The mechanical stimuli sensed include touch, body stretching and stress, position, pressure, gravity, and vibrations, including pressure changes of the air and substrate involved in sound transmission and hearing.

4.1.1 Tactile mechanoreception

The bodies of insects are clothed with cuticular projections. These are called microtrichia if many arise from one cell, or hairs, bristles, setae, or macrotrichia if they are of multicellular origin. Most flexible projections arise from an innervated socket. These are sen-silla, termed trichoid sensilla (literally hair-like little sense organs), and develop from epidermal cells that switch from cuticle production. Three cells are involved (Fig. 4.1):

1 trichogen cell, which grows the conical hair;

2 tormogen cell, which grows the socket;

3 sensory neuron, or nerve cell, which grows a den-drite into the hair and an axon that winds inwards to link with other axons to form a nerve connected to the central nervous system.

Fully developed trichoid sensilla fulfill tactile functions. As touch sensilla they respond to the movement of the hair by firing impulses from the dendrite at a frequency related to the extent of the deflection. Touch sensilla are stimulated only during actual movement of the hair. The sensitivity of each hair varies, with some being so sensitive that they respond to vibrations of air particles caused by noise (section 4.1.3).

4.1.2 Position mechanoreception (proprioceptors)

Insects require continuous knowledge of the relative position of their body parts such as limbs or head, and need to detect how the orientation of the body relates to gravity. This information is conveyed by propriocep-tors (self-perception receptors), of which three types are described here. One type of trichoid sensillum gives a continuous sensory output at a frequency that varies with the position of the hair. Sensilla often form a bed of grouped small hairs, a hair plate, at joints or at the neck, in contact with the cuticle of an adjacent body part (Fig. 4.2a). The degree of flexion of the joint gives a variable stimulus to the sensilla, thereby allowing

Fig. 4.1 Longitudinal section of a trichoid sensillum showing the arrangement of the three associated cells. (After Chapman 1991.)

monitoring of the relative positions of different parts of the body.

The second type, stretch receptors, comprise internal proprioceptors associated with muscles such as those of the abdominal and gut walls. Alteration of the length of the muscle fiber is detected by multiple-inserted neuron endings, producing variation in the rate of firing of the nerve cell. Stretch receptors monitor body functions such as abdominal or gut distension, or ventilation rate.

The third type are stress detectors on the cuticle via stress receptors called campaniform sensilla. Each sensillum comprises a central cap or peg surrounded by a raised circle of cuticle and with a single neuron per sensillum (Fig. 4.2 b). These sensilla are located on joints, such as those of legs and wings, and other places liable to distortion. Locations include the haltere (the knob-like modified hind wing of Diptera), at the base of which there are dorsal and ventral groups of campani-form sensilla that respond to distortions created during flight.

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