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 sensilla, 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 dendrite 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).

Figures 4.1. Longitudinal section of a trichoid sensillum showing the arrangement of the three associated cells.

(After Chapman 1991)

Chapter 4