Mechanoreception is the perception of any mechanical distortion of the body. This may result from touching an object or from the impact of vibrations borne through the air, water or the substratum, and thus mechanoreception includes the sense of hearing. It also includes distortions of the body which arise from the stance of the insect and from the force exerted by gravity, so some mechanoreceptors are proprioceptors. Three broad structural categories of mechanoreceptor are present in insects: cuticular structures with bipolar neurons; subcuticular structures with bipolar neurons, known as chordotonal organs; internal multipolar neurons which function as stretch receptors.

Definition: The term sensillum (plural: sensilla) refers to the basic structural and functional unit of cuticular mechanoreceptors and chemoreceptors. It includes the cuticular structure, the neuron or neurons, the associated sheath cells with the cavities they enclose and the structures they produce. Fig. 23.1 shows the basic elements of a sensillum. Sensilla often occur singly, but may also be grouped together to form functional units.

Review: McIver, 1985

CUTICULAR MECHANORECEPTORS

Cuticular mechanoreceptors fall mainly into two classes: hair-like projections from the cuticle with a basal socket, and dome-like campaniform sensilla. Both types have similar arrangements of neurons and sheath cells.

Structure

Cuticular components

Hair-like structures Most of the larger hairs on an insect's body are mechanoreceptors. There are no pores (apart from the molting pore, see below) in the cuticle of hairs that function solely as mechanoreceptors, and they are, therefore, known as aporous sensilla.