The structure and function of abdominal stretch receptors in insects

Abstract

The morphology of dorsal abdominal stretch receptors in representatives of the Orthoptera, Odonata, Hymenoptera and Lepidoptera is described on the basis of dissections, serial sections and whole mounts. The receptors consist of sensory neurons in association with connective tissue strands or muscles. The organs are found to be slung between the intersegmental folds (Lepidoptera), between one intersegmental fold and the dorsal body wall (Orthoptera, Odonata, Hymenoptera), or between the dorsal body wall and a nerve branch in the abdomen (Odonata). The mode of anchorage of the organs and other morphological features suggests a proprioceptive function. This can be demonstrated by oscillographic recording from the axons of the sensory neurons. An apparatus is described in which isolated preparations containing the receptors could be subjected to mechanical stimulation by stretch of controlled intensity. Over a certain range the frequency of impulse discharge from the sensory neuron is linearly proportional to the tension applied to the receptor. Adaptation is slow, and the organs in all insects investigated behave similarly and in a way typical for static receptors. Three pairs of receptors per segment are present in the Odonata (dragonfly larva), and an attempt is made to correlate their responses with the chief abdominal movements of the insect. In comparing the anatomical features of the various receptor organs, it is pointed out how they can be arranged in a series leading from neurons closely associated with the body wall, to those specialized to register stresses in connective tissue strands which are slung between suitably placed points in the segment, and finally to sensory neurons in closer association with the abdominal musculature. The series culminates in the lepidopteran receptor which shows striking points of resemblance to the vertebrate muscle spindle.