Central Control of an Insect Segmental Reflex I. Inhibition by Different Parts of the Central Nervous System

An earlier paper (Rowell, 1961 a) described a reflex cleaning or grooming movement by the front leg in Schistocerca gregaria; it is evoked by stimulation of the sensilla of the prothoracic sternum. In the unrestrained animal this response can only rarely be seen, but under experimental conditions it can be obtained reliably if the connectives uniting the prothoracic ganglion to the rest of the C.N.s. are cut, provided that the tarsi of the front legs are not allowed to contact anything. Under all other conditions the reflex is partially or completely inhibited. The need for central control of 'peripheral' reflexes is especially obvious in insects, with their complex behaviour and segmentally organized nervous system. Some broad conclusions about this control have been reached on the basis of earlier lesion experiments (see Roeder (1953, 1958) for reviews). Successive removal of the supraand suboesophageal ganglia cause first hyperactivity and then depression of behaviour such as locomotion and of many reflexes, suggesting inhibitory and excitatory functions for these two areas. The terminal abdominal ganglion of many insects is influenced by inhibition from the head ganglia, and inhibitory relations between different thoracic segments are known in the locomotion of insects. Little is known about the mechanism of this control. The inhibition of the prothoracic cleaning reflex in Schistocerca is being studied, and the present paper deals with lesion experiments. A brief note on some of the results has been published elsewhere (Rowell, 1963).