Observations on the nervous system of the flight apparatus in the locust Schistocerca gregaria

Abstract

ABSTRACT The hair sense-organs of the head are part of a sensory system affecting the activity of motor neurones to the flight muscles. They possess curved hollow hair shafts inserted in a complex socket. A large neurone is present beneath the socket and is partly surrounded by a large formative cell, the trichogen-tormogen cell. The distal process passes up into the expanded base of the hair shaft. Fine connexions between the outer region of the formative cell and the inner part round the neurone process, possibly limiting angular sensitivity, can be seen in some specimens, although the form and fine structure of the hair shaft is almost certainly important in this respect. The axons from the 5 areas of hair organs are collected together into a dorsal tegumentary nerve, those from area three forming a short subocellar nerve. Electronmicrographs of this nerve show that there are a number of large fibres (1 to 5 μ), and many more smaller fibres (1· to 0·1 μ) with no sheaths. There were estimated to be 5,500 fibres in each dorsal tegumentary nerve. Within the central nervous system, the dorsal tegumentary fibres may follow one of 4 routes, as follows. They may (i) pass forward into the protocerebrum, (ii) end in zones of terminals in the deutocerebral region, (iii) a few thick fibres pass down into the suboesophageal ganglion and then cross over to the opposite side giving off collaterals before descending to the pterothoracic ganglia, (iv) most of the fine descending fibres probably end at the suboesophageal level, a proportion of them crossing over here. The motor neurones to the longitudinal indirect muscles M81 and M82 consist of 4 anterior and 1 posterior cell respectively, and possess large and striking cell bodies, whose collaterals could be seen in the dorsal zones of motor terminals. The probable internuncial links between the sensory and motor arcs are outlined.