MECHANORECEPTION 47 



present in the abdominal segments of pupae and adults but have not 

 been found in the thorax. The receptor consists of a multipolar re- 

 ceptor cell, a modified muscle fibre, and motor innervation to the fibre 

 (Fig. 37a). The neuron is enclosed in the usual connective-tissue sheath . 



Fig. 37. A. Diagram of the structure and innervation of a lepidopteran 

 stretch receptor, based on a preparation from the right side of the fifth 

 abdominal segment of a pupa of Antheraeapernyi. The receptor is short 

 because no tension is applied. B. Diagram showing the relative sizes 

 and positions of the two giant nuclei, the neuron sheath, and the fibre 

 tract of a lepidopteran stretch receptor. (Redrawn from Finlayson and 

 Lowenstein, 1958.) 



Its dendrites sometimes run on finger-like extensions of the sheath, 

 which is continuous with a tubular sheath extending along the edge of 

 the receptor (Fig. 37b). The muscle which constitutes the bulk of the 

 receptor has fainter striations than normal muscles. At the centre there 

 is a clear swelling with a giant nucleus (100-360 ijl). Adjacent to this 

 is another large nucleus. This muscle has its own separate motor 



