VIII 



Nervous System 

 i. Nezvly hatched larva 



The nervous system of an insect consists of a brain, ventral 

 nerve cord, and a complex of ganglia associated with the brain, 

 the stomatogastric system. The ventral nerve cord and the brain 

 of the young larva — Stage XV, — have already been briefly de- 

 scribed but before entering upon an account of their development 

 it will be desirable to examine their structure more closely. The 

 ventral cord, as already said, consists of seventeen ganglia, one 

 for each of the primary trunk segments. These ganglia are act- 

 ually double, each representing a pair of simple ganglia united in 

 the mid-line. The first three of these double ganglia, belonging to 

 the mandibular and the two maxillary segments, are closely united 

 to form a large elongated compound ganglion, the sub oesophageal 

 ganglion (Figs. XV, 38, 39, 42, 44 and 45, SoeGng), situated in 

 the lower half of the head, below the oesophagus, and supplying 

 nerves to the mouth parts. Its compound nature is clearly indi- 

 cated by the presence of two transverse constrictions dividing the 

 ganglion into three subequal parts (Fig. 39). Springing from the 

 anterior end of the ganglion is a pair of stout connectives leading 

 to the brain and embracing the oesophagus, the circumoeso phageal 

 commissures, while from its posterior end two similar connectives 

 or commissures unite it to the ganglion next following, the first 

 thoracic. The eleven ganglia following, including the first thor- 

 acic, are essentially alike except as to their size, the first thoracic 

 being the largest, the second and third thoracic ganglia a trifle 

 smaller, while the abdominal ganglia diminish slightly in size to- 

 ward the posterior end. The last three ganglia are united to 

 form a compound ganglion, but the last of the series, the eleventh 

 abdominal, is so slight that it may be considered as vestigial. Each 

 of the ganglia (Fig. 37, A, C and D) is flattened in form, about 



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