Brain 43 



3. The Nervous System. 



The nervous s^^stem of the Chironomns-larva (figs. 34-39) (iangim. 

 consists of a brain or supra-oesophageal ganglion, a sub- 

 oesophageal ganglion, three thoracic, and eight abdominal 

 ganglia, the last of which supplies two segments, and is 

 closely applied to the last but one. 



The brain is, as usual, two-lobed, the lobes bulging Brain, 

 outwards and downwards. Oesophageal connectives can 

 hardly be said to exist, and the sub-oesophageal ganglion 

 lies not behind, but beneath the brain. Both the brain 

 and the sub-oesophageal ganglion properly belong to the 

 head, in which they are actually lodged in most insects. 

 In the larvae of D'lptera Nemocera their position varies ; 

 they may lie in the head (Culex, Simulium, Phalacrocera, 

 some species of Chironomus), lialf in and half out (Tipula. 

 Ptj^choptera), or altogether behind it (some species of 

 Chironomus, Dicranota). In the ' acephalous ' larva 

 of the blow-fly they occupy the metathoracic segment. 

 In the embryo and very young larva of Chironomus 

 dorsalis the brain lies in the head, from which it gradually 

 shifts backwards during the first few days after hatching ^ 

 After the first moult the small larval head is almost 

 entirely filled by the jaw-muscles. Hence the nerve- 

 centres, as well as the rudiments of the head of the fly, 

 which begin to form in a later stage, can onlj^ find the 

 room which they require in the thorax. 



The first thoracic ganglion of the Cliironomus-larva Ganglia 

 lies in the prothorax, the second and third in the meso- nectives .>f 

 thorax. The first abdominal is shifted forwards from its eord' * 

 proper segment to the metathorax (fig. 35). 



The connectives between the ganglia, though really 

 double, appear to form a single cord behind the first 

 abdominal ganglion, except in very young larvae, where 



' Weisniiimi, 1863. 



