62 THE LARVA OF THE BLOW-FLY. 



numerous granules of dark brown pigment, of a substance 

 deeply blackened by osmic acid, and of a colourless material 

 which stains with carmine, but is unaffected by osmic acid. 

 Normally there are no crystals, either in the cells or in the 

 secretion, although some preparations exhibit crystals which 

 resemble stearin or tyrosin. These are apparently the result of 

 post-mortem changes. 



7. THE NERVOUS SYSTEM, 

 a. Comparative Morphology. 



The nervous system in insects consists of two parts, a somatic 

 and a splanchnic system of nerves and ganglia, which have 

 been properly compared with the cerebro-spinal and sympa- 

 thetic systems of the vertebrata, respectively. The former 

 supplies the integument, the organs of special sense, and the 

 skeletal muscles ; and the latter innervates the visceral muscles, 

 and probably has a trophic influence on the secretory epithelia. 



The somatic nervous system consists of a series of ventral 

 ganglia, united with each other by one or two longitudinal 

 commissures. In the primitive embryonic condition all the 

 ganglia are distinct, and there may be as many as seventeen 

 pairs— four cephalic, three thoracic, and ten abdominal (Brandt 

 [30]). As development progresses, several of the ganglia 

 become fused together, and this fusion is more marked in the 

 higher types of insects. Most larvae have only two centres in 

 the head, a pre-oral and a post-oral centre, with three thoracic 

 and seven or eight abdominal ganglia. In the imago, the two 

 cephalic centres are frequently fused, and the thoracic form but 

 two centres. The greatest concentration occurs in the Muscidae, 

 which have only two centres in the imago, the cephalic and 



Bibliography : — 



28. Newport, G., ' On the nervous system of the larva of Sphinx ligustri 



and the changes it undergoes in the earliest stages of the pupa.' 

 Phil. Trans., 1832, p. 3S3. London. 



29. Brandt, E., ' Vergleichend. Anatomische Skizze des Nervensystems 



der Insekten.' Horte Soc. Ent. Ross., toni. xv., 1879. 

 \y 30, Brandt, E., ' Ueber die Metamorphosen des Nervensystems der 

 ZweiflUgler.' Horaj Soc. Ent. Ross., tom. xv., 1879. 



