473 



The Ventral Nerve Cord and Terifheral Nerves of the 

 ll^ASONiA Larva. 



In the newly hatched larva the ventral nerve cord is 

 visible through the transparent cuticle as a thick column, 

 not very distinctly marked off into ganglia, and passing from 

 the third segment backwards along the mid-ventral line to 

 the eleventh (fig. 1). In front, the nerve cord communicates 

 by a pair of circum-oeeophageal connectives with the brain, 

 which occupies a large part of the second segment. From the 

 brain a pair of minute nerves is given off to the small rudi- 

 mentary sense papillae (antennae) on the first segment. Other 

 nerves doubtless leave the brain, and supply various parts of 

 the head, but I have not been able to observe them definitely. 



It is only with the greatest difficulty that ganglia can be 

 observed in the ventral nerve cord at this period, and the 

 presence of lateral nerves is the best indication of their posi- 

 tion. These nerves are quite prominent and are even clearly 

 visible through the transparent cuticle (fig. 1). The posterior 

 ones are the largest and supply the greater part of the hinder 

 region of the larva. Lying in front of the brain, just dorsal 

 to the oesophagus, is a minute rounded stomotogastric ganglion 

 (fig. 117), connected by a pair of nerves that surround the 

 narrow oesophagus with the circumoesophageal connectives 

 near the junction of these with the first ventral ganglion. 



During the growth of the larva there is a corresponding 

 increase in the size of the nervous elements, and it is not till 

 a considerable time after hatching that the various ganglia 

 can be clearly observed. Of these, twelve, not including the 

 brain, are present (fig. 225, a). The last one can be seen in 

 longitudinal sections to be composed, apparently, of three very 

 closely fused ganglia, so that the larva possesses at least 

 fifteen of these. No account is taken here of the possible 

 multiganglionic nature of the brain. 



Covering the central nervous system and the peripheral 

 nerves of the newly hatched larva is a very delicate membrane, 

 composed of two kinds of cells : the purely larval cells, and 

 the embryonic imaginal cells which will replace them during 

 the metamorphosis. Both these kinds of cells are very flat- 

 tened and embrace the masses of new cells closely; they con- 

 stitute a part of the splanchnopleural portion of the "peri- 

 toneum." 



The nerve cells are of two kinds. Lying usually on the 

 outside, but sometimes also scattered partly within the nerve 

 cord, are large cells, devoid of a fibre, with big nuclei, con- 

 taining a large karyosome and several scattered chromatin 

 o 



