27 



The fatty tissue of Chironomus-larvre is divided into an inner and 

 an outer layer, and I think that the longitudinal strand and the fat-sac 

 must be classed as a special development of the former, hut what 

 special function they perform I have not been able to find out. 



From the point of view of comparative anatomy the fat-sacs are, The air*sacs of 

 I think, of some importance for they probably show us how Coretlim 

 (Phantom Larva) has come to possess those remarkable organs, the 

 air-sacs, whose hydrostatic function is well-known.* These organs consist 

 of a pair of large sacs in . the thorax, connected by long transparent 

 tubes containing fluid to a similar pair near the end of the abdomen. 

 In order to convert one system into the other, we have only to suppose 

 a second enlargement in the dorsal region of C. pusio and replace the 

 fat-drops by an air space. 



We have already seen that Chironomns pusio possesses a rudimentary 

 tracheal system in the neighbourhood of the fat sacs, though quite 

 independent of them; larva No. 12 possesses, in addition to its large 

 sacs, a complete tracheal system extending without interuption from 

 head to anal gills, and also in one species of Corethra I found rudimentary 

 tracheae in head and anal region. These facts suggest that the air-sacs of 

 Corethra are not, as generally supposed, a modification of the tracheal 

 system, but of the internal layer of the fatty tissues. 



THE NERVOUS SYSTEM. 



The nervous system of C. pusio, as in C. dorsalis, consists of a 

 brain, a subcesophageal ganglion, three thoracic and eight abdominal 

 ganglia, the last two being partly fused together (fig. 5). The brain 

 is bi-lobed and much larger than is usual in Chironomus larvae, and 

 in dorsal view completely hides the suboesophageal ganglion ; tracheal 

 branches penetrate into its substance. Paired connectives unite the 

 thoracic ganglia, but in the abdomen the connectives become fused 

 into single strands along the greater part of their course. The distribution 

 of the nerves as far as I have been able to trace them may be seen 

 in fig. 5. No. 1 8 is another large-brained larva ; in this species, as 



* MIALL. Aquatic Insects, p. 118. 



