106 



GNATS OR MOSQUITOES — CHAPTER VI 



extremely elastic and distensible tracheae, which swell out 

 into bubble-containing dilatations, wherever their mutual 

 pressure permits of their doing so. Apart from a few loose 

 connective elements, the sacs consist of nothing else than 

 these curiously modified tracheje. 



The circulatory and nervous systems also present no 

 peculiarities beyond the few points that will be found noted 

 in the section on the anatomy of the larva and its meta- 

 morphoses. In the imago, however, the fused cephalic 



Fig '25. — Diagram of the Nervous System. 

 a n, antennal nerve ; o I, ophthalmic lobes ; s g, supra-cesophagoal 

 ganglion, or brain ; s b g, sub-oesophageal ganglion ; t g, mass consisting 

 of the third thoracic and first dorsal ganglion ; a g, abdominal ganglia. 



ganglia, or brain, is very large, and although they do not 

 appear to possess any specially great amount of insect 

 intelligence, its relative weight as compared with that of 

 the body, cannot fall far short of that of even the human 

 subject. One striking point, however, is the manner in 

 which almost the whole of the branches of the last ab- 

 dominal ganglion are supplied to the internal generative 

 organs. So much is this the case, that a successful pre- 

 paration of either the ovaries or testes almost always carries 

 away with it this ganglion. 



