72 GNATS OR MOSQUITOES — CHAPTER V 



ones that undergo any marked change. These widen round 

 their separated and collapsed intima, and a new and strongly 

 thickened intima is developed. In the main trunks no new 

 intima is formed, and when the imago escapes no portion 

 of the intima is shed, saving the portions connecting the 

 syphons and the first abdominal stigmata with the main 

 trunks. These fragments are, in the case of the syphons, 

 well developed, and have a fully-developed spiral thickening. 

 The portions connected with the first abdominal stigmata, 

 though better developed than the other abdominal branches, 

 have the spiral thickening only slightly developed. The 

 terminal portion is beset with very numerous small spines. 



The nervous system is particularly interesting. Within 

 the short space of four days, certain ganglia increase enor- 

 mously in size by the addition of cells, apparently derived 

 directly from the epidermis ; and other ganglia shift their 

 positions bodily and sometimes fuse with others. 



In the larva each of the first eight abdominal segments 

 has a pair of ganglia ; and yet a pupa, only half escaped from 

 the larval cuticle, has four in the thorax and none in the 

 first segment of the abdomen. During pupal life these four 

 ganglia fuse into one compact mass. During the first two 

 days of pupal life the eighth ganglia migrate into and fuse 

 with those of the sixth segment. In the female the change 

 goes further. A pupa almost ready to burst and give exit 

 to the imago has still the arrangement already described ; 

 but an imago killed immediately after its escape is found to 

 have no ganglia in the seventh or eighth segment, but in the 

 sixth segment are two masses ; the first the pair properly 

 belonging to the segment, lying at its anterior end ; the 

 other a double mass, formed of the seventh and eighth 

 gangha, lying in the hinder end of the segment. 



In the male imago the arrangement is the same as in the 

 advanced pupa. In the head the supra-ocsophageal ganglion 

 increases enormously in size. The epidermal (" hypo- 

 dermal ") cells, especially those near the borders of the 

 eyes, proliferate freely, and the cells budded off from their 

 inner surfaces migrate inwards and form the new cells of 

 the ganglia. By this process the ganglia, w^hich at the 



