DIPTERA, 435 



of these segments except the first bears two tufts of tracheal 

 gills ; but those of the last segment are united. The 

 head, which forms the front end of the first of the seven 

 larger divisions, bears a pair of slender antennae ; each of 

 these consists of a very short basal segment and two long 

 segments ; at the tip of the last of these there is a pair of 

 minute appendages and a bristle. The suture between the 

 head and the remaining part of the first division is best seen 

 on the ventral side of the body. On the dorsal side a 

 suture may be seen dividing the last division into two 

 segments. 



The pupa-state is passed in the same place as the larval. 

 Like the larvae the pupae are very conspicuous on account 

 of their black color, and are apt to occur like the larvae 

 closely clustered together. The pupa is not enclosed in the 

 larval skin, and differs greatly in form from the larva. On 

 the dorsal side the skin is hard, forming a convex scale over 

 the body (Fig. 508, c) ; and the thorax bears a pair of breath- 

 ing-organs ; on the ventral side the skin is very delicate, soft, 

 and transparent ; so that the developing legs and wings may 

 be easily seen when the insect is removed from the rock. 

 The pupae cling to the rock by means of six suckers, three 

 on each side near the edge of the lower surface of the abdo- 

 men ; and so firmly do they cling that it is difficult to re- 

 move specimens without breaking them. 



We have watched the midges emerge from their pupa- 

 skins and escape from the water. The pupae occurred in 

 groups so as to form black patches on the rocks. Each one 

 was resting with its head down stream. Each midge on 

 emerging forced its way out through a transverse rent be- 

 tween the thorax and abdomen. It then worked its body 

 out slowly, and in spite of the swift current held it vertical. 

 The water covering the patch of pupae varied from one 

 fourth to one half inch in depth. In the shallower parts 

 the adult had no trouble in working its way to the surface 

 still clinging to the pupa-skin by its very long hind legs. 



