INSECTA : DIPTERA 



355 



surrounds them. When the larva is full grown it is about 



half an inch long, and two dark spots become visible on the 



thorax (Fig. 280, B) a sign that pupation is about to occur. 



At this time the larva spins a little oval-shaped 



nest or cocoon, attaching the silk threads on 



each side to a leaf, and inside this the larval skin is 



thrown off. The pupa retains a 

 good deal of power of movement, 

 and it breaks away one end of 

 the cocoon, so that it finally lies 

 supported in a silken pocket, but 

 with its head-end and branched 



FIG. 281. Pupae of two species 

 of Simulium ( x 2). 



a, Pupal cocoon with a central pro- 

 jection from the upper margin of 

 the pocket ; b, cocoon of Simu- 

 lium sericeum ; the pupa has 

 four two-branched gill-filaments 

 on each side, and the cocoon is 

 without a central spike. 



FIG. 282. Pupa of Simulium 

 removed from the cocoon. 



respiratory filaments projecting into the current of water 

 (Fig. 281). The pupae in the pockets with a central spike 

 on the margin are like those shown in Fig. 282, having 

 four unbranched filaments on each side, whilst those in the 

 pockets with simple margin, have four two-branched hairs on 

 each side. The pupae are constantly changing their positions 



