TRANSFORMATIONS OF RUFFALO GNATS. 23 



THE PUPAL STAGE. 



METHODS OF PUPATION. 



The histoblasts of the pupal and adult organs are formed in 

 Simulium larva? some considerable time before pupation takes place, 

 and can be discerned when the larvae are half grown, shortly after 

 the second molt. The pupal respiratory organs, composed of long 

 tubelike filaments (PI. V, fig. 1), can be seen underneath the larval 

 skin, on the sides of the anterior portion of the thorax, lying coiled 

 up and visible as rounded darkened areas which become almost black 

 as the pupal stage approaches. (PI. Ill, fig. 7, A, A.) Shortly after 

 the molt preceding pupation the chitinous hooks on the dorsal and 

 ventral surfaces of the abdomen of the pupa become well developed 

 underneath the larval skin. 



Before commencing to pupate the larva spins over itself a pocket- 

 shaped pupal case, formed of the strong silken threads supplied by 

 the salivary glands. The shape and texture of the pupal cases vary 

 according to the species. Of the five species discussed in this paper, 

 four have pupal cases of the shape that may be described as the 

 "wall-pocket" type, the texture finely spun, almost leathery in ap- 

 pearance, the cases being spun as a rule well separated from one 

 another, and the distal end wide and rather open. (PL V, fig. 2.) 

 S. pictipes. is the exception, having the pupal case shaped after the 

 fashion of a boot (PI. V, fig. 5), the heel always pointing down- 

 stream, the pupal cases often overlapping one another in a coral- 

 like formation, the texture coarsely spun, very tough, and the distal 

 end narrow. 



Shortly before the larva has finished making the pupal case the 

 air from the tracheae, entering the extension leading from the base 

 of the main trunk from which the branch filaments arise, slowly 

 creeps along the entire length of the filaments. The skin splits along 

 the dorsum of the thorax and the filaments are at once projected 

 into the water. The skin behind the head is then worked off toward 

 the caudal end of the larva and the old tracheae are withdrawn 

 through the spiracular openings. The inner lining of the hind gut, 

 together with the anal gills, which are formed of the same epithelial 

 layer, is then cast out through the anal slit which is situated just 

 above the X-shaped sclerites on the dorsal surface at the posterior 

 end of the larva, and may be seen attached in the interior of the 

 cast larval skin, which often remains for a while within the pupal 

 case. The skeleton of the head of the larva is then pushed off down- 

 ward to the front, the wall of the alimentary canal, about as far as 

 the junction of the pharynx and the oesophagus, coming away with it. 

 The pupa seems to lock itself in the pupal case by means of the 



