TRANSFORMATIONS OF BUFFALO GNATS. 23 
THE PUPAL STAGE. 
METHODS OF PUPATION. 
The histoblasts of the pupal and adult organs are formed in 
Simulium larvae 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 (PL 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. (PL III, 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 
u 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 (PL 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the caudal end of the larva and the old tracheae are withdrawn 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 
