180 BLACK-FLIES. 
Constantly feeding, these larve grow very rapidly, and are 
soon ready to transform into pupe. But before they can 
do so they have to construct some protection for the inac- 
tive pupz, otherwise this would be swept away by the cur- 

Fig. 148.—Simulium tribulatum, cocoon, Fig. 149.—Simulium tribulatum, pupa. 
Greatly enlarged. Original. Greatly enlarged. Original. 
rent. They construct for this purpose a neat little cradle- 
like house, made loosely of silk, and open at the top. This 
cradle, shown in fig. 148, is built against a stone, or upon 
submerged plants. In it the larva fastens itself by a little 
loose silk in the closed part, and now transforms to a pupa, 
as shown in the illustration, and again, removed from the 
cradle,in fig. 149. Buta great change has taken place, and 
the insect no longer breathes through an organ located near 
the tail, but by an entirely different one near the head. 
These breathing-organs differ greatly in the different species, 
but are always composed of a number of fine thread-like 
tubes closed at the end, and which unite near the base in a 
7!) 
