Ufe-Mstory of Pericoma canescens (Psychodidss). 143 
pupa from the larval skin. The ventral surface is more 
uniformly covered with tubercles and setge. A pair of 
small and distinct plates, which appear to be more solid 
than the neighbouring integument, defend the middle 
annulus of most of the abdominal segments on the 
ventral side, lying within the shield. The sides of the 
body bear a flexible armour of regular and close-set 
setae, with here and there a larger plate. There is a 
lateral fringe of much longer setae, the first annulus of 
the segment having, as a rule, a pair of single setae, the 
second annulus none, and the third a pair of large double 
ones. It seems probable that these long and stiff hairs, 
all pointing backwards, like those which project from the 
dorsal shields, save the larva from being swept away by 
a sudden rush of water. Buried, as it commonly is, in 
well-rooted, filamentous algae, the larva has only to keep 
its head up stream to offer a most eff'ective resistance to 
the current, while it will not be seriously hindered in 
travelling head-first through the weeds. 
The eighth abdominal segment is defended above and 
on the sides by a single large shield, which bears a 
median pair of long setae, and three pairs of lateral ones. 
The ventral side is more flexible, but is protected by a 
centrally placed scutcheon and two small lateral plates. 
Two pairs of long setae are borne upon the hinder edge 
of the scutcheon. Close behind this is the anus, which 
is shut in on the sides and in front by a pair of plates. 
The extremity of the abdomen is truncated. It bears 
four large anal processes, two dorsal and two ventral. 
Between the bases of these processes, but nearer to the 
dorsal pair, are the posterior spiracles. The anterior 
spiracles are borne on the sides of the second annulus 
of the prothoracic segment. They project as cylindrical 
tubes with circular, terminal openings, and wrinkled 
bases. Anterior spiracles are unusual in aquatic Dip- 
terous larvae, but Eristalis furnishes us with another 
example. In Eristalis the anterior spiracle is probably 
not functional ; in Pericoma it apparently is. 
The two pairs of anal processes are similar in con- 
struction, but the ventral ones are rather longer than 
the dorsal. Each process consists of a stout chitinous 
rod bearing a fringe of fine filaments, which project 
from the sides and tip. The terminal filaments are the 
longest, and those on the inner side towards the base of 
the rod the shortest. The anal processes stand out from 
