THE Briotocy or EpHyprRA suBOPACA LOEW 569 
retraction of the first two segments, they are entirely concealed within 
the fold of the integument. The thoracic segments are footless, while 
each abdominal segment bears a pair of prolegs. These prolegs are 
‘nipple-shaped, are fused at the basal third, and bear a number of claws 
on their blunt tips. These claws are arranged in three rows, usually with 
five in. the first, four in the second, and four or five in the third. The 
- number varies and the size of the claws decreases row by row. In addition, 
there is one more row of rather insignificant small claws. The last pair 
of prolegs are more prominent than any of the preceding ones, and the 
claws upon them are much larger. The claws here are opposed in position 
to those on the other prolegs. This enables the larva to grasp an object 
by means of these and the two preceding pairs, when pupation is impending. 
Behind the anal opening is a pair of more or less rounded pads which 
are considered as parts of the prolegs, and a number of small claws are 
borne on them. The caudal process is a cylindrical sheath, into which 
its two branches can be withdrawn. It is longer than any segment in 
the body of the larva, being about three or four times as long as the 
twelfth segment. At the end of each branch of the process is a chitinous 
cap with one large round pit situated at the center and four small openings 
on a chitinous knob surrounding this pit. At the inner border of each 
of these openings is attached a fan-shaped thin membrane, which can be 
seen best when the larva sticks its caudal tips to the surface of the water 
for respiration (Plate LIV, 7). 
Internal structures 
The tracheal system.— The tracheal system consists mainly of two 
pairs of longitudinal trunks, one dorsal and one ventral. The dorsal 
trunks are large and are the true trunks, while the ventral are more 
delicate and are made up merely by the connection of the outer branches 
of the dorsal pair (Plate LIV, 8). The dorsal pair begins in the second 
segment where the prothoracic stigmas open out through the body wall, 
and extends posteriorly to the caudal tips, which bear the spiracles. 
Connecting these two main trunks in the fourth segment is a commissure, 
overlying the cephalopharyngeal skeleton. Traigardh states that in Hphydra 
riparia this is the only commissure, but the writer has found in Ephydra 
subopaca, in the caudal region close behind the twelfth segment, a very 
short commissure concealed by the crossing of the two tracheal trunks, 
