18 
BULLETIN OF THE BUREAU OF FISHERIES. 
opment transverse grooves begin to appear and increase until in the fully matured 
larva each segment has three or four secondary grooves, and it is very difficult to 
select the true intersegmental sutures. 
There is a pair of spiracles on each segment, the anterior ones being larger 
than the posterior, with the exception of the last pair, which are much the largest 
of all. These open on the dorsal surface in the bottom of the groove already 
described and are the only ones that function during larval life. 
LOCOMOTION. 
The larvae live in the water until ready to pupate and show two methods of 
locomotion, walking and swimming. The legs are strong, and in spite of the long 
abdomen the larva can crawl actively about over the bottom of the pond and even 
out in the open air. When endeavoring to escape impending danger, it moves 
with considerable rapidity, but hardly fast enough to be called running. 
When pond 2E was drained, the Hydrous larvae continued crawling about 
over the bottom for two days after the water had entirely disappeared. By that 
time the bottom had become so thoroughly dried that they could no longer find 
any shelter from the hot sun. One hundred or more were gathered and preserved, 
and probably all the others perished. During their endeavors to find shelter some 
traveled 100 feet, surmounting various obstacles in their way, such as stones, 
water-logged fragments of wood, and the debris usually found on the bottom of a 
pond. This shows that they can travel long distances, if necessary, when hunting 
for a suitable place to pupate, and the fact that they usually select a spot near the 
water’s edge would indicate that the presence of moisture is the chief essential. 
Swimming is accomplished by both leg and body movements. To increase 
the power of the legs as swimming organs, the femora of all three pairs have a 
fringe of heavy setae along their outer margins, the tibiae are strongly flattened 
and have a slightly heavier fringe along both outer and inner margins, and the 
tarsi are also flattened and have a very short fringe along the inner margin, each 
tarsus ending in a single long and stout claw. The body in swimming moves 
rhythmically up and down in a manner very similar to that of a leech; but the 
combined effort of body and legs produces only a slow progress, with frequent 
stops, and it is a very simple matter to catch these larvae in a dip net 
BREATHING. 
The larva breathes air through the two tracheal trunks that extend the whole 
length of the body. So far as known there are no tissues or organs that extract 
air from the water. Hence it must come frequently to the surface in order to 
renew its air supply and is easily seen and captured at such times. When seeking 
fresh air, it swims almost vertically upward until close to the surface, then turns 
head downward and thrusts the posterior end of the abdomen and the cerci above 
the surface film. The cerci fall onto the surface film at right angles to the body 
axis, thus supporting the larva in position and at the same time opening the mouths 
of the air trunks. When it has finished taking in air, the tip of the abdomen is 
drawn beneath the surface film, thereby straightening out the cerci parallel with 
