18 



BULLETIN or 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 



