240 



BULLETIN OF THE BUREAU OF FISHERIES. 



fied antennae, whose enlarged club-shaped terminal joints are covered with a dense 

 coat of fine hair, the groove opening into the dorsal reservoir, and an enlargement 

 of the meso and meta thoracic and first abdominal spiracles. 



In the Haliplidse the elytra are held firmly in place by groovings in the plem'a 

 and by knoblike structm'es at the outer ends of the posterior coxae. The latter 

 are not soldered fast to the metasternum as in the other families, but are free and 

 project backward far enough to cover from three to five abdomen segments. The 

 posterior end of the body is thrust out of the water, and air is admitted to the 

 space between the broad posterior coxae and the ventral surface of the abdomen. 

 This air finds its way to the reservoir beneath the elytra through a groove in the 

 pleurum at the anterior end of each coxa. The structural modifications in this 

 family thus include the peculiarly modified posterior coxae, the grooves leading to 

 the dorsal reservoir, and the enlargement of the metathoracic and first abdominal 

 spiracles. 



In the Gyrinidae the lateral or outer margin of each elytron is turned down- 

 ward and a little inward and its free edge is grooved. The lateral margins of the 

 meso and meta thorax and the anterior abdomen segments are fused and raised 

 into a longitudinal ridge, which fits into the groove along the edge of the elytra, 

 the two locking together like the lateral hinge on a fresh-water mussel. Opposite 

 the junction of the meso and meta thorax is a rounded peg just inside the edge of 

 each elytron, which fits into a socket in the thorax and holds the elytron securely 

 in place. In Dineutes at the posterior end the under surface of each elytron and 

 the upper surface of the abdomen segment immediately beneath it are covered 

 with short hairs, which make a tight joint when the elytra are closed. In Gyrinus 

 the same closing of the joint is accomplished by a transverse ridge, which, runs 

 around the posterior end of each elytron on the under siu'face. The air enters 

 the dorsal reservoir through a groove just inside the posterior end of the turned- 

 down lateral margin of each elytron. In this family, accordingly, the structural 

 modifications for breathing consist in the interlocking margins of the elytra and 

 the body, in the peg for fastening the two together, and in the posterior groove 

 for the admission of air. The spiracles are all about the same size. 



In the larvae there are two methods of supplying the lateral tracheae with air — 

 one by direct admission above the surface of the water, the other by infiltration. 

 For the former it is necessary that the tracheae open externally at the posterior 

 end of the body; that this opening be closed in some way while the larva is beneath 

 the water; and that some provision be made for holding the larva at the surface 

 long enough to thoroughly aerate the tracheae. In the dytiscid larvae the tracheae 

 open at the extreme posterior end of the body close to the dorsal surface. Beneath 

 them on the ventral surface are given off a pair of cerci which are heavily fringed 

 with setae. The extensor muscles operating these cerci run from the base of the 

 latter to the dorsal surface of the abdomen, around the tracheae. They are so 

 arranged that when the cerci are extended parallel with the body axis or nearly so 

 the posterior openings of the tracheae are closed by the contraction of the muscles. 



When the larva thrusts the posterior end of the body above the surface of the 

 water to breathe, the cerci are turned down at right angles to the body axis on 



