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 pleura 
and by knoblike structures 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 surface. 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 
