WATER BEETLES IN RELATION TO PONDFISH CULTURE. 
239 
Diminished resistance to the water has been effected first and most notably by 
this very compacting together of the body parts, thereby getting rid of gaping 
joints, interstices, and irregularities of the surface; by the rounding and smoothing 
of the body contours into a boat-shaped form; by the reduction of spines, hairs, 
and sculpture on the body, especially on the dorsal surface; by the reversal of the 
antennas, and in the Dytiscidae and Gyrinidse by the folding of the first two pairs 
of legs into concavities on the ventral surface of the thorax, and by the conspicuous 
flattening of the hind legs. 
The increased swimming efficiency of the hind legs has been brought about 
by the transformation of the coxae into large flattened plates, securely welded to 
the ventral surface of the metasternum, thus furnishing an exceptionally rigid 
supporting base; by the flattening of the remaining joints of the leg into one hori- 
zontal plane; by the shortening of the proximal and the lengthening of the distal 
joints; by the development of swimming fringes of long hairs along the margins of 
the tarsus and sometimes of the tibia, and by the reduction of the terminal claws. 
FOR RESPIRATION. 
Since both the larva and the adult live in water, some provision has to be 
made to supply them with oxygen. In discussing the various methods by which 
t his is accomplished it will be convenient to separate the two stages. 
In the adult beetles the modifications of structure for the purpose of breath- 
ing are as interesting as those concerned in locomotion. They consist in a migra- 
tion of the spiracles onto the dorsal surface of the body, so that they are entirely 
covered by the elytra. Attention has already been called, in the compacting of 
the body, to the tight joints made by the elytra with the pro thorax, with each other, 
and with the sides of the abdomen. A reservoir for air is thus formed between 
the elytra and the dorsal surface of the body, which when filled enables the beetle 
to breathe while it is beneath the water. This reservoir is supplied in various ways. 
In the Dytiscidae the posterior end of the abdomen is projected above the sur- 
face of the water and the elytra are lifted slightly, enabling the air to enter directly. 
According to Brocher (1911a) the beetles in this family breathe by drawing in air 
through the last two adbominal spiracles and expelling it through the others, par- 
ticularly the anterior pair. He also found in Cybister air pockets in the muscles 
of the meso and meta thorax, directly connected with the anterior spiracles, but 
he did not think any large amount of air was stored in them. 
In the Hydrophilidse the elytra are strongly arched, thus creating a relatively 
large reservoir for storage of air. This is not filled directly by thrusting the pos- 
terior end of the body above the surface, as in the Dytiscidae. On the contrary, 
the angle between the head and the prothorax on one side is brought to the surface 
and the club-shaped antenna on that side is carried outward until it comes in con- 
tact with the air above the water. It is then folded back again, carrying a bubble 
of air with it. Some of this air is spread over the ventral surface of the body in a 
thin, silvery film, and some of it finds its way to the reservoir under the elytra 
through a groove between the meso and pro thorax. Thus, the structural modi- 
fications for respiration in this family include the arching of the elytra, the modi- 
