WATER BEETI^S 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 

 antennae, and in the Dytiscidse 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 

 fiattening of the hind legs. 



The increased swimming efiiciency of the hind legs has been brought about 

 by the transformation of the coxse 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 

 this 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 prothorax, 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 Dytiscidse 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 Hydrophilidae 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- 



