WATER BEETLES IN RELATION TO PONDFISH CULTURE. 



241 



top of the surface film. This is done by contracting the flexor muscles and relaxing 

 the extensors, thereby opening the tracheae. In the hydrophilid larvae the tracheae 

 open in the bottom of a transverse groove, which is connected with the base of the 

 cerci in such a way that it is kept closed when the cerci are extended but is opened 

 when they are flexed. Incidentally the turning down of the cerci onto the sm-face 

 film of the water supports the larva while it is breatting. The operation of the 

 cerci in both these families has been well described and figured by Brocher (1913, 

 Dytiscidae, p. 125, text figs. 2 and 5) and d'Orchymont (1913, Hydrophilidae, p. 202, 

 text figs. 22 and 23). 



An infiltration of air occurs in the haliplid and gyrimd larvae and in excep- 

 tional genera of the Dytiscidae (Coptotomus) and the Hydrophilidae (Berosus). 

 This is accomplished by the presence in the larvae of lateral gills along the sides of 

 the abdomen and in the haliplid larvae by jointed spines on the dorsal and lateral 

 surfaces of the thorax and abdomen. Branches of the lateral tracheae run out 

 into these gills or spines and there give off fine tracheoles, through whose thin 

 walls the air infiltrates from the water. 



FOR FLOTATION. 



Formerly the film of air clinging to various parts of the bodies of adult beetles 

 was universally regarded as designed for breathing purposes. Brocher (1914a), 

 however, has clearly demonstrated that it is not used for that pm-pose but for some- 

 thing very different, and his idea has been accepted by Leng (1913) and others. 



According to this interpretation the air film of a beetle corresponds to the air 

 bladder of a fish and is concerned with flotation or the maintenance of equilibrium. 

 In the Dytiscidae the dorsal surface of the abdomen behind the elytra is covered 

 with a heavy pubescence to which a bubble of air clings when the beetle dives. 

 But this is an exhaled bubble and not one that is to be inhaled. 



In the Hydrophilidae the terminal joints of the antennae, the ventral surface 

 of the thorax, the whole of the first and varying amounts of the succeeding ab- 

 domen segments are covered with a fine pubescence, which retains a film of air 

 when the beetle is submerged. Since the hair by entangling the air keeps the 

 body dry, it has received the name of hydrofuge pubescence. Its real use, how- 

 ever, is not to keep the water away from the body, but by entangling the air to 

 lessen the specific gravity of the beetle and thus aid it in locomotion. 



In the Gyrinidae also the ventral surface of the body is covered with short 

 hairs, which entangle a film of air, and this aids in supporting the body upon the 

 surface of the water. This film seems intended entirely for flotation, and none of 

 it, as far as known, is used for respiration. 



In the Haliplidae, as in the Dytiscidae, there is often an air bubble at the posterior 

 end of the elytra. This must be exhaled air, however, since the air supply enters 

 the dorsal reservoir by way of the coxal plates and the pleural grooves. The tip 

 of the abdomen is hairy on the dorsal surface, and the pleural grooves are lined 

 with fine hairs. These seem rather to prevent the entrance of water than to retain 

 an air film, and the latter is not present in any of the genera. Evidently there is 

 enough buoyancy in the air carried under the coxal plates and beneath the elytra 

 to obviate the necessity of an air film. 



