XVII 



INSECTA: COLEOPTERA 



253 



almost black, dorsally, though in the water the thorax and 

 front part of the abdomen appear silvery beneath, owing to 

 the film of air held by the closely- 

 set short hairs which cover this 

 part of the body. The air film 

 also extends on the dorsal side 

 round the neck and between the 

 thorax and elytra. The beetle 

 swims rather irregularly in the 

 water, using the legs of the two 

 sides alternately, instead of to- 

 gether as in Byticus. The two 

 back pairs of legs are large and 

 feathered and are used in swim- 

 ming, whilst the front legs are 

 much smaller. In the male, the 

 last joint of the foot (tarsus) is 

 flattened out into a triangular plate 

 (Fig. 168, d). 



Below the long elytra are large 

 membranous wings (Fig. 168), by 

 the aid of which the beetle flies 

 from pond to pond, usually at 

 night. On the ventral surface the 

 body slopes to a ridge in the middle line, and the thoracic 

 ridge is prolonged behind into a strong spine, which some- 

 times inflicts a prick when the beetle is handled. 



Like Dyticus, Hydrophilus has to come to the 

 ' surface to breathe, but whilst the former tilts its 

 tail up into the air, and takes in its fresh supply through the 

 abdominal spiracles, refilling the air space below the elytra 

 from behind, the Silver Water Beetle swims to the surface, and 

 lies there almost horizontally, just lifting one " shoulder " out 

 of the water and opening a little air passage between the head 

 and thorax ; this passage is bounded on its outer side by 

 the curiously curved antenna, the end joints of which are 

 densely covered with hairs, and thus prevent the water 

 from obtaining entrance.^ The air passes through this 

 channel and along the air films at the sides of the thorax and 



' For a very clear detailed account of the process, refer to Aquatic Insects, 

 by Professor Miall. 



Fig. 184.— The Silver Water 

 Beetle {Hydrophilus piceus 9 ). 



Drawn from dead specimen with 

 antennae, a, spread out (in life 

 tliey would be hidden in this 

 view) ; ins-p, maxillary palp. 



Respiration. 



