INSECTS 



169 



of the beetle are situated along each side of its back, 



underneath the wing-covers, so that 



the air held there readily enters the 



body. The water scavenger-beetle 



when at the surface keeps its head 



uppermost. It carries most of its air 



supply on its under or ventral sur- 

 face, where it is held in a coat of 



fine short hairs. The air gives the 



under side of the beetle a shining 



silvery appearance. It is held by the 



fine hairs by virtue of the surface 



film. If you dip a bit of cloth having 



a pile, as velvet, into water, you will 



see that it retains underneath the water 



a nearly complete coating of air. The 



under side of the water scavenger- 

 beetle is covered in places with a 



fine pubescence which acts like the 



pile of the velvet. 



The water-bugs are about half an inch long, and are 



grayish or black and white in color. There are two com- 

 mon kinds, one called back- 

 swimmers (fig. 124), which 

 swim with under side upper- 

 most, and have the back 

 black with large creamy 

 patches, the other called 

 water-boatmen (fig. 126), 

 which swim with back upper- 

 most, and are greenish gray. 



Fig. 125. — W.iler-tiijer, 

 the larva of the pre- 

 daceoub water-beetle, 

 Dytiats sp. (Natural 

 size ; from specimen. ) 



Fig. 126. — A water-boatman, Corisa 

 sp. (Twice natural size; from 

 Jenkins and Kellogg.) 



with fine black 



mottling. 



Both kinds come to the surface for air, and carry a supply 

 of it down with them. Observe this, and note the differ- 

 ence in the disposition of the air (revealed by its silvery 



