i6o 



THE ANIMALS AND MAN 



ing vegetation in the water. The first have slender thread- 

 like antennae, while the second have antenna? with thickened 

 or club-like tips. As neither kind has gills both have to 

 come to the surface to get air, but they always carry down 

 with them a supply sufficient to last some time. They do 

 this in two different ways. The predaceous diving-beetles 

 force the posterior tip of the body above the surface (they 

 always hang head downward when at the surface) and 



slightly lift the tips of the horny black 

 wing-covers which lie on the back. Air 

 rushes in under the wing-covers and is 

 held by the closing of the tips. The 

 breathing pores or spiracles 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 

 surface, 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 hav- 

 ing a pile, as velvet, into water, you will see 

 that it retains underneath the water a 

 nearly complete coating of air. The under 

 tie. Dyticus sp. s ide of the water scavenger-beetle is cov- 



(Natural size.) ered 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 common 

 kinds, one called back-swimmers (fig. 68), which swim with 

 under side uppermost, and have the back black with large 

 creamy patches, the other called water-boatmen (fig. 70), 



FIG. 69. Water- 

 tiger, the larva 

 of the predace- 

 ous water-bee- 



