ARTHROPODS AND MOLLUSCS 



159 



the pool are small, oval, flattened, shining black insects 

 that dart swiftly about in curving paths on the water. These 

 are whirligig beetles. Do they run on the w r ater or swim? 

 Do they ever dive and swim beneath the surface ? Examine 

 one with a magnifier, and note that it has four compound 

 eyes instead of two, the usual number in insects. Where 



FIG. 68. Predaceous diving beetles, Dyticus, and back-swimmers, Nota- 

 secta. (Slightly less than natural size; from living specimens.) 



is the extra pair situated? Note the peculiar shape of the 

 legs. What are the legs specially fitted for? 



Swimming about below the surface may sometimes be 

 found large, shining, black beetles (fig. 68) from half an 

 inch to an inch and a half long. There are two principal 

 kinds, the predaceous diving-beetles, which kill and eat other 

 insects, and the water scavenger-beetles which feed on decay- 



