312 A BOOK OF INSECTS 



have grooved mandibles, somewhat like those of the ant- 

 lion, and suck the juices of their victims. When full-fed 

 they leave the water, and form cells in the earth, where 

 they change to pupa?. 



The chief problem which confronts an aquatic insect 

 is that of obtaining the supply of oxygen necessary for 

 its vital processes. We know that the typical insect 

 breathes by means of trachea?, which communicate with 

 the atmosphere through openings called spiracles ; but it 

 is obvious that this plan could not succeed under water 

 without considerable modification. The variety of ways 

 in which the difficulty has been surmounted is extremely 

 interesting. In Dytiscus the spiracles are not placed along 

 the sides of the body, as is the case with most terrestrial 

 insects, but open upon the back, beneath the elytra, More- 

 over, the two posterior pairs of spiracles are unusually 

 large ; and the elytra fit perfectly against the abdomen, 

 so that an air-tight space is formed between them and 

 the insect's back. When the beetle desires to take 

 breath, it poises itself in the water, thrusts its tail-end 

 through the surface-film, and slightly depresses the tip of 

 its abdomen, with the result that air rushes into the four 

 large spiracles, and fills the space between the elytra and 

 the back. The chink is then closed tightly, and the insect 

 is once more ready for a diving excursion. Dr. Sharp 

 tells us that the male Dytiscus rises to the surface to take 

 breath once in every eight minutes and twenty seconds 

 on an average, and remains poised for about fifty-four 

 seconds. The longest interval that the insect was observed 

 to pass without rising to the surface was nineteen minutes, 

 although the female, being less active in habit, rises less 

 frequently than her mate. 



The whirligig beetles dive to escape danger, but do 

 not stay long beneath the surface. They carry with 



