36 INJURIOUS AND USEFUL INSECTS 



through the mandible into the mouth. The inner orifice of 

 the tube lies upon the expanded base of the mandible. In 

 our own time an American naturalist. Burgess, has shown 

 that the mouth of the Dytiscus-larva is capable of being ab- 

 solutely closed by a "mouth-lock." When the mandibles are 

 fixed in the prey and the mouth-lock closed, a completely 

 watertight channel leads from the body of the victim into 

 the stomach of the devourer. So completely is the ordinary 

 outlet closed that when a larva preserved in alcohol is placed 

 in boiling hot paraffin, the vapour escapes in the form of trains 

 of bubbles by the tips of the mandibles, by the spiracles, and 

 by every other opening, however minute ; but none from the 

 mouth. I find that the mouth-lock acts automatically, open- 

 ing when the mandibles open, and closing -when they close. 

 Thus the larva can adapt its mouth, by a slight movement of 

 the mandibles, either to sucking or to the gnawing of solid 

 morsels. 



When pupation is at hand the larva leaves the water and 

 makes a cell in moist earth. The larval skin is cast, and the 

 form of the body changes, becoming thicker and shorter than 

 before. The new organs which are to serve the adult beetle, 

 such as the wings and long legs, are bent towards the under 

 side. Many of the segments now bear prominent spines, 

 which are common in subterranean beetle-pupae. They may 

 be used in the limited movements of which such pupae are 

 generally capable. The pupal stage is brief, lasting only 

 two or three weeks. But at the approach of winter any full- 

 fed larvae which survive creep into the earth, where it is 

 probable they remain for months as resting larvae, pupating 

 only when spring is at hand. The subterranean life of 

 Dytiscus is not yet well ascertained in all particulars. 



The adult beetle is quite able to withstand the cold of winter, 

 and can be seen swimming about beneath thick ice. How 

 it manages to breathe under these conditions is a question of 

 some interest, to which we have no answer ready. 



The modifications of the adult beetle to aquatic life are far 

 less profound than those of the larva. The same thing may 

 be said of aquatic insects in general, to whatever order they 

 may belong. The larva has only to feed and grow, but the 

 adult female has to seek a mate, and to lay her eggs, occasion- 

 ally at least, in a new site. To sacrifice the advantage of 



