64 BRITISH INSECTS 



example. It is more heavily built than a tiger-beetle, 

 while its eyes are smaller and far less prominent 

 The little bombardier-beetle 1 must also be mentioned, 

 because of its singular method of self-defence. It is 

 preyed upon by larger members of the family, and 

 when chased ejects a drop of volatile and explosive 

 fluid from the tip of its abdomen, thus disconcerting 

 its pursuer. Most ground-beetles and their larvae are 

 nocturnal in habit, and hide by day under stones or in 

 crevices of the soil. The majority are predaceous, 

 but there are a few remarkable exceptions, several 

 species feeding on rootlets and seeds. 



The carnivorous water-beetles 2 may be typified by 

 Dytiscus marginalis^ often called the " great water- 

 beetle." Its structure agrees very closely with that 

 of a ground-beetle, but the details are beautifully 

 adjusted to meet the requirements of a life that is 

 spent chiefly in the water The insect is an expert 

 swimmer and diver, and preys exclusively on aquatic 

 creatures. Nevertheless, it has well-developed wings, 

 and frequently flies by night from one pond or lake to 

 another. Its spiracles open beneath the elytra, the 

 edges of which fit closely against the sides of the 

 abdomen, enclosing an air-tight space above the back. 

 This space serves as a reservoir for air. Moreover, 

 the two posterior pairs of spiracles are uncommonly 

 large, so that by thrusting its tail-end through the 

 surface-film into the atmosphere, and slightly depres- 

 sing the tip of its abdomen, the insect can rapidly 

 discharge the vitiated air from its tracheal system 

 and take in a fresh supply. The Dytiscus larva is 

 extremely rapacious. Its mandibles are pointed and 

 tubular, and it feeds by suction, after the manner of 

 1 Brachinus crepitans. 2 Dytiscidce. 



