*V: 



274 THE BIOLOGY OF INSECTS 



either resting or moving about on the surface film, or diving 

 and swimming through the water. The whirligig beetles 

 (Gyrinus) may be seen darting or circling rapidly over the 

 surface ; they are then on the water but entirely in the 

 atmosphere, for they depress without breaking the surface- 

 film. A beetle thus skimming will then dive suddenly and 

 cleave the water with rapid strokes of its oar-like middle- 

 and hind-legs, carrying with it, between the extremity of 

 its closed wing-covers (elytra) and the tail-end of its body, 

 a bubble of air, the boundary of which gleams like a sphere 

 of silver. Tliis tiny enclosure from the upper atmosphere, 

 in contact with the beetle's spiracles, enables it to continue 

 breathing normally for a considerable time although sub- 

 merged. The larva of Gyrinus, an elongate firm-coated grub 

 with well-developed legs, has a series of paired thread-like 

 hollow abdominal appendages which serve as gills for 

 ensuring gaseous exchange between the cavity of its closed 

 air-tube system and the air dissolved in the water. The 

 Gyrinus grub is thus so specially adapted for aquatic life 

 that it could not survive out of the w^ater. Yet at the close 

 of the larval period it crawls up the shoot of an aquatic plant 

 and spins a silken cocoon between the leaves ; within this it 

 pupates and the pupa with its paired series of open spiracles 

 breathes atmospheric air in the manner usual with insects. 

 Into the air therefore the beetle, when developed, must 

 emerge, though, as we have seen, it spends most of its Hfe 

 on or beneath the surface of the w^ater. Its possession of 

 functional wings ensures the power of migration in case 

 of need. 



Gyrinus, like its much larger and well-known relation 

 Dyticus, is a creature of prey. Another group of aquatic 

 beetles with very different modifications for their special 

 mode of life are the species of Donacia and allied genera 

 belonging to the family of the leaf-beetles (Chrysomelidae) 

 and vegetarian in their habits. While Gyrinus and Dyticus 

 have smooth boat-like contours to the body and the hinder 

 legs broadened like paddles, the Donaciine beetles are of an 

 aspect like that of many members of their order which never 



