AQUATIC BEETLES 



49 



fully to one another than to the chitinous hairs. 

 Chitin, the hard substance of which Insect integuments 

 are formed, is not, as a rule, easily wetted by water. 

 A vigorous effort is required when the larva seeks to 

 draw its tail under water again. While it remains 

 suspended, air can be taken in by two spiracles which 

 occupy the extreme tip of the tail, and lead to the 

 great longitudinal air-tubes. There are seven other 

 pairs of spiracles on the sides of the body, but these 

 are rudimentary and closed. It is only at the time of 

 pupation that they become functional. 



At length the larva ceases to feed, creeps into 

 moist earth near the edge of the 

 water, makes a roundish cell there, 

 and changes to a pupa. It is then 

 soft, yellowish in colour, and much 

 shorter and broader than before. In 

 summer the pupal stage lasts only 

 a fortnight or so. In cold weather 

 advanced larvae bury themselves in 

 the banks, probably pupating only 

 when spring is at hand. There is 

 however some difficulty in getting 

 full information respecting the pro- 

 cess of pupation in the winter or 

 early spring. The full-grown Beetles 

 live through more than one winter, 

 and except in very cold weather 

 they swim about and feed. In this 

 Insect, as in most others, the parts 

 of the imago are all present in the pupa, and are at 

 least externally perfect, though still shrouded in a 



E 



FIG. 7. Pupa of Dy- 



tiscus, dorsal view. 



From Schiodte. 



