40 NATURAL HISTORY OF AQUATIC INSECTS CH. 



many species and several genera in the family of 

 Dytiscidae, but it will be best to devote our space 

 chiefly to the' life-history of D. marginalis, one of the 

 biggest and the commonest of them all. 



Both the larva and the imago are aquatic ; the 

 pupal stage is however passed in the earth. This 

 is the case with other aquatic Coleoptera too. The 

 explanation is not hard to discover. These Insects 

 are air-breathers throughout their whole life. As 

 larvae or adults they can come to the surface at 

 pleasure, and renew their supply of air. But the 

 pupa is almost motionless. Its limbs are being 

 reconstructed, and cannot effect more than a slight 

 change of attitude. Under these circumstances, the 

 Insect passes its pupal stage where air can be 

 procured at all times. For the sake of protection it 

 buries itself in the earth, but so near to the surface 

 that air can easily reach it. 



Lyonnet observed that captive females of Dytiscus 

 laid their eggs at random in the water. 1 For several 

 years in succession he watched the eggs so laid, and 

 none were ever hatched. At last one egg produced a 

 larva, but this soon died. Lyonnet was not aware that 

 the eggs of his captive Dytisci had been laid under 

 unnatural conditions. The female, when at liberty to 

 obey her own instincts, lays her eggs in rushes or 

 other aquatic plants beneath the surface of the water. 

 She makes an incision in the stem, which reaches the 



1 This would agree better with Acilius than with Dytiscus. 

 Acilius lets the eggs drop upon the mud while swimming about. 

 Colymbetes arranges its eggs upon leaves, c., to which they 

 adhere strongly (Regimbart). 



