222 NATURAL HISTORY OF AQUATIC INSECTS CH. 



serve as oars ; others again use the hinder part of 

 the body as a fish does its tail. Here we find no such 

 arrangements. In both cases the head is broad ; in 

 Prestwichia the hind legs have, indeed, some scattered 

 hairs, but not more so than its terrestrial allies ; nor in 

 Polynema do the wings appear in any way modified 

 to adapt them to their new function. In conclusion, 

 if Polynema natans and Prestwichia aquatica had 

 been extinct species no palaeontologist would have 

 suspected that they were aquatic ; in the present 

 state of our knowledge there is nothing in their 

 structure which would have suggested such an idea." l 



Both Polynema and Prestwichia are minute, about 

 a millimetre long. 



Ganin l has traced the development of Polynema 

 (Anaphes) natans or some closely allied species. 

 The eggs are laid and the early stages passed inside 

 the eggs of a common Dragon-fly, Calopteryx virgo. 

 The female Calopteryx lays her eggs in the cellular 

 tissue of the leaves of the white water-lily (Nymphaea), 

 and it is to discover and penetrate them that the 

 Polynema enters the water. As a rule one egg is 

 laid by the parasite in one egg of the Dragon-fly. If 

 more than one is laid in the same egg, only one 

 attains complete development. The egg of Polynema 

 is flask-shaped, having a short and slender neck at 

 that end where the tail of the larva will ultimately 

 appear. The young larva is motionless and of very 

 simple form. It is enclosed within a transparent 

 cuticle, which again is enclosed by the egg of the 



1 Lubbock, loc. cit. 



2 Zeits. /. wiss. ZooL, Bd. XIX. p. 417 (1869). 



