462 ANIMAL RESPIRATION 



which are shown by their structure to be half-way between hind- 

 gilled snails and lung -snails. In one member of the family 

 {Siphonaria) the gill-cavity is partly converted into a lung and 

 the gill is reduced, while in an allied form {Gadinia) the gill has 

 gone altogether. 



AMPHIBIOUS INSECTS (Insecta) 



Certain cases have already been described of insects which 

 live in water either when adult or during early stages of exist- 

 ence (see p. 439). But all the aquatic forms of the kind so far 

 dealt with are strictly air-breathers, either carrying about a supply 

 with them under water, or coming up to the surface from time to 

 time for the purposes of respiration. There are, however, insects 

 which are far more thorough-going water animals than this, being 

 able to breathe the air dissolved in water during the early part of 

 their lives. And since such forms are typical air-breathers when 

 adult, they are just as much entitled to be called amphibious as 

 frogs and newts. There is, nevertheless, a very great difference 

 between the ancestral history of amphibious vertebrates and am- 

 phibious insects. The former are terrestrial forms emerging, as 

 it were, from the aquatic mode of life, and dependent upon a 

 damp atmosphere even when adult. Amphibious insects, on the 

 other hand, are members of a group which is thoroughly terres- 

 trial, and, like birds, has even conquered the realms of air. There 

 can be no doubt that in the very remote past insects sprang from 

 aquatic ancestors, but there is no reason to suppose that these 

 were in the least like the water-inhabitingr stashes of such members 

 of the class as are amphibious. It is, in fact, a case of the re- 

 acquirement of aquatic habits in the history of a group, i.e. it is 

 a secondary phenomenon, while the amphibiousness of frogs and 

 the like is a primary phenomenon, due to the fact that they have 

 been specialized from fish-like creatures, which are recapitulated, 

 as it were, by the tadpole stage. 



Amphibious insects chiefly, if not entirely, belong to one or 

 other of two orders, Net-winged Insects i^N europterd) and Two- 

 winged Insects {Diptera), which it will be most convenient to 

 consider separately. 



