134 
HALF HOURS WITH INSECTS. [Packard. 
very few insects live in salt water, and it is easy to see that 
a maritime life is not a normal one to them. So in a less 
degree with the fresh water forms. The aquatic insects are 
representatives of scattered families, and though all are in 
various waj^s modified by their aquatic surroundings, yet so 
much do they differ in their modes of development and 
structure among themselves, that it is easy to see that they 
belong mainly to terrestrial types which have adopted an 
aquatic life after the type to which they had belonged had 
become fixedly terrestrial. For example, many beetles 
which are aquatic are allied to the carnivorous ground 
beetles. The Dytiscus and its allies are essentially aquatic 
Carabidse, the family comprising the ground beetles, The 
larvae of these water beetles have the same kind of feelers 
and mouth-parts as the land Carabids ; the structure of the 
adult beetle is on the Carabid type, the body being, how¬ 
ever, more ovate and modified for swimming. Both types 
(Figs. 94 and 95) may have been derived from ancestors 
of terrestrial habits. As proof of this we have the Cali¬ 
fornian Amphizoa (Fig. 96), which is said by Dr. Horn to 
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