MuttkowsM—Tke Fauna of Lake Mendota. 405 
notes great plasticity. The attempt to select the aquatic rep¬ 
resentatives of the Diptera for inclusion in the present study 
proves this strikingly. Table 3 is a compilation intended to 
show the degree of ‘ ‘ aquativeness ” of the larvae—that is, 
the extent to which larvae have adapted themselves to an aquatic 
mode of life. This is evidenced most strongly in the respira¬ 
tory adaptations,—whether the larvae have remained surface- 
breathers (i. e., come to the surface to breathe the air above it), 
or whether they have become water-breathers (i. e., obtaining 
their oxygen directly from the water.) (See p. 472.) 
It is surprising to find that seven of the families are only in 
part aquatic; again, it is interesting to note that several fam¬ 
ilies have members under the extremes of terrestrial and water¬ 
breathing habits (Leptidae, Ceratopogoninae) ; or, that of a 
partial and a total aquativeness as in the Psychodidae, Coreth- 
ridae, and Anthomyiidae. 
On the one hand, this classification indicates that the transi¬ 
tion from a purely terrestrial habitat to an aquatic one is not 
very difficult; it also indicates that the final step from surface¬ 
breathing to complete water-breathing is, after all, not so very 
great. On the other hand, the presence of water-breathers 
alone in the lake complex demonstrates the dominating in- 
fiuence of respiration; while a further specialization of the 
aquatic habit is found in the limitation of some species to a 
constant current. 
In the present paper only two of the Dipterous families can 
be considered, the Corethridae and the Chironomidae. 
Family Corethridae. (Table 5, p. 475). 
A single species of this family occurs in the lake, Corethra 
punctipennis Say. The Corethridae are of interest in more 
ways than one. In the first place, the presence of semiaquatic and 
holaquatic larvae distinguishes the family. Secondly, the fact that 
Corethra is one of the few insect genera whose species are pelagic 
is of considerable interest. In addition the peculiar anatomy 
of the larva, its remarkable transparency, its physiology, and 
its importance as an item of fish food make it noteworthy. 
Corethra punctipennis occurs in the lake in enormous num¬ 
bers. Due to its pelagic habits, it is possible to give only an 
approximate estimate of its numbers. The figures given in the 
