Muttkowski—The Fauna of Lake Mendota. 
413 
K. Order Coleoptera. (Table 5, p. 475). 
Adaptation to an aquatic mode of life has reached its great¬ 
est differentiation among the aquatic Coleoptera. Table 4 aims 
to show the progressive adaptations for the families listed. It 
is difficult to class the Coleoptera into surface-breathers and 
water-breathers. For on the one hand some of the adult Col¬ 
eoptera are able to effect a gaseous interchange while sub¬ 
merged ; on the other hand some of the water-breathing larvae 
have stigmata which they are able to use at the surface, al¬ 
though in the latter case primarily in the prepupal stage. In 
effect, therefore, the adults mentioned (some Dytiscidae, etc.) 
are true water-breathers, yet in a manner different from that 
of the larvae. (See p. 473). 
Again, many regular surface-breathers are intermittently 
submerged, for a long period during the winter, and are able 
to effect a partial oxygenation during their submersion 
(Wesenberg-Lund, Bge, etc.). This complicates the situation 
and allowance must be made in a table. The adaptations to 
the aquatic environment are so manifold among both larvae 
and adults, that it seems as if each species has developed indi¬ 
vidual means and methods of aquatic respiration. Among the 
list of families noted as breathing ‘‘submerged air” there is 
hardly one which follows precisely the same method as any of 
the others. 
Now, with all their manifold adaptations to an aquatic life, 
aquatic beetles, except for a few species, are scarce in the lake 
complex. Surface-breathing larvae are entirely absent from 
the lake; water-breathing larvae, while common to a certain 
extent, are limited through their adults, which usually are sur¬ 
face-breathers. Beetles, in one or all stages, are surface- 
breathers; and it is through the surface-breathing habit that 
the molar agents exert their dominating influence on the lake 
complex. ! 
The Hygrobiidae, Dysticidae, Hydrophilidae (except Bero- 
sus), Donaciinae, and Gyrinidae, the latter despite the water¬ 
breathing larvae, are practically absent form the lake commu¬ 
nity. The typical lake Coleoptera are the Haliplidae in the 
vegetation zone, and the Dryopidae in the rocky and gravelly 
areas, while several species of adult Dytiscidae are locally 
abundant. 
