Muttkowski—The Fauna of Lake Mendota. 
447 
Yet the community of forms and adaptations of both habitats 
is sufficiently pertinent to indicate a probable common’ origin 
(Steinmann 1907). Both faunas consist largely of appressed 
dingers and some burrowers, all are well protected or sheltered 
forms (except Simulium, which is specialized in an entirely 
unique direction.) Both complexes do not admit of air- 
breathers, that is, as residents; for the air-breathers so fre¬ 
quently found in the slack water of rapids are not residents in 
the sense of permanence. They exist because the current does 
not reach them; and if it should they would be unable to main¬ 
tain themselves. No emergent plants or submerged sperma- 
tophytes will be found; Cladophora and some other filamentous 
algae are able to maintain an appressed growth where the cur¬ 
rent is not too strong. 
The current considerably modifies the shelter, and the food 
and breeding habits of the biota. The constant current pre¬ 
vents the biota from wandering about; as a rule an individual 
remains fixed for the greater part of its life. Strong fastholds 
in the way of strong claws, suckers, and other means are pos¬ 
sessed by most of the biota, while the few swimmers (Hydrach- 
nida, fan-tail darter) have especially elaborate swimming 
structures. Shelter must be sought under rocks, where the 
force of the current is somewhat broken. The eddies behind 
the rocks are favorable feeding grounds, since much of the 
flotsam passing over the rapids finds its way into the tiny eddies 
and whirlpools about the rocks. Some animals have elaborated 
their holding devices to trap this flotsam; such are the caddis- 
worms of the rapids, whose webs are generally supplied with 
sieves and nets for food-capture. Other animals of the rapids, 
such as Simulium and the may-flies, have elaborated their mouth- 
parts into ‘^nets” through which the water is strained for the 
micro-organisms which comprise their food. 
Most of the insects of the rapids at the time of their final 
ecdysis float to the surface, where the pupal skin opens at once 
and the adult flies upward, an adaptation insuring the safety of 
the species. In oviposition some species (Odonata) dip their 
abdomen into the water and let the current carry the eggs 
away; others (caddisflies) may lay their eggs at the water’s 
edge or enter the water and fasten the eggs to rocks. The lat¬ 
ter applies specially to stenophilic species. 
