446 Wisconsin Academy of Sciences^ Arts, and Letters. 
The most general methods of concealment adopted have un¬ 
doubtedly been subterranean and endophytous shelter,—chiefly 
among the holometabola; endophytous life is in certain respects 
(plant-borers and miners)' a variant of subterranean life. Both 
modes are successful in that they offer shelter from extremes of 
climate and protection from enemies, less successful in that they 
largely restrict the self-feeding larvae to a limited supply of 
plant food. 
The ease of burrowing in soft or moist ground led animals to 
a wet substratum, and finally to a purely aquatic environment. 
We find within several orders, even within families, practi¬ 
cally all of these transitions (see tables 3 and 4, Diptera and 
Coleoptera). Other species may have adopted an aquatic life 
more directly. 
The advantage of aquatic life is that this environment offers 
a greater uniformity of conditions and a plentiful food supply. 
Conditions are not extreme, changes are slow, and the animal is 
easily able to adjust itself to these without any special 
morphological adaptations. But the aquatic environment 
necessitated certain physiological adjustments, especially along 
respiratory lines, and it is by the perfection of these adapta¬ 
tions that species indicate the degree of aquativeness (i. e., ad¬ 
justment to aquatic life) attained by them. This aquativeness 
has been developed to its highest point in lake life and in rapids, 
where molar agents have forced a complete separation from 
aerial respiration and a resultant total aquativeness. 
Rapids .—When comparing other hydrobiota with the lake, we 
find a rather marked resemblance between the rapids and cer¬ 
tain exposed portions of the lake,—i. e., the shore and the rachion 
(Wesenberg-Lund 1908, Steinmann 1907). The same may-flies, 
leeches, beetle-larvae, and caddis-worms are found in the rapids. 
On the other hand, certain stenophilic species such as Simulium, 
Chironomus tenellus, Baetis pygmaea, Hydropsyche alternans, 
and others, are conspicuously absent from the lake shore. For 
while the latter offers current at intervals, these stenophils de¬ 
mand a persistent current, hence are homoiophilous. Experi¬ 
mentally, I have placed Simulium and Hydropsyche alternans 
larvae in a jar where the current would sweep around the jar, 
but not into it; most of the larvae died within two or three hours, 
despite thorough oxygenation and repeated change of water. 
