448 Wisconsin Academy of Sciences, Arts, and Letters. 
More than any other fauna that of the rapids is in danger of 
extinction during the winter period. Frequently the rapids 
may freeze solidly, and with the friction of slowly moving 
rocks and ice pressure all animate life is destroyed. As a mat¬ 
ter of fact, the insects do not hibernate in the larval stage in 
the rapids; long before the rapids freeze up, the fauna has be¬ 
come reduced, and the larvae present are comparatively few. 
Springs .—The outlets of springs are frequently miniature 
rapids and may approximate the conditions of rapids. But 
they are nearly always characterized by an abundant growth of 
water cress, among which one is certain to find air-breathers 
such as Hemiptera and Tipulid larvae. This is a striking 
example of the dependence of surface-breathing species on plant 
growth for their maintenance. 
The most important character of springs is the temperature, 
which as a rule is constant throughout the year. The numer¬ 
ous springs about the Madison lakes are all homoiothermal, with 
a temperature of 8-10° C. the year round. The outlets con¬ 
tain an abundance of water-cress, in which caddis-worms, snails, 
Crustacea, and planarians are exceedingly plentiful. Some of 
the typical representatives are: the caddis-worms Neophylax 
autumnus, Platyphylax designatus (more truly a rapids form), 
Limnephilus rhombicus, the latter with both vernal and autum¬ 
nal cases at the same time, an interesting persistence and over¬ 
lapping of a seasonal instinct. Among planarians Dendro- 
coelium lacteum and Polycelis nigra are especially common, 
while Planaria doritocephala and P. agilis are quite frequent, 
all four species occurring also in the lake; Physa gyrina and P. 
heterostropha among Gastropoda; and Gammarus limnaeus 
among Crustacea. 
It is difficult to decide which factor controls the fonticolar 
association, whether it is current, food supply, temperature, or 
chemical composition of the water. This much is certain: the 
springs about the lakes vary as to their biota. Thus for in¬ 
stance, Merrill Spring outlet contains an abundance of cad¬ 
dis-worms and Planarians, while Merrill Creek close by, which 
is fed by springs, contains not a single Trichopteron or Plana- 
rian. The conditions appear to be similar to Merrill Spring, 
but differ in the chemical makeup of the water, due to addi¬ 
tional drainage water and the composition of its basin. In 
places the Merrill Creek rapids exceed those of the spring’s 
