440 Wisconsin Academy of Sciences, Arts, and Letters. 
This dormancy appears to be a definite seasonal instinct and 
persists even among the surface-breathers frequenting 
homoiothermal waters which never freeze. But curious to say, 
while the Gyrini, Dytiscidae, Tipulid larvae, and other surface- 
breathers occurring in these waters may be evident a little longer 
than their relatives of the frozen ponds, sooner or later they, 
too, burrow into the mud or silt of the homoiothermal waters and 
remain so for several months. 
For the shore animals winter brings a new problem which is 
as difficult as any offered by more temperate conditions. This 
is the problem of ice,—of the freezing of their habitat. Many 
forms such as some species of caddisworms, may-fiies, and 
leeches, appear to recognize the danger by migrating downward 
toward the breaker line or even below that point. This mi¬ 
gration apparently occurs between 4° and 1° C. But to my 
surprise I have found that many other species, including 
Psephenus lecontei, Stenelmis crenatus, Erpobdella punctata, 
Nephelopsis obscura, Heptagenia, Sparganophilus eiseni, and 
others, make no attempt at all to escape downward. I have 
found them close to the shore margin, with less than 2 cm. of 
water between the ice (then 15 cm. thick) and the bottom; later 
on I have found them packed for days and weeks in mush ice 
which had been forced under the ice-sheet proper by heavy 
winds. The mush ice in the shore area would be practically 
continuous with the surface ice. 
It can therefore be supposed that they die by freezing, or that 
^ they can resist the influence of cold, or that their survival is 
purely accidental. One thing is certain,—and that is, when 
spring comes the identical shore will have its normal biota, and 
apparently without numerical impairment. Whether this biota 
is a ^ ‘ survival, ’ ’ or newly recruited, I have been unable to ascer¬ 
tain with certainty, although I believe that the former is the 
case. There is always the possibility that the ground tempera¬ 
ture is sufficient to keep a thin sheet of water between the ice 
and the bottom proper, in which case a ‘‘zone of safety’’ would 
be established; this would explain the survival of the shore 
dwellers, and some condition like this “zone” seems to be in¬ 
dicated by various examinations made in the winter of 1914-15. 
Before the ice goes out in the spring, animals become active 
again. Ground radiation raises the temperature of the water 
during late winter, so that it approximates 4° C. at the bottom. 
