562 Wisconsin Academy of Sciences , Arts, and Letters. 
from the upper stratum of water on the approach of day in 
order to avoid the intense light. The weight of evidence in 
these observations favors this explanation. The results show 
pretty conclusively that light, is the primary factor in causing 
the descent of the Crustacea in the morning and in keeping them 
from the surface during the day. It is more difficult to state, 
however, just why they move upward so promptly when the in¬ 
tensity of the light is sufficiently decreased. 
The importance of light is shown by the fact that the time at 
which the upward movement takes place in the evening and the 
time of the. downward movement ini the morning are re¬ 
lated more or less closely to the time of sunset and of 
sunrise. Some Crustacea also were found nearer the sur¬ 
face on cloudy than on clear days. In some instances 
forms which were absent, from the upper stratum of water 
on clear days were found there on cloudy days in nearly 
as great abundance as at night. This was also found to 
be true of the Daphnias in Winona lake (8b). In this connec¬ 
tion, Birge (2b, p. 426) describes a set of observations on Lake 
Mendota made .on a, day when there was, an alternation of cloud 
and sun. During the sunny periods, the Daphnias were at a 
depth, of one meter and during the cloudy periods one-half 
meter. ITe says, “It was as though the Daphnias were de¬ 
pressed by a force against which they were contending, and 
they rose with the promptness of a, compressed spring relieved 
of weight.” Burckhardt (4) also found that the poor zone of 
Daphnia hyalina increased directly proportional to the strength 
of the daylight. In my results,, the limnetic Crustacea fall into 
two groups with respect to the effect of sun and cloud on their 
vertical distribution in the daytime in the upper stratum. 
Daphnia hyalina and D. retrocurva, were nearer the surface on 
cloudy than on clear days. Also the distribution of Diaptomus 
and Cyclops in the upper stratum was affected by the obscuring 
of the sun by clouds, for they frequently occupied this region 
in nearly as great, numbers on cloudy days as at night. The 
day position of Daphnia pulicaria, Leptodora, and Epischura, 
however, was the same on cloudy as on clear days. This seems 
to show that light is a more important factor in producing diur¬ 
nal movements in the former group than in the latter, or per¬ 
haps it is better to say that the influence of light is modified by 
other factors not so much in the former as in the latter group. 
