OBSERVATIONS ON LARVJE OF CORETHRA PUNCTIPENNIS. 2J9 



ment indicate that the intensity of the illumination at a depth of 

 23 meters on a clear day, between n : oo A.M. and I : oo P.M., is 

 about equal to that produced by full moonlight at the 'surf ace of 

 the lake. During the early forenoon and the late afternoon, as 

 well as on cloudy days, the illumination is much smaller than this. 

 For some time before sunset, the bottom stratum must be sub- 

 stantially in total darkness, yet the observations show that the 

 emergence of the larvae from the mud is very closely correlated 

 in time with the setting of the sun. 



Not only does the illumination in the bottom water become 

 very small in the late afternoon, but there is a further protection 

 from light afforded by the bottom ooze in which the larvae remain 

 concealed during the day. To what depth the larvse penetrate 

 the loose mud is not known, but in the laboratory they readily 

 burrow down to a depth of a centimeter or more. The dim light 

 which reaches the bottom in the deeper portions of the lake can 

 penetrate the ooze only to a very slight extent at most, even dur- 

 ing the brightest part of the day, and this raises the very interest- 

 ing question as to what stimulus causes the larvae and pupae to 

 emerge from the mud so promptly and regularly about the time 

 of sunset. No definite data bearing on this point have yet been 

 obtained. 



These larvae are eaten with avidity by many fishes and their 

 habit of occupying the mud in the daytime may thus serve a very 

 important purpose from the standpoint of protection from such 

 enemies. A further protection is afforded by the disappearance 

 of the dissolved oxygen in the hypolimnion. Usually by the first 

 of August very little free oxygen remains in this stratum, which 

 makes the lower water unfit for the permanent occupation of the 

 larger forms which prey upon these larvae. In spite of the lack 

 of oxygen, however, P'earse and Achtenberg found that the yel- 

 low perch Perca flavesccns (Mitchill) invades the lower 

 water and feeds upon these larvae. While these fish survive for 

 a period of two hours in water that contains no dissolved oxygen, 

 these authors state that it is doubtful whether a perch is able to 

 feed for more than a few minutes at a time under such conditions. 

 It seems probable, therefore, that the Corethra larvae are not 

 eaten as freely as they might be if anaerobic conditions did not 



