OBSERVATIONS ON LARWE OF CORETHRA PUNCTIPENNIS. 2J7 



meter at 8:00 P.M., while the samples obtained during the next 

 three hours yielded from 1,600 to 2,100 larvae. The latter num- 

 ber was also found at 3:00 A.M., bvit it rose to 2,665 at 4:00 

 A.M. and to slightly more than 3,000 per square meter at 5:00 

 A.M. Thus the mud contained from one half to two thirds as 

 many larvae at night as were found there in the daytime. 



For a certain period after they hatch out, the behavior of the 

 young larvae is very different in the daytime from that of the full- 

 grown individuals ; that is, the former occupy the lower water 

 during the daylight hours instead of the mud, being found in the 

 lower part of the mesolimnion and in the hypolimnion. The 

 young larvae migrate into the upper water at night just as the 

 full-grown ones do. It has not been definitely determined just 

 how long this difference in behavior lasts ; only rarely was an 

 individual found in the mud which was estimated to be only one 

 third as large as a full-grown larva and frequently individuals 

 were obtained from the water w r hich were recorded as half 

 grown. Thus, it appears that the young larvae inhabit the lower 

 water in the daytime instead of the mud until they are approxi- 

 mately one third grown, or perhaps a little larger. Muttkowski 

 states that the larval period lasts from six to seven weeks in the 

 summer broods; on this basis it may be estimated that the dif- 

 ference in behavior between the young and full-grown larvae con- 

 tinues for the first ten days or two weeks of the larval period. 



A series of catches w r as made with a plankton trap on August 

 2, 1917, for the purpose of ascertaining the vertical distribution 

 of the small larvae. The results are shown in Table II. No larvae 

 were found in the upper 8 meters, but they appeared at 10 meters 

 and at greater depths. The maximum number, 489 individuals 

 per cubic meter of water, was obtained at a depth of 18 meters, 

 which was about the middle of the hypolimnion. Somewhat 

 more than 88 per cent, of the total number of individuals occu- 

 pied the 15-20 meter stratum. 



Some results obtained on Devils Lake, Wisconsin, show that 

 the behavior of the full-grown larvae of Corcthra plumicornis 

 Fabricius 1 differs in the daytime from that of C. punctipennis in 



i Dr. ]. R. Malloch kindly identified this larva. 



