OBSERVATIONS ON LARV.E OF CORETHRA PUNCTIPENNIS. 28 1 



that at 20 meters gave nearly ten times as many as the latter. On 

 September 24, the differences were fiftyfold and more than a 

 hundredfold, respectively, and on October 24 the number was 

 nearly two hundred times as large at 20 meters as at 12 meters. 



Just how these larvae are able to constantly maintain such a 

 marked difference in numbers in favor of the deep water is a 

 puzzling question. Their method of locomotion would not lead 

 one to expect them to travel very far of their own accord should 

 they reach the shallow areas, yet it seems probable that many of 

 them are carried into the shallow water by the currents when 

 they migrate into the upper strata at night. This would be true 

 especially on windy nights. Table III. shows that a very small 

 number of larvae is found at 10 meters as compared with the 

 deep water and 39 per cent, of the area of the lake lies outside the 

 10 meter contour line. The number is usually not much larger 

 at 12 meters than at 10 meters and the former divides the area of 

 the lake approximately into halves. The outer or shallower half 

 of the lake, then, is very sparsely populated by these larvae, but 

 it is not clear just how the number is kept so small in comparison 

 with the inner or deeper half of the lake. 



In a large proportion of the former area the bottom does not 

 consist of material in which the larvae can readily conceal them- 

 selves in the daytime, being composed of sand, gravel, and rock, 

 so that the tendency would be to avoid these areas. On the other 

 hand, the larvae are no more abundant in the shallow portions of 

 protected bays where a muddy bottom suitable for concealment is 

 found at a depth of only 5 meters or 6 meters. The difference 

 can scarcely be attributed to a proportionally unequal distribution 

 of eggs between the two regions because very large numbers of 

 egg bearing females are found over the shallow water as well as 

 along the shore ; it seems probable, therefore, that enormous num- 

 bers of eggs are deposited in the shallow areas. 



When the larvae migrate into the upper strata of the lake at 

 night, the direct currents tend to carry them into the shallow 

 water, but the return currents, on the other hand, will aid more or 

 less in bringing them back to the deep water. Apparently the 

 chief factors governing the distribution of the Corcthni larvae 



