LIFE HISTORY OF HYDROUS ( HYDROPHILUS) TRIANGULARIS. 
13 
Laker’s suggestion that the eggs did not hatch in two of the cocoons because 
he cut off the spike may or may not be true, but evidently the immediate cause 
was the sinking of the cases, or “ cocoons” as he calls them. The egg case must 
float at the surface of the water if the eggs are to develop properly. One of those 
obtained by the present author had sunk to the bottom of the pond, and although 
the spike and every other part was normal and intact, yet the eggs were unde- 
veloped. 
It seems reasonable to conclude, therefore, that the spike or mast, if it has any 
function, is concerned with floating the egg case and with keeping it right side up 
in the water, since both of these conditions are essential to the successful hatching 
of the eggs. 
The number of eggs in a case varies considerably, but may be given as usually 
between 100 and 130. Garman (1881) found 107 eggs in the single case he exam- 
ined, Matheson (1914) counted 112 and 130 in two cases, and the present author 
obtained 121 in one case and 117 in another. The larger end of the egg is next 
the roof of the case, the lower end being somewhat narrowed and also in the outer 
row of eggs considerably tapered on the outer side, as is admirably shown in the 
figures given by Miger and Lancret (1809). 
When the eggs hatch, the larvae issue from the lower ends and escape into the 
chamber beneath the egg mass. Here they crawl about for several (about 12) 
hours before venturing out into the open water. 
THE LARVA. 
On leaving the egg case the young larva is about 9 mm. long, measured from 
the tips of the mandibles to the tips of the posterior cerci. At first its color is an 
almost uniform light yellowish brown, but the dorsal surface quickly becomes 
darker with the exception of two irregular lines, one on either side of the median 
fine and quite close to it, which extend from the anterior margin of the thorax to 
the tip of the abdomen. The outer sides of the light dorsal lines are accentuated 
with black on the thorax and darker brown on the first abdominal segments. On 
entering the pupal chamber these color distinctions are practically obliterated 
and the entire surface, dorsal and ventral, becomes a uniform brownish black. 
The skin is densely clothed with short hairs, giving it a velvety appearance, 
and scattered among them are much longer hairs or setse irregularly arranged. 
Along the lateral margins where the dorsal and ventral surfaces meet is a double 
ridge or fold of the skin, which stands out quite prominently. On the dorsal half 
of this ridge is a row of small papillae, one and sometimes two on each segment. 
There are two other rows of similar papillae, one on the dorsal and one on the ventral 
surface, close to the ridge. Each papilla carries a tuft of long setae, varying in 
number from three or four to seven or eight. This double ridge remains light 
yellowish brown like the ventral surface until the final change when the larva is 
ready to pupate. Along the dorsal margin of this ridge on either side are the 
spiracles, but they are closed in the larva and take no part in respiration. 
At the posterior end of the abdomen, opening dorsally upward and backward, 
is a deep transverse groove, from the bottom of which open the large longitudinal 
