LIFE HISTORY OF HYDEOUS ( 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 

 line 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 setae 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 haK 

 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 



