40 Bulletin American Museum of Natural History [Vol. XLII 



Hydrous triangularis Say 



Plate I, Figure 7; Plate VIII 



It hardly seems necessary to go into the biology of this species. 

 However, the main events occur as follows. The egg-cases are found 

 mostly in June, and, although always attached to floating leaves or 

 other debris, have never been reported as fastened to living plants. 

 Over one hundred eggs are enclosed within it and the larvae when newly 

 hatched are very ungainly as compared with the full-grown larva. The 

 usual two molts take place and the pupa appears in about one month. 

 The duration of this latter stage is about eleven days. 



Egg-case. — Light brownish with its mast and plate at base of mast almost 

 black. It measures 24 mm. long, 15 mm. high, and about 22 mm. wide. The horn-like 

 process, which arises from the cap end of the case, is almost at right angles with the 

 upper side of the case and is usually 7-8 mm. long. Below the plate is a lunar-shaped 

 opening which leads into a chamber below the eggs. 



Newly Hatched Larva. — Length, 8 mm.; width at the thorax, 1.7 mm. 

 Light brownish with appendages whitish darkening with age. Integument with fine 

 dark hairs. 



Head, without mouth-parts, 1.14 mm. long dorsally and 1.82 mm. long ventrally; 

 2.1 mm. in width; broadly ovate, large, depressed, strongly elevated; fronto-clypeal 

 suture well marked; frontal sutures widely separated, converging only slightly and 

 not uniting to form an epicranial suture; gula fairly prominent, arched, semicircular 

 behind; gular sutures confluent and distinct. 



Labro-clypeus reduced, with its anterior margin slightly concave and bearing a 

 few microscopic projections towards each side. Lateral expansions of the epistoma 

 similar, acute, not prominent. 



Ocular areas in groups of six, elongate and arranged in two parallel rows, the 

 first three nearly vertically while the posterior three are horizontally placed. The 

 sixth or outer one of the posterior row distant from the fifth. 



Antennae three-segmented, exceeding the right mandible by almost the length 

 of the terminal segment, and left by its last and hah of its penultimate segment; 

 first segment longer than the second and third together, almost as long as the stipes, 

 bent inwardly near the base and fringed on the inside, except the basal third, with 

 slender seta?; second segment swollen distally and not quite as long as the third, 

 which bears no distinct terminal sense-cones. 



Mandibles asymmetrical, prominent, sharply pointed at their tips and with 

 their inner surfaces grooved; each mandible with a single inner tooth, the inner tooth 

 of the light mandible with a large tooth on its anterior margin forming an unequally 

 bifid tooth; the left mandible stouter and shorter than the right and with a membra- 

 nous area in the region of the molar surface. 



Maxillae with joint-like palpifer; stipes slender, slightly bowed, longer than the 

 palpifer and palpus together and its inner surface with a row of four stout setae; 

 palpifer with a chitinous finger-like appendage at its interno-distal angle; about the 

 same length as each of the palpal segments but slightly wider; terminal segment of 

 palpus with no distinct sense-cones. Articulating maxillary piece well developed. 



