LIFE HISTORY OF HYDEOUS (HYDROPHILUS) TRIANGULARIS. 



17 



outer margin of the base of each mandible is flush with the lateral margin of the 

 head, and the two mandibles shut down across the mouth like the blades of a pair 

 of scissors, the right mandible being ventral to the left. In consequence of the wide 

 interval between their bases the mandibles have no power of chewing, but can 

 only bite or cut. 



The maxillae {mx, figs. 8 and 9) are a trifle longer than the antennae and are 

 attached directly below them on the ventral surface of the head. Each is appar- 

 ently six-jointed, the basal joint very short and wide and the second joint twice 

 the length of the four terminal joints combined and about twice the diameter of 

 the third joint. The four terminal joints diminish regularly in length and diameter. 

 On the inner margin of the third joint near the distal end is a small papilla, tipped 

 with a single bristle. The second joint is covered with widely scattered hairs; the 

 other joints are naked (fig. 12). 



Matheson (1914, p. 342) said of this maxilla: ^'The cardo is greatly elongated, 

 the lacinia being reduced to a mere joint. The palpus is three-jointed." It seems 

 better to interpret the basal joint as the cardo, the second joint as the greatly 

 elongated stipes, and the third joint as 

 the palpifer. The three terminal joints 

 then become the maxillary palp, while 

 the subgalea, the galea, and the lacinia 

 are represented by the tiny knob on the 

 palpifer. This interpretation corre- 

 sponds exactly with that given by 

 Dimmock (1909, p. 13) for the larval 

 maxilla of a Carabid beetle. 



The labrum or upper lip is cut Fig. is.— Ventral (left) and dorsal views of labiiun. Z,ligula; Zp, 



squarely across between the bases of the ^^P' ^' ^' p^'p'^'^' submentum. 



mandibles and apparently renders but little actual service. The labium or lower 

 lip, on the contrary, is well developed and plays an important part in feeding. 

 The submentum is not visible in ventral view but is clearly seen from the dorsal 

 surface. The mentum is prolonged into a triangular lobe on either side of the 

 palpiger; the latter has a well-defined ligula and carries on either side a two-jointed 

 labial palp (fig. 13). 



THORAX. 



The thorax is made up of three segments that increase slightly in width back- 

 ward. The first one is longer than the others and has a well-defined transverse 

 groove at about its center; the other two have only lateral indentations and short 

 transverse wrinkles that do not form continuous grooves. The second and third 

 segments each carry a pair of spiracles, but there are none on the first segment. 



ABDOMEN. 



The abdomen is composed of eight segments, the first one a little wider than 

 the last thorax segment, the second wider still, and the remaining six decreasing 

 regularly in width, the terminal one being only one-fifth the width of the second. 

 At first the segments are smooth and free from wrinkles, but with advancing devel- 



