LIFE HISTORY OF HYDROUS ( 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 ex acijigg ^With that given by 
■?^.i v vl) imnmck3|(|(p^ p .< 13)' for the larval 
maxilla of a Carabid beetle. 
The labrum or upper lip is cut Fig. 13.— Ventral (left) and dorsal views of labium. Z,ligula; Ip, 
squarely across between the bases of the labial palp: m > mentum; p ’ palpiger; si ’ su “ um ' 
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- 
