THE LIFE-HISTORY OF HYDROBIVS FUSCIPES, L. 319 



of the type characteristic of the group, serving the double purpose of tearing and 

 grinding. Seen from above, both jaws show a two-pointed apex, the outer point of the 

 left jaw being slightly thickened and having also a double point slightly marked. On 

 the inner edge of the left jaw just below the apex is a thick tuft of long bristles, and 

 immediately below this tuft is a second series of bristles of which those nearer the apex 

 are longer than those nearer the base. The uppermost bristle of this second series has 

 a thickened base. 



The inner edge of the right jaw shows only a very small tuft of a few bristles in 

 place of the upper tuft on the left jaw, but the second series is similar in both jaws. 

 Seen from above, the base of each jaw ends in a triangular projection of which the 

 surface is smooth, as is also the margin of that projection in the right jaw, but in the 

 left the margin is " feathered " by a series of short hairs. Seen from below, the jaws do 

 not show much more than when seen from above. They are, however, three-sided, possess- 

 ing not only a dorsal and ventral but also an inner face, broad at the base and narrow- 

 ing to a point at the apex ; and the inner face of the one jaw is, of course, opposed to 

 the inner face of the other. The basal part of this inner face of the jaw is the grinding 

 region, the food being brought here by the action of the maxillae and the points of the jaws 

 and then ground down between the bases of the two jaws before being finally swallowed. 

 And this grinding apparatus is very efficient, as the two halves of it differ from one 

 another in accordance with different functions. The grinding region of the left jaw is 

 somewhat concave, and has a series of strong ridges running down it parallel to the 

 length of the mandible. These ridges do not cover the whole of the concavity, but taper 

 off towards the dorsal side of it and give place to what appear to be a number of minute 

 tubercles. The grinding region is fringed all round with short hairs; and it is these 

 which are partly visible in the dorsal view of this mandible. 



On the right jaw the corresponding region is somewhat convex, obviously to fit the 

 concavity on the left jaw. There are no fringing hairs or deep grooves or ridges, but 

 the whole surface is sculptured like a fine file. 



In a general way this description would probably apply to the grinding apparatus 

 of all the Hydrophilidae, but there are probably slight variations in the different genera. 

 For instance, in Hydrophilus piceus, L. , most of the concavity of the left mandible is 

 file-like in sculpture, the strong ridges being confined to a small portion of the upper 

 border of the concavity. 



The maxillae (fig. 3) are brush-like organs. The galea of each is crowned by five or 

 six rows of stiff hairs incurved towards the mouth. These rows are placed one behind 

 the other and no doubt make a very effective brush. The lacinia also bears a strong 

 tuft of hairs, and there are also stiff hairs on the free edge of the stipes along the upper 

 third of its length. The rest of the free edge of this part gives off a very fine trans- 

 parent membrane as a narrow strip along it. The palpi are long and composed of four 

 simple segments, the basal one being very short. 



The labium (fig. 4) has the galeae and lacineae feathered along their free edges with 



