1904] McCLENAHAN-.—RHYNCOPHOROUS COLEOPTERA. 93 



III. Imago. 



The most striking peculiarity of the adult beetle, a characteristic of all Rhyn- 

 cophorous Coleoptera, is a prolongation of the anterior part of the head which 

 forms the rostrum. It is with this part that we are chiefly concerned. An under- 

 standing of its development will be best had if its structure in the adult beetle is 

 first clearly understood and compared with that of the larva, just described. 



The rostrum of Rhyncophora is unique in that it is not made up of elongated 

 mouth parts* as in Hymenoptera, Hemiptera, Diptera and Lepidoptera ; not even 

 in part, as in scorpion files (Mecoptera) ; but it is the greatly elongated fore part 

 of the head, the part between the eyes and the mouth ; and it bears the mouth 

 parts, which are similar in shape to those of other Coleoptera, at its apex. It 

 varies from 3^ to 7 mm. in length in Balaniniis nasicus, being generally longer than 

 the entire body of the weevil. It is gently decurved and has a deep groove 

 on each side for the reception of the reflexed scape of the antenna. In it the 

 usual sutures between the head sclerites are entirely obliterated by fusion and the 

 scanty external evidence as to its composition is derived from the position of eyes, 

 antennae and the mouth parts. This alone indicates that it is composed of fused 

 frons, genae and gula of the normal insect head — a view which ontogeny substan- 

 tiates, as we shall see under a subsequent heading. 



Mouth Parts. 



In these two weevils at least the mouth parts, generally speaking, show rather 

 close correspondence with those of the larvae. They are more elongate and have 

 sharper cutting edges. 



Each mandible bears also at its base a singular process as long as (Plate IX, 

 fig. 18) or longer than (Plate IX, fig. 15) the body of the mandible. This process 

 projects j30steriorly down the pharynx to the entrance of the esophagus. It is 

 thickly beset with posteriorly directed prickles similar to those of the surrounding 

 pharyngeal wall. 



This process doubtless serves for the propulsion of the food down the narrow 

 passage-way of the anterior end of the alimentary canal. With the swinging of the 

 mandibles upon their hinges the two pharyngeal processes appended to their inner 

 basal angles, lying side by side in the pharynx are drawn forward and pushed back- 

 ward. Such is the direction of the prickles that the backward movement carries the 

 food along. 



After discovering these pharyngeal processes, I searched for some account of 



