292 



ARTHUR D. WHEDON. 



ductors and it is possible that these two masses represent them at 

 this stage. No histological examination has as yet been made. 



The tracheae which are to remain in the adult are seen wound 

 vertically up and down between the main muscles. The T-shaped 

 chitinous support mentioned in connection with the larval labium 

 is very conspicuous, and bears a large flattened tooth-like ventral 

 process. 



THE LABIUM OF THE ADULT. 



General Structure. 



The labium of the adult is much shorter, thinner, and more ex- 

 panded than that of the larva. When flattened out it measures 

 approximately five millimeters in length by eight millimeters in 

 diameter, including the lateral lobes. That is, it is less than a 

 third as long as, and only a third wider than, the larval structure 

 from which it arose. The added width results from the expan- 

 sion of the lateral lobes. The latter with the greatly enlarged 

 median lobe, form the mask which covers the mouthparts. In 

 their normal positions the lateral lobes and the median lobe rise 

 almost vertically from the borders of the floor-like mentum-sub- 

 mentum to enclose the sides and front of the mouth; the whole 

 structure is nearly cup-like. 



The function of the labium is changed to a considerable extent. 

 In the larva it is a remarkable organ of prehension, in the imago it 

 has become mainly one for the retention of prey caught in the 

 basket of the spiny legs. Its moveable hooks now resemble palps 

 possessed of a single small terminal tooth or spine each. While 

 formidable in size and appearance the end hooks are weakly j oined 

 to the lobes (Fig. 4). They would be of little use without the 

 pressure and support applied to their outer faces by the flexing 

 of the median lobe (Fig. 7), converting them into a sort of fork 

 to hold the food in the mouth. 



The mentum is relatively short and very wide due to the forma- 

 tion of a pair of squames laterally by the development of longi- 

 tudinal folds and chitinous ridges along the areas of muscle at- 

 tachment. The hinge between it and the submentum is less 

 flexible than in the larva, but possesses a distinct enlargement along 

 its median half. Most of the muscle attachments are grouped on 



