416 



most cases indistinct. The latter, however, seems to be represented in 

 species having the deeper median clefts. The palpi are present in the 

 so-called "lateral arms", the distal segments being represented by the 

 movable hooks (Figs. 8, 9; lp 2 ). The lateral arms also bear a 

 number of raptorial setae in some species (Fig. 10). In capturing 

 prey, which consists of mosquito and other soft aquatic larvae, the 

 nymph swings out the hinged labium, opening and closing the lateral 

 arms like a pair of jaws. The victim is then drawn back toward the 

 mouth and the heavy maxillae and mandibles finish the work. 



Labial Muscles. — The muscles operating the labium have been 

 studied by few, and studies that have been made seem incomplete. It 

 has therefore seemed advisable to examine them in detail. The struc- 

 tures have been determined by means of cross and longitudinal sections 

 and verified as far as possible by gross dissections. 



The median lobe (Figs. 1, 2; ml) contains four large muscles for 

 operating the lateral arms. These are attached proximad directly to 

 the dorsal wall of the submentum, dorsad of the hinge (Fig. 4). At 

 the point of attachment to the labial palpi, the muscles are usually 

 modified to form tendons. With the submentum there are also two 

 pairs of muscles, which, though not as large as those of the median 

 lobe, play an even more important part in the operation of the piece. 

 The points of insertion of the menta^ muscles are of especial interest 

 and give clues to the actual function of each muscle. The dorsal 

 inner pair (Figs. 2, 4) is attached proximad to the tentorium and distad 

 just above the hinge which is between the median lobe and the sub- 

 mentum. The remaining, or ventral, pair is attached to the chitinous 

 ligament or rod described below — a structure present only in the 

 Odonata. The rod (Figs. 3, 4, 7; cr) is unpaired and is attached to 

 the ventral wall of the head just caudad of the hypopharynx. It 

 extends obliquely caudad and dorsad in the plane of the meson and is 

 attached again to the dorsal wall of the submentum, at which point it 

 expands in such a way as to form the top of a T. The ventral pair of 

 muscles are inserted on the base of the chitinous rod (Fig. 4). From 

 this point they extend over the mento-submental hinge and attach 

 themselves to the ventral wall of the median lobe. A third pair of 

 muscles, present at the base of the submentum, is attached to the ventral 

 wall of the head caudad of the hypopharynx, extends caudad and 

 dorsad, and is inserted on or near the tentorium. 



The exact function of each of the muscles contained in the sub- 

 mentum is difficult to explain, and it is probable that no single pair 

 can be said to produce a given result with the exception of those operat- 

 ing the labial palpi. Thus it can be seen that the oblique muscles at 



