426 



segmented appendage being the palpus, the broader one, the fused 

 galea and lacinia. The palpus has a number of large setae scattered 

 over the surface. The galea-lacinia is more or less compressed, and 

 the distal margin has about six irregularly placed hooks arranged in 

 two rows. A marginal fringe of heavy setae extends proximad from 

 the hooks. In Hetaerina, if the galea-lacinia be placed on edge, there 

 will be seen a strong indentation between the two rows of hooks, an 

 indication of the fused condition of the piece. A study of Plecoptera 

 (Fig. 31) and Ephemerida (Morgan, '13) offers convincing reasons 

 for the interpretation of this piece as galea and lacinia fused, as com- 

 pared with a belief in the reduction of the galea, or in the fusion of 

 this with the palpus instead of the lacinia, or in the reduction of the 

 palpus. All degrees of fusion, from complete separation (Fig. 31) to 

 complete fusion and disappearance of the suture, may be had in series 

 selected from these two orders. 



Mandibles (Fig. 30, md). — All of each mandible is hidden beneath 

 the labrum and labium except the lateral surface. The teeth are strong 

 and heavily chitinized and the distal margins are divided into two pro- 

 jections, the cephalic one bearing a number of teeth, the caudal one 

 with a number of teeth and cutting edges arranged in the shape of a Z. 



Hypopharynx (Fig. 30, hp). — The visible portion of the hypo- 

 pharynx appears as a semicircular part between the tips of the max- 

 illae. It is much more heavily chitinized than that of the nymph, and 

 usually has a number of long setae attached to each lateral surface. 



Pro pharynx. — The propharynx is closely applied to the interior of 

 the labrum and clypeus and presents no features of interest. 



Microthorax (Figs. 27, 29, 36, 39). — The microthorax comprises 

 the neck sclerites, and is much reduced in the Zygoptera. The dorsal 

 and ventral sclerites (notum and sternum) and the episterna are want- 

 ing. The only portions remaining are the conspicuous lateral plates, 

 the epimera (min). In many species the epimera are much widened 

 on the caudal third, and this portion is almost completely divided by 

 a deep cephalic indentation. The indentation separates from the main 

 part of the epimera a bell-shaped dorsal part which serves as a buffer 

 for the head and is to some extent freely movable. The ventral part 

 is slightly larger than the dorsal buffer, but is thrown into folds, and 

 the cephalic part of the ventral piece is drawn out into a long tapering 

 point. The tips of the epimera are fastened together by ligaments and 

 the head rotates upon the apices of the two together, which rest against 

 the body of the tentorium. 



Thorax. — The thorax comprises the three body-segments caudad 

 of the microthorax. The first conspicuous ring is the prothorax. The 



