196 PHILIP p. CALVERT. 



distal extremity of this second piece bear at either angle, a lateral 

 joint (II), the lateral labial lobe of the imago. On the outer side 

 of the lateral lobe is one or more articulated spines, of which the 

 apical one (tp) is probably the free terminal joint of the labial palp, 

 while on the inner side are teeth or spines (figs. 8, 9, 10). The sub- 

 mentura is directed backwards, and, in rest, lies in contact with the 

 ventral thoracic surface, between the bases of the legs. The articu- 

 lation of the subnientum with the second piece is hinge-like, the latter 

 being directed forwards. The planes of the lateral lobes are inclined 

 to that of the second piece. The labium can thus be protruded to a 

 considerable distance in front of the head, while the movement of 

 the lateral lobes is from side to side, their teeth and spines completing 

 the already formidable armament of the mask. The feet are very 

 long in proportion to the body, thus giving a spider-like appearance 

 to the nymph. Their main divisions are clearly expressed, and are 

 sparsely clothed with hairs, but the joints of the tarsus are indistinct, 

 and the tarsal nails are not toothed. Owing to the flattened form of 

 the body, the bases of the legs of the right and left sides are much 

 farther apart than in the imago. The shape of the abdomen varies 

 in the different groups. At its hind end there are in the Zygoptera, 

 three long, sparsely hairy, caudal tracheal gills, of which one is me- 

 dian and dorsal, the other two lateral and more ventral. In the 

 Anisoptera, in the same positions as those occupied by the gills, are 

 three chitinous pieces which act as valves to guard the anal oj^ening. 

 Between these three pieces — which are called middle and lateral ap- 

 pendages by Hagen — there is also in the Anisoptera, a pair of chiti- 

 nous parts, which in life are constantly moving in a horizontal plane 

 to and from each other in the process of respiration. Their function 

 is probably to constantly renew the water in the rectum, in which are 

 situated the rectal tracheal-gills. No traces of external genital organs 

 are visible. The alimentary canal is complete, but there are as yet no 

 salivary glands. Packard (19) distinguished the dorsal vessel in re- 

 cently-hatched nymphs ofDiplax, where the " flow of blood to the head 

 and the return currents through the lacunar or venous circulation 

 along the sides of the body are easily observed." On either side 

 of the body is a large longitudinal trachea (dorsal trunk?) from 

 which branches proceed to the various parts of the body and the 

 appendages, and are especially numerous in the rectal region. The 

 ventral nerve cord and the cephalic ganglia are present ; in the 

 youngest nymphs of Ischnura vertiealis and Libellula pulchella, the 



