NORTH AMERICAN ODONATA. 197 



writer finds eight distinct pairs of abdominal ganglia in segments 

 1 to 8 ; the last pair being the largest. 



The subsequent changes which take place in the nymph up to 



the time of imagination, are as follows : 



The relative increase in size of thorax and abdomen is much 

 greater than that of the head, so that the latter, which, in the young 

 nymph, was one-fourth or more as long as the trunk, in the last 

 nymphal stage is only one-sixth to one-ninth of that length. The 

 eyes soon become relatively larger, but never in any of the groups 

 do they meet upon the top of the head. The epicranium is conse- 

 quently of large size. The clypeus does not become differentiated 

 into post- and ante-clypeus. The ocelli can be distinguished in old 

 Agrionine nymphs at least. The long terminal antennal joint of 

 the young nymph divides into a number of joints, in most of the 

 subfamilies. Mandibles and first maxillse closely approach the form 

 of those of the imago. The labium throughout retains its large 

 proportionate size ; the shape of its lateral lobes is different in the 

 young and old nymphs of the same species. 



Thorax. — In the young nymph the prothorax is almost, or fully as 

 large as either of the other two thoracic segments, but as the nymph 

 increases in size, the meso- and metathorax become relatively larger, 

 due to the development of the wing-covers and the wing-muscles. 

 The wing-covers appear after the third or fourth moult (Poletaiew), 

 as small triangular flaps, one on either side of the meso- and meta- 

 notum. They increase in size with each successive moult, are di- 

 rected backwards, and lie upon the dorsal surface of the thorax and 

 anterior abdominal segments, the hind wing-covers lying over and 

 concealing the greater part of the front wing-covers ; the courses 

 of the chief longitudinal veins become quite apparent in the old 

 nymphs. As the wing-covers increase in size, the right and left 

 mesepisterna, which at first were entirely separate, extend dorsally 

 and meet each other, first at the front margin of the mesothorax, 

 their union extending gradually backwards towards the base of the 

 front wing-covers. The boundaries of the pleural sclerites may be 

 distinguished in the old nymphs. The bases of the legs remain 

 quite far apart up to imagination ; the length of the legs although 

 great throughout nymphal life, is relatively less than in the first post- 

 oval stage. The hairs which clothe the legs of the young nymph 

 are soon lost; in their stead may be seen longitudinal rows of very 

 minute spines, rudiments of those of the imago. Although the tarsi 



TEANS. AM. ENT. SOC. XX. AUGUST, 1893 



