NORTH AMKRICAN ODONATA. 203 



Still greater inactivity precedes the moults of the nymphal period, 

 which are accomplished in the water. The fissures by which the 

 nymph emerges from the exuvia are two, a transverse dorsal curved 

 fissure passing across the top of the head through the middle or in 

 front of the eyes, and a median longitudinal dorsal fissure extending 

 from the first fis.-ure as far back as the base of the hind wing-covers. 

 The exuvia remains in one piece. From his studies on Odonate 

 nymi)hs, Chatin (23) concludes that the "epidermal cells produce 

 the chitinous coat not by secretion, but by a special process, which 

 is to be considered as a direct emanation of their protoplasm trans- 

 forming itself into chitinified strata." 



When the time for the final transformation (imagination) arrives, 

 the nymph crawls out of the water upon some object. The skin 

 splits in the same lines as described above. In the Agrioninse, at 

 least, the first part of the imago to emerge is the anterior portion of 

 the thorax, then the head, the feet, the wings and the abdomen. The 

 last two are of course much smaller at this time than they will be. 

 The imago lingers near where the transformation has taken place, 

 until the wings and abdomen are stretched to their full extent. Bel- 

 lesme (22) thinks, from his observations on Platetrum depresmmi, 

 that this inflation is accomplished by swallowing air and storing it 

 in the alimentary canal. 



The nymph-skin, left attached to the object by its feet, almost per- 

 fectly preserves its size and shape, and sometimes closes at the fissures 

 so as to quite conceal them. The following measurements give the 

 size of a nymph-skin and of the fully expanded imago, a female 

 Anax Junius, which came from it, the first dimension in each case 

 being that of the exuvia : 



Total length (including appendages) 45 nun., 67. Length of the head 7, 7. 

 Greatest height of head 3.5, 8.5. Length of thorax 8, 14. Length of abdomen 

 (incl. app. ) 32.5, 46. Length of hind wing (cover 8.5;, 49. Length of third femur 

 7.5, 10. 



In recently transformed imagos the colors are pale, and a yellowish 

 hue predominates; such are called teneral Im&gos. The characteristic 

 color of the species appears after some hours. A sign of increasing 

 age is a bluish or whitish powder, which appears on the body, espe- 

 cially of the males, and is known as pruinoseness. While the colors 

 of the nymphs are dull, in harmony with that of their surroundings, 

 and therefore protective, those of the imagos are usually bright, 

 attractive and frequently brilliant. 



TRANS. AM. KNT. SOC. XX. AUGUST. 1893, 



