398 SYNOPTIC COLLECTION. 



In Chloeon as many as twenty moults may take place. 

 Up to the ninth moult there is no indication of wings, 

 according to Lubbock. At the twelfth stage the posterior 

 angles of the mesothorax and metathorax have grown out 

 (fig- 3)- The mesothoracic angles grow faster than the 

 metathoracic and cover the latter completely (fig. 4). 

 These figures are interesting as showing the origin of the 

 wing pads of the pupa. When the last skin but one is 

 thrown off, the winged insect (fig. 5) flies away, but still 

 another skin must be shed before full size is attained (fig. 

 6, C. dipterum Lubb.). The last skin is often got rid of 

 while on the wing. We shall see that in most insects this 

 slow process of development is more or less shortened by 

 the law of acceleration. While there is no resting period, 

 there is a metamorphosis which transforms the wingless 

 insect into the winged creature. It is evident that this 

 metamorphosis is more specialized than the primitive 

 development of the Thysanura. 



The generalized characters of the adult (PI. 958, fig. 6 ; 

 see also No. 959, Hexagenia bilineata Say) are a long, 

 unconsolidated body ; an abdomen without an ovipositor 

 and with the genital openings paired ; a thorax without 

 secondary sutures and with wings that are simple in their 

 venation. 



The adaptive .and secondary characters, on the other 

 hand, are seen as soon as the young larva develops tra- 

 cheal gills (PI. 958, fig. 2 ; see also figs. 3, 4) on each 

 side of the abdomen. These fit it for an aquatic existence 

 and the larval and pupal life may last three or four years. 

 The long caudal setae also aid as respiratory organs, and 

 the terminal blood vessel in the body is so made as to 

 drive the blood backward into the canals of the setae, 1 

 where it is purified by the air in the water. 



1 Zimmermann, Zeitschr. f. wiss. Zool., XXXIV, 1880, p. 404. 

 See also Creutzburg, Ann. and Mag. Nat. Hist., (5), XV, 1885, p. 

 494. 



