DEVELOPMENT OF THE IMAGO FROM THE NYMPH. 345 



closely resemble cilia. These form the down-like hairs which 

 cover the body of the imago (Fig. 47). 



Subsequently the cellular integument is seen to consist of 

 two layers of cells : a superficial layer of very small ones, which 

 ultimately become converted into a chitinous membrane ; and 

 a deep layer of large cells, the trichogenic cells beneath the 

 great setas, and of medium-sized cells derived from them, which 

 form the hypodermis and the deeper layers of the cuticle de- 

 veloped after the escape of the imago from the pupa (see p. 280). 



The corrugation of the integument, which is especially 

 marked in the abdominal region, permits of its expansion after 

 the escape of the imago from the pupa-case. 



By the end of the seventh day the articulations between the 

 limb segments begin to assume definite characters, and the 

 pads and claws of the tarsi are formed from their terminal lobes. 

 The larger wing nervures are present as distinct ridges, and 

 the oral Icbes of the proboscis are so completely developed that 

 the similarity of the mouth to that of the Hemiptera, so marked 

 at an earlier period, is no longer obvious. 



The second period, from the end of the seventh day to the 

 escape of the imago from the pupa, is marked by scarcely any 

 changes of external form, but is chiefly remarkable for the 

 rapid development of the internal organs, which make very slow 

 progress during the first period. 



Throughout this second period the development of the 

 contents of the head- capsule exhibits a marked advance over 

 that of the thoracic and abdominal organs, and the contents 

 of the thorax are further advanced than those of the abdomen. 



Whilst the thoracic ganglia remain comparatively rudi- 

 mentary, the brain exhibits great complexity, and scarcely 

 differs from that of the adult. The least developed organs of 

 the head, on the tenth day of the pupa, are the compound 

 eyes, in which pigment first appears on the ninth or tenth day ; 

 but the simple eyes are already fully developed, and are ap- 

 parently more perfect than in the adult imago. These organs 

 in the pupa are more like the simple eyes of spiders than those 

 of the adult insect. 



23—2 



