344 



THE DEVELOPMENT OF THE NYMPH. 



3. THE DEVELOPMENT OF THE IMAGO FROM THE 

 NYMPH. 



From the fifth day of the pupa state to the escape of the imago. 



The development of the nymph, as already stated, may be 

 regarded as complete when the pupa sheath is separated from 

 the subjacent cellular layer. From this period until the imago 

 emerges from the pupa, the process of development may be 

 conveniently divided, as Weismann [2] suggested, into two 

 stages. The first commences on the fifth day, and terminates 



Fig. 47. — A section through the .iljdominal integument of a nymph from a pupa 

 seven clays old: e f, outer celKilar layer of small epiblast cells ; /(, inner cellular 

 layer of hypoderm and trichogenic cells. The cuticular layers are developed 

 l)etween the outer and inner cells. 



about the end of the seventh. During this period the external 

 form of the nymph undergoes rapid evolution, and by the end 

 of the seventh day differs but little from that of the young 

 imago when it is ready to escape from the pupa-shell. The 

 integumental setae are developed from the large trichogenic 

 cells, and the small cells of the epiblast between them increase 

 in number so rapidly that the whole integument becomes 

 minutely corrugated, the great seta; arising from the ridges. 

 The hollows of the rugae are occupied by minute setie which 



