338 THE DEVELOPMENT OF THE NYMPH. 



reticulum in which the muscles lie — a connective reticulum 

 which, like the connective tissues generally, is, therefore, of 

 parablastic origin. 



i. The Tracheal System of the Nymph. 



The investigation of the changes which occur in the tracheal 

 system in the nymph and pronymph stages is most difficult. 

 The vessels are not easily traced in sections, in which of course 

 no air remains after successful imbedding, and it is not until 

 the nymph stage is far advanced that the tracheal vessels are 

 easily demonstrated. 



The earlier stages of their development have been described 

 by Weismann, and the methods he adopted — those of ordinary 

 dissection — although far from satisfactory, throw more light on 

 the process than the newer methods of section-cutting. I am 

 inclined to agree in most points with Weismann, but confess 

 that I should feel more satisfied if my sections showed the 

 conditions, which he described, clearly. 



Such rough dissections as can be made bear out Weismann's 

 statements, and the difficulty of tracing the tracheae in sections 

 in the early stages of the pupa is so great that, until some 

 further methods of research can be invented, I fear little pro- 

 gress is likely to be made. In pupae a few days older, however, 

 the evidence of sections is unequivocal. 



In the earliest sections of pupae I have only found the large 

 tracheal trunks in various stages of degeneration and re-forma- 

 tion, with an occasional group of small twigs cut through 

 transversely, which are recognisable by their bright, highly- 

 refractive intima. 



Weismann says : ' The tracheal system of the pupa is very 

 peculiar, and differs not only from that of the larva, but from 

 every known variation of the respiratory system in insects. 

 Only a small portion of the tracheae of the pupa is formed in 

 relation with those of the larva ; the greater part is developed 

 independently. 



' Two great respiratory trunks are common to the larva and 



