THE DEVELOPMENT OF THE NYMPH. 339 



pupa, but there is this difference, whilst those of the larva 

 extend the whole length of the body and end in a stigma 

 at each pole, those of the pupa are short and possess only 

 anterior openings, the stigmatic cornua. From these the 

 main trunks extend backwards for a short distance, and then 

 suddenly break up into small twigs, which without further sub- 

 division form a tuft comparable with a horse-tail (' Pferde- 

 schwanz') and float freely in the pseudo-yelk' [2, p. 169]. 



It is these vessels which were first observed by Viallanes in the 

 rudimentary wing, to which allusion has already been made 

 (p. 160). They do not last long, but soon disappear, or only a 

 small number remain, as I have sought for them in vain on the 

 fifth or sixth day of the pupa. 



Weismann adds : ' The main trunks in the nymph give off 

 side branches, and are united in front by a transverse vessel.' 



These are clearly the vessels to the several discs, and are 

 developed from the tracheae of the larva, which are distin- 

 guished by the small embryonic cells of their external coat 

 (see p. 85 and PL IV.). The transverse commissure does not 

 unite the main trunks, as Weismann thought, but the branches 

 to the head discs, which become the anterior extremities of the 

 main trunks of the nymph. 



The separated intima of the larval tracheae is shed by being 

 withdrawn through the stigmatic cornua during their evertion, 

 and remains attached to the pupa-case. Almost directly after 

 this the intersegmental spiracle of the nymph is seen to be 

 connected with a diverticulum of the wall of the main trunk 

 close behind the stigmatic cornu. 



During the separation of the pupa-sheath from the cellular 

 ectoderm of the nymph, the intima of all these tracheae 

 separates from the cellular wall, a distinct space filled with fluid 

 intervening, and soon after this a new intima appears outside 

 the old one. This is the intima of the trachea; of the imago. 



The intima of the tracheae of the nymph, the stigmatic 

 cornua, and the intersegmental spiracles of the nymph separate 

 with the pupa-sheath, but the shed intima of the tracheae is 

 not withdrawn from the tracheae of the imago until the latter 



