AND DEVELOPMENT OF COMYS INFELIX. 241 



continued, finally pi-ojecting out of the body as withered atrophied remains of the tail- 

 apparatus (PI. 11. fig. 14). Between these two transverse connectives are eight pairs of 

 side branches, more marked than the finer anastomosiog branches of trachesB that, as 

 stated, are given off in each segment. Of these eight pairs of conspicuous side branches, 

 the first and last have each a definite spiracle, but the six pairs between have no spiracles, 

 or else they are so small as to be unf unctional. This is an interesting condition between 

 the peripneustic and the amphipneustic arrangements. 



Alimentary system. — The mouth and mandibles show no alteration, and the alimentary 

 canal is unchanged except that its contents are more marked in colour and quantity, 

 and that the communication witli the anus is now probably established. The coloured 

 refringent granules in the stomach are most noticeable. 



iii. Third Stage. 



■ External form. — The larva now has become more swollen and measures just over 

 3 mm. in length (PI. 11. fig. 15), and it is only very occasionally one finds ti'aces of the 

 tail-tracheae outside the body. 



Imaginal discs — It is now possible to discern in outline the early rudiments of the 

 future organs of the imago : these rudiments Weismann called " imaginal discs " ; 

 Kiinckel d'Herculais (34) called tliem " histoblasts," and Miall refers to them as 

 " imaginal folds." Gradually they assume a form (PI. 11. fig. 15) in which the antenna 

 can be seen as a pair of plate-like bodies near the mouth. Behind and almost above 

 these is a pair of circular discs from which the eyes will arise. The buds "which will 

 ultimately form the buccal folds are also discernible. 'Jlie leg-rudiments (PL 11. 

 fig. 15, l}~^) come behind the eyes, and on the outer margin the wing-rudiments are 

 plainly traceable. Nearer the posterior end of the body are two oval masses of cells, 

 representing the future sexual glands (PI. 11. fig. 15, s.g) — ovaries or testes; and 

 following closely on these are three pairs of bud-like bodies, which are the rudiments of 

 the sting (PI. 11. fig. 15, si!.'"'). 



Tracheal system. — The tracheal trunks still have a lateral course, CDunected by an anterior 

 and posterior transverse commissure. The stigmata are as in the pnjceding stage, but in 

 connection with each of the four spiracles described as functional in tliat iustar, there 

 is an apparatus by which intercommunication is apparently set up betweea the respiratorv 

 systems of the host and parasite (PI. 11. fig. 15). From each of these four spii'acles 

 there is a double tracheal tube running out into the host's body ; these two branches 

 become subdivided and ramify in the host-tissue, and by this means, it seems, the 

 parasite obtains its air-supply. Whether tliese ramifying tracheue originate from the 

 tracheal system of the Coccid host, or from that of the parasite for whose benefit we may 

 presume they are functioning, is quite obscure. If the former be the correct inter- 

 pretation, we must conclude that the host develops a respiratory system for the benefit 

 of the parasite that is destroying it. If, on the other hand, the structures originate 

 with the parasite itself, we have to deal with the remarkable fact that they constitute a 

 tracheal system entirely outside of the creature to whicli they belong. Buguion says tiiat in 



33* 



