368 THE TRACHEAL SYSTEM OF THE IMAGO. 
they do not enter the muscle-fibres and do not generally 
penetrate the epithelial walls of the body or viscera. 
There are, however, some exceptions to this: trache pene- 
trate the nerve-centres, and dioptron ; ramify between the large 
epithelial cells of the rectal papilla, and also penetrate the 
thicker portions of the proventriculus. 
I know no other parts in which the trachee enter the 
epithelial layer. Whenever they do there are distinct pro- 
longations of the reticular tissue of the ccelom between the 
epithelial elements. 
3, THE DEVELOPMENT OF THE TRACHEAL SYSTEM IN 
THE BLOW-FLY. 
The manner in which the first tracheal trunks are developed 
in the embryo has been already described, but, owing to the 
difficulties which beset the investigation, it is satisfactory to 
find that the subsequent changes, which occur in the larva and 
pupa, bear out the views of Weismann on the development, 
and of Gegenbaur on the morphology of the tracheal system. 
Development in the Larva.—As Weismann observed [2, p. 115] 
the tracheal system is profoundly modified during the growth 
of the larva and the development of the nymph. In the larva 
the changes in the tracheal system are: (1) a continual increase 
in the number of arborescent tracheze, together with a corre- 
sponding increased complexity of the tracheal network, which 
invests all the internal organs and supplies the hypodermic 
layer of the integument; (2) The development of the anterior 
spiracles or stigmatic cornua ; and (3) The re-development of 
the posterior spiracles. 
New Ramifications of the Trachew first appear as buds or solid 
outgrowths of the peritoneal coat of the old ones; these buds 
become hollow and are at first filled with fluid; a new intima 
is then developed, but no air finds its way into them until the 
next ecdysis, when the intima of the new branches is found to 
be continuous with the new intima of the older trunks. The 
capillaries are developed in the interior of the stellate cells of 
