THE TRACHEAL SYSTEM OF THE BLOW-FLY. 365 
sized trachez, although it soon disappears from the tracheal 
capillaries. I have specimens made thirty years ago, in which 
the silvery lustre, due to the contained air, still remains as a 
characteristic of the tracheal vessels. 
Although the trachez consist in all insects primarily of an 
external cellular or peritoneal coat and of a cuticular intima, 
similar to those described in the larva (p. 48), the defini- 
tive condition in the imago varies considerably in different 
insects, and depends upon the relative development of these 
coats. 
In some insects, especially in the Lepidoptera, the peritoneal 
coat persists as a continuous layer of thick cells, which become 
loaded with fat granules; whilst in others, as the Blow-fly, the 
external coat becomes exceedingly thin, and is seen to consist 
of stellate cells, which leave considerable lacunz between their 
branches, where the wall of the vessel consists of cuticular 
intima only. 
The intima of the larger cylindrical trachez always exhibits 
the well-known spiral fibre in the Blow-fly imago, but this is 
lost in the smaller branches and in the great dilated air-sacs. 
It only differs from the spiral fibre of the larval trachez in 
being finer and more closely coiled. 
The Air Sacs have an exceedingly fine silvery-looking intima, 
thrown permanently into minute ruge, which give it an irides- 
cent appearance under certain conditions of illumination. 
The ruge are more marked and larger in the vicinity of the 
cylindrical trachez, which arise from the sacs; they are there 
seen to be due to a thickening of the membrane. This 
thickening is more marked in the vessels which are in contact 
with the brain, great eyes, and thoracic ganglion; and these 
exhibit a dark-brown tint. This thickening of the walls of the 
air-sacs apparently serves to protect the soft structures with 
which they are in relation from undue pressure. 
In the nymph the air-sacs are deeply plicated and covered 
by a thick peritoneal coat, consisting of closely-united cells, 
which give off processes extending between the plice of the 
intima. In the imago, however, the great distension of the 
