THE TR AC HEAL SYSTEM OF THE BLOW-FLY. 365 



sized tracheae, 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 tracheae 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 lacunae between their 

 branches, where the wall of the vessel consists of cuticular 

 intima only. 



The intima of the larger cylindrical tracheae 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 tracheae in 

 being finer and more closely coiled. 



The Air Sacs have an exceedingly fine silvery-looking intima, 

 thrown permanently into minute rugae, which give it an irides- 

 cent appearance under certain conditions of illumination. 



The rugae are more marked and larger in the vicinity of the 

 cylindrical tracheae, 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 plicae of the 

 intima. In the imago, however, the great distension of the 



