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terminal parts of the branches are extremely fine and 

 it is from these that the supply of oxygen to the body 

 is maintained. 



These tracheae, in keeping with their origin from the 

 outer layer of the body, are lined with a substance 

 termed chitin. This is strengthened on the inner side 

 by fine, more or less spiral, closely set ridges, which 

 serve to maintain an open passage in these minute 

 channels. In certain regions these tubes are dilated 

 so as to form what are termed air sacs. In consequence 

 of the very direct conveyance of air to all parts of the 

 body by means of this system, the blood is relieved of 

 the function of carrying oxygen and is nutritive merely, 

 not respiratory. Contrary to what takes place in man, 

 inspiration is a passive act, while expiration is active, 

 and is effected by the muscles mainly of the thorax and 

 abdomen. 



The paired openings at the surface of the body are 

 known as spiracles or stigmata. In many insects these 

 are guarded by spines or bristles which serve to keep 

 out dust, casual wandering organisms, and true 

 parasites. Whilst in the hive bee protection of the 

 abdominal spiracles is secured by their minute size, 

 the anterior thoracic openings are larger and unfortun- 

 ately are easily accessible to Tarsonemus woodi. Not- 

 withstanding the provision for safeguarding the respir- 

 atory system of insects here referred to, there are many 

 cases in this group both of the casual intrusion of stray 

 organisms and of definitely established parasites. Well 

 known examples of this are to be found in the flies of 

 the Family Tachinidae. Rennie and Sutherland (Par- 

 asitology XII., 1920, pp. 199-211), have lately described 

 such! a case of parasitism in the common " leather 

 jacket " (larva of Cranefiy). Here, once the parasite 

 insect is within the trachea, there is developed from 

 the invaded tube as a result of the irritation set up, a 

 small blind sac which envelopes the parasite. The latter, 

 in course of time establishes an opening at the blind 

 end, feeds through it upon the host's tissues, whilst 

 utilizing at the other end of its sac or tube the air 

 supply from the host's trachea. Eventually the host 

 insect dies, the parasite pupates in the remains and in 

 due course emerges as an adult fly. Other cases of a 



