2i6 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



characteristic an appearance to these tubes (Fig. 93). 

 These tracheae are distributed to all parts of the 

 body, running alongside the nerves and entering with 

 them into the central nervous system ; they are 

 particularly well developed in the head. 



In the higher forms the tracheal orifices, which are 

 definitely known as stigmata, are not distributed over 

 the whole body ; they would appear to have been primi- 

 tively arranged by pairs in each metamere, and in some 

 Myriopods that condition is essentially retained. As 

 the stigmata diminish in number, the tracheae tend to 

 branch, and in some Myriopods (such as lulus) 

 they unite with one another and form tracheal 

 anastomoses of much the same kind as those found 

 among Hexapod insects ; in the higher members of 

 the last-mentioned group the stigmata are reduced to 

 two or three pairs, but, thanks to the anastomosing 

 branches, all the parts of the body are still well supplied 

 with air vessels, the thinnest and finest of which 

 are without the spiral fibre. Just as in Birds 

 (see page 239), we find that in flying insects the air 

 passages or tracheal tubes often pass into swellings or 

 enlargements, the so-called tracheal vesicles ; these 

 may be small and numerous as in some Coleoptera, or 

 larger and less numerous as in butterflies and bees, 

 or reduced to two, which are, as in the fly, of enor- 

 mous size, and abdominal in position. The stigma 

 or spiracle, by which the air enters, varies consider- 

 ably in form and structure, and the same is true of 

 the apparatus by which it can be closed. Little is known 

 with certainty as to the mechanism of the respira- 

 tory movements of insects, but the high temperature 

 of the bee-hive, and the activity of such restless 

 creatures as the house-fly, speak clearly enough of a 

 large amount of oxydation, and of waste of tissue ; 

 in direct relation to the activity of the cellular organ- 

 ism is the amount of oxygen which it requires. It 



