232 ZOOLOGY FOR MEDICAL STUDENTS chap. 



increased by its developing into a complicated tree-like arrangement of 

 branching tubes. These tubes or tracheae branch all through the bodvj 

 their very fine terminal twigs ending blindly in immediate relation with 

 the living protoplasm of the various tissues. Except in the case of the 

 fine ultimate twigs the tracheae are kept open by a kind of spiral spring 

 formed by a spirally running thickening of their cuticular lining. 



In performing its breathing movements the insect compresses the 

 contents of its body by muscular action. This compresses the tracheae, 

 forcing the air in their interior out through the external openings or 

 stigmata situated in a row along the sides of the body (Fig. 97, A, st). 

 When the muscles relax, the spiral thickening of the tracheal lining causes 

 the tracheae to resume their former volume and they are filled by an 

 inrush of fresh air through the stigmata. 



Whereas, therefore, in the Annelid worm the necessary supplies of 

 oxygen are conveyed to, and the excreted carbon dioxide conveyed 

 away from, the living tissues by the circulating blood, in the insect on 

 the other hand the conveyance takes place in gaseous form in the tracheal 

 tubes. In other words the tracheal tubes perform what is in the annelid 

 one of the main functions of the blood-vessels, more especially of the 

 capillary blood-vessels, and the suspicion is aroused that here may be 

 the factor which has inaugurated the degeneration of the arthropodan 

 capillaries and finer vessels. This idea goes against the commonly 

 accepted belief that the arthropods were primitively aquatic animals, 

 but it is supported by the fact that Peripatus — by far the most nearly 

 primitive of the existing arthropods — is terrestrial and breathes by 

 means of tracheae. 



The spongy packing tissue between the spaces of the haemocoele is 

 in the insects charged with droplets of fat which give it a snowy-white 

 colour. This tissue is known as the " fatty body " of the insect. What 

 its special function is, whether it fulfils a purely physical one in protecting 

 the tissues from too great cooling by the constant indraught of air 

 through the tracheae or on the other hand performs some more com- 

 plicated role in metabolism, is not yet certainly determined. 



In certain insects, such as the beetles known as fire-flies, particular 

 localized masses of fatty-body are specialized as light-producing or 

 photogenic organs. The light is apparently produced by the oxidation 

 of a special substance formed in the metabolism of the organ. And 

 in correlation with this the photogenic organs possess a very rich supply 

 of tracheal tubes. The pale-greenish light most commonly produced is 

 remarkable for its economical character, the proportion of the energy 

 wasted as non-luminous rays being extraordinarily small as compared 



