50 THE TRACHEATION OF WINGS 



the medial trachea to pecuharities of thoracic structure, as has been done 

 above. 



The invasion of the area of the radial sector by the medial trachea in the 

 Odonata was also referred to in the introduction as a condition which might 

 be due to the better air supply of the cubito-anal trachea. In the Odonata 

 the distance between the bases of the radial and medial tracheae is so short 

 that it would hardly seem that the medial trachea should be so modified as 

 to invade the area of the radial trachea in order to save the air from passing 

 so short a distance along the transverse basal trachea. The May-flies 

 illustrate the same condition of the medial trachea invading the area of the 

 radial sector, but here the connection of the cubito-anal trachea has been 

 lost and the air supply of the wing has been completely cephalized so that 

 the medial trachea is less favorably situated, with regard to the directness 

 of the air supply, than is the radial trachea, the area of which it has invaded. 



From all these considerations it would seem that the directness of the 

 air supply is not an important factor in controlling the relationships of the 

 tracheae. However, the function of the trachege is to supply air and, even 

 tho the surrounding structures may obsti-uct their way and cause them to 

 become modified and adopt new and longer courses, they must still furnish 

 an adequate supply of air to the tissues. The accessory cubito-anal tracheae 

 to the hind wings of the Odonata is a case where the least obstructed course 

 for the trachege to follow is also the shortest course, but the majority of 

 cases, such as the modified base of the medial trachea of the stonefly and 

 the single trachea to the wing of the honey-bee connecting with the trunk 

 of a vestigial spiracle, have seemed to have no regard for the distance which 

 the air must pass in reaching the tissues, but they have all adopted the 

 least obstructed course for the air to follow. The length of the tracheae, 

 therefore, does not seem to be the factor causing the modifications of 

 structure, but rather a course for the tracheae which is free from obstnrction 

 and which will allow for the passage of the air. Whether the air reaching 

 the radial trachea comes from the costo-radial or cubito-anal trachea makes 

 no difference to the tissues so long as the course of the radial trachea is 

 unobstructed. The medial trachea, on the other hand, would not be 

 expected to invade the area of the radial trachea simply because its base 

 might be a millimeter or two nearer to the source of air supply any more 

 than the radius should invade the area of the media in forms like the May- 

 flies where the cubito-anal trachea has been lost. 



CONCLUSIONS 



The material upon which this discussion is based does not constitute an 

 exhaustive study of this subject, for each order of insects presents an oppor- 

 tunity for more detailed research. However, there are certain points with 



