THE TRACHEATION OF WINGS 25 



trachea has become united with the medial trachea at the point where it 

 crosses the medial trachea and has lost its earlier basal connection. 



The second illustration is found in the more specialized Lepidoptera 

 where the base of media has been lost by atrophy and the first branch of 

 media, vein Ri, has become attached to the radial sector; correlated with 

 this shifting of the base of vein Mi to the radial sector trachea Ri appears to 

 be a branch of the radial sector trachea. In the more generalized Lepidop- 

 tera trachea Mi has preserved its primitive connection with the other 

 branches of the medial trachea. In the latter case the evidence presented 

 by the tracheation is complete in itself; in the former case there is other 

 evidence to show that the tracheation has been modified secondarily. 



In general it may be said that the evidence presented by the tracheation 

 of the wings of the more generalized insects is reliable, but in the case of the 

 more specialized insects where extensive modifications of the venation 

 have taken place the evidence presented by the tracheation should be 

 carefully considered before it is accepted. 



The basal connections of the tracheae of the wings. — It has been shown 

 on an earlier page that the tracheae of the wings are branches of two large 

 tracheae, one of which enters the wing near the humeral angle of the wing, 

 the other, in the region of the base of the anal area. In the Plecoptera and 

 in certain other groups of insects these two large trachese are distinct, but 

 in most insects they become connected by what has been termed the 

 transverse basal trachea. 



In the diagram representing the hypothetical primiti\-e tracheation of 

 the wings (Fig. 6) the connections of the tracheae of the wings with 

 these two large tracheae are represented; and the position in which the 

 transverse basal trachea is found when it exists is indicated by two dotted 

 lines. 



The two groups of wing-tracheae arising from these two large tracheae 

 have been designated as the costo-radial group and the cubito-anal group, 

 respectively. And it has been shown that although the medial trachea 

 belongs to the costo-radial group of tracheae in those forms in which there 

 is no connection between the two groups of tracheae, when the two groups 

 are connected by a transverse basal trachea the base of the medial trachea 

 tends to migrate along the transverse basal trachea towards the cubito-anal 

 group of tracheae, which it reaches in the more specialized forms. These 

 facts suggested the question : what are the causes that produce this result ? 

 Regarding this Comstock and Needham ^\'rote as follows: 



"We have found no indication that the formation of a transverse basal 

 trachea and the subsequent migration along it of the base of the medial 

 trachea is influenced at all by the flight function of the wing, as the arrange- 

 ment of the wing-veins does not appear to be modified by it. It should be 

 remembered that the transverse basal trachea and the bases of the wing 



