216 



THE WINGS OF EPHEMERIDA 



passes directly to the base of the wing. Here it divides and subdivides 

 into several branches which become the principal trachese of the wing. 



In all other forms that we have studied there are two large tracheae that 

 enter the wing, each being the origin of a group of the principal longitudinal 



Fig. 215.- 



-Wing of nymph of Epeorns hiimeralis 

 (After Morgan). 



trachese. This has been shown in the discussion of the hypothetical t3^pe 

 of wing tracheation (see page 16). The figure representing this hypotheti- 

 cal type is repeated here for convenience of reference (Fig. 214). 



The two groups of principal trachese referred to above have been 

 designated as the costo-radial group and the cuhito-anal group, respectively. 

 When the two groups are distinct, as is the case in the Plecoptera, in certain 

 cockroaches, and in some of the Homoptera the medial trachea is a member 

 of the costo-radial group. For this reason it is thus represented in the 

 hypothetical type (Fig. 214). But in most insects there has been developed 

 a transverse trachea connecting these two groups of tracheae. The position 

 of this transverse basal trachea of the wing is indicated in the figure of the 

 hypothetical type by dotted lines. Frequently the transverse basal 

 trachea is indistinguishable from the two main trunks which it connects; 

 the three forming a single, continuous, transverse trachea, from which arise 

 all of the wing tracheae. When a transverse basal trachea is formed the 

 medial trachea tends to migrate along it towards the cubito-anal group of 

 tracheae, and often becomes united with that group. Examples of these 

 different conditions are given elsewhere in this book. 



With these facts in mind let us try to understand the origin of the unique 

 condition of the air supply of the wings of May-fiies. In the wings of 

 nymphs of the genus Epeorus, Miss Morgan found a small trachea which 

 springs from the cubito-anal branch and extends inward toward the body 

 nearly parallel with the main stem (Fig. 215, cti-a). This she suggests may 

 be a remnant of the trachea from which the cubito-anal tracheae originally 

 branched. 



If this conclusion be correct, the single large trachea (Fig. 215, c-r), 

 which is now the chief if not only source of air supply for the wing, was 



