THE WINGS OF EPHEMERIDA 217 



developed from the stem of the costo-radial <^roup of tracheae; and that 

 part of the branch of this stem that extends to the origin of the cubito-anal 

 group of trachea?, and from which the medial trachea arises, represents the 

 transverse basal trachea (Fig. 215, tr). 



(c) THE HOMOLOGIES OF THE WING-VEINS OF THE FORE WINGS OF THE' 



EPHEMERIDA 



Figure 215 represents the fore wing of a full grown nymph of Epeorus 

 htimeralis, in which the developing veins were visible. Although a con- 

 siderable reduction of the tracheae has taken place, the basal portion of each 

 of the principal tracheae is preser\^ed. Figure 216 represents the venation 

 of the wing of the adult of the same species. The lettering of these, two 

 figures will ser\^e to indicate the now accepted view regarding the homolo- 

 gies of the trachea and of the wing veins. 



Costa occupies its usual position on the front margin of the wing. The 

 subcosta is closely parallel with the costa and is unbranched. The radius 

 is also parallel with the costa; and it appears to be an unbranched vein. 

 Media divides in the typical manner into four branches. The cubitus also 

 has its typical form being two-branched. And three, more or less branched 

 anal veins are present. Several intercalary veins are also present; the 

 more prominent of which are I Mi, IRg, IM3, and ICui. 



In the Ephemerida, as in the Odonata. the branches of media have 

 moved forward so as to occupy the field of the radial sector. The result 

 of this modification is that radius appears to be an unbranched vein 

 (Fig. 216, R). 



Whether a vestige of the radial sector remains or not is an interesting 

 problem. Between veins Mi and IR., there is a vein which appears to be a 



Fig, 216. — Wing of adult Epeorus humeralis 

 (After Morgan). 



brancli of vein Mi. This may be merely an intercalary \'cin; but there is 

 good reason to believe, as pointed out by Miss Morgan, that this is a \^estige 

 of the radial sector; consequently it is designated as vein R^. (Fig. 

 216, RJ. 



