THE WINGS OF EPHEMERIDA 



223 



Fig. 223.- 



-Hind wing of Chirotonetes albomanicatus 

 (After Morgan). 



features in the reduction of the hind wing are the following : the base of 

 vein Ml +2 has atrophied; veins R^ and IRj are entirely lost; and \'ein 



ICui, extends to the base 



Cy \ \J<T^ — \ \ \ \ "^' \ \ T~~^f-^ ^^ ^^® wing. 



The correctness of the 

 conclusions regarding the 

 homologies of the veins 

 of the hind wing indicated 

 above are confirmed by 

 a study of the tracheation 

 of the wings of the nymph 

 of this species as figured 

 by Miss Morgan (Fig. 

 224). Note especially 

 the absence of tracheae 

 between trachese Mi and 

 M2, correlated with the 



loss of veins Rg and IRg, and also the separation of tracheas Cui, and 



Cu2 close to the base of the wing-pad, which admits of the extension of the 



intercalary vein ICui, to the base of the wing of the adult. 



These two features, the loss of vein Rg and IRg and the basal extension 



of vein ICui are the ones that rendered difficult the recognition of the 



homologies of the veins of the hind 



wings before use was made of the 



corrugations of the wings for this pur- 

 pose. Unfortunately the value of this 



criterion did not occur to us at the 



time Miss Morgan's paper was written, 



which accounts for the differences 



between her conclusions regarding the 



homologies of the veins of the hind 



wings and those stated here. 



In concluding this account of the 



hind wings of May-flies, I wish to call 



attention to the prominent, forward 



projection of the humeral angle. This is a specialization which insures the 



synchronous action of the two wings of each side, as it causes the wings to 



overlap to a great extent. 



Fig. 224. — Tracheation of a hind wing 

 of a nymph of Chirotonetes albomani- 

 catus (After Morgan). 



