\"ol. XXviii] ENTOMOLOGICAL NEWS I73 



ideas as to how the tracheation should be used as an aid to 

 determining the homologies of veins. The tracheation of a 

 nymphal insect wing is never identical with the venation of the 

 adult wing. It may correspond closely ; it may entirely di- 

 verge. Tracheation affords complete confirmation of vein 

 homologies in some of the lower orders, such as Plecoptera and 

 Corrodentia ; it is worthless for such use in other orders, such 

 as Trichoptera and Diptera. It is an aid in most orders, but 

 needs to be used with discretion and with a regard to its limi- 

 tations. The interpretation of vein homologies by the study 

 of the antecedent tracheae is a method which, like most other 

 methods that we use in zoology, is of value only for what it 

 shows. It is not all-sufficient. Let any one who is inclined to 

 trust to tracheation too far read Miss Morgan's study of May- 

 fiy traclieation" and learn caution. 



The Odonata are not alone in furnishing examples of the 

 replacement of one principal tracheal branch by another, in- 

 dependent of adult venation. In explanation of its occurrence 

 in the Zygoptera in my paper of 1903^ (p. 713), I cite the par- 

 allel well known case of the attachment of trachea Mi to the 

 radius in Picris, a shift of tracheae which has never led Lepi- 

 dopterists to change the designation of the adult vein. Indeed 

 Tillyard himself is not consistent ; for in the second of the 

 two papers here under consideration he labels and discusses 

 as branches of A, certain tracheae that spring from the cubital 

 stem! If branches of the anal trachea may be shifted with- 

 out disturbance of the adult vein, why may not those of the 

 radial ? 



Fortunately, sufficient comparative study will enable one to 

 learn when such shif tings have occurred, so that even in spe- 

 cialized groups the testimony of the tracheae is not wholly in- 

 validated. But if we proceed to change the designation of 

 adult veins without first learning this we shall create for our- 

 selves intolerable and unnecessary confusion. 



"Morgan, A. H. Homologies of the wing veins of mayflies. Ann. 

 Entom. Soc. Amer. 5 : 89-106, 6 text figures, 5 plates, 1912. 



