1909] Homologies of Wing Veins 103 



the comparison of the veins themselves. What chance is there 

 on the basis of the mature wings alone to homologize except by 

 the merest guess the veins of the Aleurodidae with those of the 

 PsyllidcE? 



Yet fortunately and, in many instances, to my utmost sur- 

 prise, it was found that practically the w^hole story of the vena- 

 tion of the group of four families here concerned lies revealed 

 in the preceding tracheation and no one who follows that story 

 closely can fail to recognize in the two surviving veins of Aleurodes 

 for instance the radial sector and cubitus. (See page 122). 



One of the simplest but prettiest and most clearly cut of the 

 demonstrations of the value of the tracheation in this connection 

 is to be found by comparing the venation and tracheation of 

 Chermes which shows at once that it is the wavering and unstable 

 Ri which is lacking and not M as has heretofore been considered. 

 (See page 1 1 1) . x\nd the vagaries of Rj in the whole group form a 

 fascinating study by themselves. 



But exciting and interesting as is the story the tracheae trace, 

 the quest for the homologies of the wing veins of aphids has lain 

 along a path tedious in many respects and beset with many diffi- 

 culties. 



The tracheae of aphid wings are very delicate, and when filled 

 with the medium in which the wing is mounted they become 

 invisible, so that many of the examinations and sketches made 

 had to be finished with rapidity, and often many mounts pre- 

 pared to verify a single point. 



The wings studied in connection with this problem were pre- 

 pared in two ways. Part of them were mounted in glycerine 

 jelly after the method described by Comstock-Needham* and 

 part of them were mounted in xylene damar. With wings so 

 small and delicate as the aphids wings the latter was found in the 

 main more satisfactory. It took the balsam less time to penetrate 

 and render the trachecC invisible which would have been an objec- 



*" In making mounts of this kind our usual procedure was to spread a drop 

 of melted glycerine jelly on a slide and allow it to cool; then to dissect oflf the 

 wings (generally under water), taking with them just enough of the thorax to 

 include the basal attachments of the tracheae ; then to place these wings upon 

 the solidified glycerine jelly on the slide ; then to lower upon the wings a heated 

 cover glass, causing the jelly to melt enough to envelope the wings; and then 

 to cool the mount speedily on a cake of ice, a marble slab, or in a draught of cold 

 air. Rapid cooling is imperative, for in melted glycerine jelly the trachete 

 soon become filled and the smaller ones are then invisible." The Amer. Nat. 

 Vol. XXXII, page 45. 



