30 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 



The portion of the costa proximal to the apex of the first vein is known 

 as its first section; that between the apices of the first and second 

 veins as its second section, etc. The portion of the fourth vein lying 

 between the anterior and posterior cross-veins is known as its third 

 section. The division-point between the first and second sections is 

 the junction between the fourth vein and the cross-vein (absent in 

 Drosophila) that separates the second basal and discal cells. The 

 relative lengths of the sections of these veins are of taxonomic im- 

 portance, and are expressed by the following indices : 



Costal index: Length of second section of the costal vein divided by length 

 of its third section. 



Fourth-vein index: Length of fourth (distal) section of the fourth vein 

 divided by length of its third section. 



4c index: Length of third section of costal vein divided by length of third 

 section of fourth vein. 



ox index. Length of third (distal) section of fifth vein divided by length 

 of posterior cross-vein. 



The costal vein, up to a point between the apices of the third and 

 fourth veins, bears on its outer surface a series of short black hairs. 

 On the first costal section these hairs form a double row ; on the second 

 and third sections there is only a single row. Just before the distal 

 costal break there is a larger pair of bristle-like hairs (in Mycodrosophila 

 and in Drosophila immigrans there is only a single bristle here) . The 

 surface of the wing is covered with much smaller pale hairs that are 

 discernible only under considerable magnification. These small hairs 

 are missing in a narrow band that marks the position of the lost cross- 

 vein that, in some forms, separates the discal and second basal cells. 



IMcEwen (1918) has figured the small (sensory?) organs that occur 

 on the wing-veins of Drosophila melanogaster. These are minute ring- 

 shaped structures. There is a group of them near the base of the 

 wing, on the conmion base of the first, second, and third veins. Seven 

 larger ones occur farther out on the wing, as follows: two at the junc- 

 tion of the first and costal veins, one near the base of the third vein, 

 one near the middle of the anterior cross-vein, and three on the distal 

 section of the third vein, dividing it into three subequal proximal 

 portions and a slightly longer distal one. The last three organs 

 mentioned are attached to the posterior surface of the third vein, i. e., 

 to the surface facing the fourth vein. The other four organs are on 

 the upper surfaces of the veins. 



Examination of balsam mounts of 22 species of Drosophila, two of 

 Scaptomyza, and one each of Aulacigaster, Curtonoium, Zygothrica, 

 ZaprioniLS, Leucophenga, Chymomyza, and Mycodrosophila, shows that 

 the number and distribution of these organs on the main part of the 

 wing is rather constant. The organ on the basal section of the third 

 vein is often near the middle of that section, and in Drosophila inversa 



