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, ete. ‘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. 
5x 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. 
McEwen (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 common 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, 2. 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, Curtonotum, Zygothrica, 
Zaprionus, 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 vnversa 
