234 THE AMERICAN NATURALIST. [Vou. XXXII. 
The rvadio-medial cross-vein. This is a cross-vein extending 
from radius to media, usually near the center of the wing, and 
is designated by the abbreviation +. When in its typical 
position, this cross-vein extends from R4+5 to Mr+2; this 
results in one end being opposite cell Æ, and the other end 
opposite cell 1st M2. The cells are defined a little later. 
The medio-cubital cross-vein. This is a cross-vein extending 
from media to cubitus, usually near the center of the wing. It 
is designated by the abbreviation m-cu. When in its typical 
position this cross-vein extends from a point near the base of 
M4 to a point near the base of Cz1. 
The medial cross-vein. This is a cross-vein Uate from 
media-two (M2) to media-three (M3); this sis designated 
by the abbreviation m. The presence or absence of this 
cross-vein is often a character of considerable taxonomic 
importance. 
The arculus. In many insects there is what appears to be a 
cross-vein extending from radius to cubitus near the base of 
the wing. This has been termed the arculus by writers on the 
i Odonata, and we propose 
to extend the use of the 
term to all orders in 
which there is a similar 
arrangement of the veins 
in this part of the wing. 
The arculus is designated by the abbreviation ar. Usually 
when the arculus is present the media appears.to arise from it. 
The fact is, the arculus is compound, being composed of a sec- 
tion of the media and a cross-vein. The structure of this part 
can be clearly seen in the Odonata (Fig. 7). 
In descriptions of wings it is often desirable to refer to one 
or more of the cells. It is necessary, therefore, to have a 
nomenclature of the cells of the wing, as well as of the wing- 
veins. Certain of the cells have received special names; but as 
no effort has been made by those proposing them to trace the 
homologies of the cells beyond the limits of a single order, the 
names proposed are not available for our present purposes. 
single example will serve to illustrate this. We find the term 
Fic, 7.— The arculus, diagrammatic. 
