78 THE GENERAL FEATURES OF WINGS 



veins can be most frequently recognized are the following: Plecoptera, 

 Trichoptera, Lepidoptera, Diptera, Hymenoptera, Hemiptera, and Hom- 

 optera. The names applied to these cross-veins are as follows: 



The humeral cross-vein. — This extends from subcosta to the costa near 

 the humeral angle of the wing, and is designated by the abbreviation h 

 (Fig. 65). It is the most constant of all of the named cross-veins. 



R^■M'^ 



I \«- a a 



Mi 

 Cn 



Fig. 66. — Diagram of an arculus of a dragon-fly. 



The radial cross-vein. — This extends between the two principal divisions 

 of radius, i. e. from vein Ri to vein Rg, and is designated by the abbreviation 

 r (Fig. 65). 



The sectoral cross-vein. — -This extends between the principal divisions 

 of the radial sector, i. e. from vein R2+3 to vein R4+5, or from vein R3 to 

 vein R4, and is designated by the abbreviation 5 (Fig. 65). 



The radio-medial cross-vein. — This extends from radius to media, usually 

 near the center of the wing, and is designated by the abbreviation r-m 

 (Fig. 6s). 



The medial cross-vein. — This extends from vein M2 to vein M3, dividing 

 cell M2 into cells ist M2 and 2d M2. The presence or absence of this cross- 

 vein is often a character of considerable taxonomic importance. It is 

 designated by the abbreviation m (Fig. 65). 



The medio-cuhital cross-vein. — This extends from media to cubitus and 

 is designated by the abbreviation m-cu (Fig. 65). 



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 Odonata, and we have extended 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 

 (Fig. 66). Usually when the arculus is present media appears to arise 

 from it. The fact is, the arculus is compound, being composed of a section 

 of media and a cross-vein. MacGillivray ('12) has designated that part 

 of the arculus which is a section of media the anterior arctilus (a a) and that 

 part formed by a cross-vein, the posterior arculus (p a). 



The costal cross-veins. — In some cases it is desirable to refer to certain 

 cross-veins in many veined wings. This most often occurs in the case of 

 those cross-veins that, extend between the costa and the subcosta. These 

 are commonly designated as the costal cross-veins. This name 'is appropriate 



