ARTHROPOD A— INSECTA. 



421 



circulating fluid. The principal veins arise by cuticular thicken- 

 ing around the principal trachese which lie almost free within the 

 simple sac-like wings of the immature insect. In some cases, how- 

 ever, the nervures are formed before they become tracheated 

 (Hymenoptera, Coleoptera?)."^ 



The principal veins or nervures have received distinct names, 

 those most generally adopted being from before backwards (see 



3d A 



Fig. 1724. Diagram of the tracheation of a primitive insect wing. (Hypothetic. 

 After Comstock and Needham. ) C, costa ; Sc, subcosta ; Sc^^ Sc^, first and second 

 subcostal branches ; Rs, radial sector ; R^-R^, branches of radial groups ; J/, media ; 

 M^-M^, branches of media ; Cu, cubitus ; CV/p Cu^, branches of cubitus ; is/A-jdA, 

 first, second and third anal veins. 



Fig. 1724) : (i) costa — C, (2) subcosta — Sc, (3) radius — R, (4) 

 media — M, (5) cubitus — Cu, (6) anal veins — A. The costa gen- 

 erally forms the front margin of the wing, and has also been called 

 the marginal vein. The subcosta is more or less parallel to the 

 costa, and in much-veined wings gives off numerous small branches 

 to the costa. This vein has also been called the mediastinal (Fig. 

 '^72>7)' The radial group, also known as the scapular veins (Fig. 

 '^72>7)j is the most prominent of the wing- veins. Typically it is 

 five-branched, the main vein separating into two divisions, the 

 radius proper, R, and the radial sector, Rg. The latter is four- 

 branched (i?2 to Rr^, from before backwards, Fig. 1724), in primi- 

 tive groups, though the number of branches may be modified by 

 the development of new ones (generally between R2 and R^), 

 and by the reduction of the number through coalescence of the 

 older ones. The first radial vein or radius proper may also be- 



*See the important papers **The Wings of Insects," by J. H. Comstock, and J, G. 

 Needham, American Naturalist, Vol. XXXII., 1898, and XXXIII., 1899. 



