95 



The nodal furrow. — This is a transverse suture beginning 

 at a point in tiie costal margin of the wing, corresponding 

 to the nodus of the Odonata, and extending towards the 

 inner margin of the wing. It crosses a varying number of 

 veins in different orders of insects. 



The furrows of the wing are in no sense homologous or even analogous 

 to veins. More than this, the relative positions of the furrows and of the 

 wing-veins are not constant ; for it frequently happens that the course of a 

 vein has been so modified that it crosses the line of a furrow and the rel- 

 ative positions of the two are thus reversed. If this fact had been under- 

 stood by Adolph we would have been spared his misleading theory of alter- 

 nating concave and convex veins. * 



Margins of wings. — An insect's wing is more or less 

 triangular in outline ; it, therefore, presents three margins ; 

 the costal margin (Fig. 3, a-b) ; the outer margin (Fig. 3, b-c), 

 and the inner margin (Fig. 3, c-d). 



Angles of wings. — The angle at the base of the costal 

 margin (Fig. 3, a) is the humeral angle ; that between the cos- 

 tal margin and the outer margin (Fig. 3, b) is the apex of 

 the wing ; and the angle between the outer margin and the 

 inner margin (Fig. 3, c) is the anal angle. 



The typical branching of the wing-veins.— In order 

 to determine the homologies of the branches of a forked 

 vein when some of the cells have been obliterated by the 

 coalescence of branches, it is necessary to know what was 

 the primitive type of branching of this vein. For this rea- 

 son an effort has been made to determine the probable 

 structure of the wing of the primitive winged insect. 



In the study of the development of wings, it has been 

 found that in the more generalized insects the cavities of 

 the longitudinal wing-veins are formed about tracheae, and 



* G. Ernst Adolph, Ueber Insectenflugel. Nova Acta der ksl. Leop. -Carol- 

 Deutschen Akadefnie der Naturforscher, Bd.xli, pp. 215-251, 1879. 



