Vol. 1.] Woochvorth . — Wing Veins of Insects. 105 



The venation of the two wings never exactly corresponds, 

 but approaches correspondence most nearly when the wings 

 are of approximately the same size and shape, as is true in 

 some species of Perla and Nemoura. The family possesses a 

 few quite characteristic venations and shows certain very 

 evident tendencies, but there are intermediate forms so com- 

 pletely connecting the various types that, with the diversity, 

 it is difficult to define the Perlid type of venation except by 

 negations. Even such a wing as that possessed by the Paleo- 

 zoic Protoperla would not be particularly anomalous in this 

 family, if it existed on an insect at the present day. 



One of the most evident and most constant tendencies notice- 

 able in these wings is the production of a strong cross vein 



FIG. 41. Venation in Nemon.ra. 



beyond the middle, corresponding quite strictly with the 

 principal connective in the higher groups. This [connective is 

 not very evident in Pteronarchys, where the venation is of the 

 primitive type, nor in such forms as Eusthenia, where the veins 

 have come to lie parallel with the front margin, forming long 

 rectangular cells arranged as bricks in a wall. It is most 

 evident in Capnia and in the hind wings of Perla. 



The ladder-like arrangement of veins is a very characteristic 

 thing in a Perlid wing. This consists of a series of numerous, 

 closely placed cross veins, usually occupying two interspaces, 

 as in Fig. 41. This ladder serves to locate the first posterior 

 vein, the sub-median of Pictet, which is the middle vein of the 

 ladder. There is practically no difference of opinion regarding 

 the homologies of the veins in this group. 



TERMITID,^. 



The peculiarity of the venation of the Termitidas is in part 

 dependent on the extremely short period the wings are used — a 

 structure that will wear well not being required - and in part 

 on the manner of specialization, whereby the posterior two 

 thirds practically becomes veinless; all the formative material 

 is apparently appropriated by the primary and the veins 



