44 University of Califoruia Publicatious. [Entomology 



This condition is seen in all wings in the delicate vein that 

 borders the posterior basal membrane and connects with the 

 edge of the postscutellum. It comes out sometimes very dis- 

 tinctly in parts of the vein where folds occur (Fig. 16). In 

 many cases a whole vein will become tracheoid, and occasion- 

 ally all the veins of a wing take on this character. Whether 

 a vein is tracheoid or not is a matter independent of its homol- 

 ogy, as the character may occur in wings of any group of insects 

 or in any particular vein. 



Tracheoid thickenings occur in both membranes independ- 

 ently of each other, as may be most clearly seen when halves 

 of the vein fail to correspond exactly in position, as is not 

 uncommon in Psocida?. In the accompanying figure (Fig. 17), 



I 



u ^ 



FIG. 17. Portion of Pfocid 

 FIG. 16. Bulla at outer, lower cross vein in wing, showing nonconformity 



Vespa, showing tracheoid markings. of halves. 



Avhere, to prevent confusion, the tracheoid structure of only 

 one lamina is indicated, the tracheoid ridges of the two do not 

 correspond. The shape of the two forks also shows that in 

 this particular case the half veins themselves could not have 

 been made to superpose. 



In their ontogenetic origin the tracheoid markings of veins 

 are due to excessive chitinization of the outer edges of the 

 transverse folds of the layer of cells that in the pupa produce 

 the veins. The markings consist of thickenings of the cuticle 

 which project on the convex side of the vein. It is these ridges 

 that, in the wings of crickets, are made use of in the formation 

 of the stridulating organ. They are developed in the same 

 position as is the hairy or scaly clothing of the veins, and are 

 related to the spines which sometimes replace the hairs on the 

 wings of insects. The wing-clasp of the front wings of the 

 species of Psocus illustrated in Fig. 18 shows a very interest- 



