226 Sellards — Structure of Paleozoic Cockroaches, 



as Scudder maintained, a comparatively recent acquisition. It 

 is to be noted that the plaiting as well as the fold of the hind 

 wing of cockroaches developed subsequent to the differentia- 

 tion of these insects as a distinct phylum. A comparatively 

 broad anal expanse was, however, common to the early Orthop- 

 tera, but the plaiting itself has originated independently in 

 more than one division of the order. The same is true of 

 cross veins. It is probable that at the time cockroaches were 

 differentiated, well-marked cross veins were entirely lacking. 

 Now, on the contrary, cross veins are numerous and not 

 unlike those of other Orthoptera. 



Mechanical Principle. — The plications are doubtless devel- 

 oped largely in response to mechanical need. Mechanical 

 principles seem also to have had an influence in developing 

 cross veins. * The interchange of the circulating fluids of the 

 wing, tending to follow within established paths, probably also 

 influenced the development of cross veins. 



37 



Figure 1. — Gerablattina arcuata Sellards; illustrating venation of a 

 typical Carboniferous adult cockroach wing. 



Figure 37. — Wing of a recent nymph ; showing venation. (After Packard.) 



Recapitulation of Ancestral Characters. — The recurrence 

 during the ontogeny of the individual of characteristics found 

 in the adult condition of earlier representatives of the phylum 

 is familiar to the student of any group. The insects are not 

 an exception to the general rule. In the accompanying figures 

 the nervation of the wing of a modern nymph cockroach is 

 brought into comparison with a typical Carboniferous adult. 



*It is interesting to notice an analogous progressive evolution in the vege- 

 table kingdom. The Carboniferous flora contains a great majority of simple 

 veined leaves, while at the present time netted-veined leaves predominate. 

 In the case of plants, both mechanical and physiological factors have doubt- 

 less operated. 



