1926 ] 
Tilvyard’s Work on Insect Phytogeny 
95 
further back in geological time and have indicated the steps in 
the evolution of the clavus. The original position of the wings at 
rest in these primitive Hemiptera was almost certainly stegop- 
terous. The folding of the wings flat on the back was accom- 
panied by the shortening and widening of the clavus and the 
thinning of the membrane distal of a line drawn transversely 
from the tip of the clavus to a point on the costal margin, that is, 
of those portions of the fore-wings which overlapped in the 
resting position. Thus was developed the hemielytral condition 
reaching its culmination in the typical heteropterous fore-wing. 
Conversely the lengthening and narrowing of the clavus led to 
the evolution of the type which reaches its highest development 
in the tegmen of the Homoptera Auchenorrhyncha. 
The venation of the most primitive of the Homoptera from 
the Kansan lower Permian is derivable from a condition similar 
to that seen in Copeognatha from the same beds. Thus for the 
first time we have palaeontological evidence for the view originally 
advanced by Borner from a consideration of the head structure 
and mouth-parts, that the Psocids and Hemiptera are related 
groups and that the latter with the Thysanoptera were derived 
from mandibulate ancestors by way of a Psocoid intermediate 
type. In the same complex obviously belong also the Anoplura, 
which are, however, much more closely related to modern Psocids 
through the Mallophaga. 
