122 
Psyche 
[December 
wing membrane of the Protoperlaria, in size and distribu- 
tion almost identical to those of Sialis. 
D. The Thorax. 
The sternum of the endopterygote thorax differs from 
that of the primitive Exopterygota in having a reduced 
furcasternum. The coxae of the Endopterygota have 
acquired a third point of articulation, on the furcasternum. 
In the Plecoptera, the sternum is highly variable. Md&t 
studies of the thorax in this order have been made of 
Pteronarcys , or the large perlidae. In these stoneflies, 
the sternum is large and broad, and has slight resemblance 
to that of Sialis. But in some of the smaller stoneflies, such 
as Taeniopteryx , this is not the case. - v, 
The furcasternum (Fs) of Taeniopteryx ^Eig. 10 p is 
small, about the size as is that of Sialis. On both the meso- 
thorax and metathorax, it bears on each side a process 
which extends close to the coxa; these have' been termed 
furcasternal arms by Hanson (1946), who states “Al- 
though they are not articulated with the coxae, they appear 
to be adapted to offer them very strong support in the 
movement of the legs.” The sternal coxal articulation 
(ST-CX ART) of the neuropteroid thorax is probably its 
most distinctive feature ; one would expect this condition to 
have been derived from exactly such a stage as occurs in 
some stoneflies, by gradual strengthening of the association 
of the coxa and furcasternum. Once a sternal coxal articu- 
lation had become established, the thorax could be greatly 
strengthened by an infolding along the midline, reducing 
the exposed area of the furcasternum, so that the coxal 
bases become nearly contiguous. That such an infolding 
has taken place is indicated, not only by the structure of 
the furcasternum of neuropteroids, but also by the median 
longitudinal sutures on the basisterna (Fig. 11). 
It is not necessarily to be inferred from the close re- 
semblance of the thorax of Taeniopteryx to that of Sialis, 
that taeniopterygids are more primitive or more closely 
related to the Endopterygota than are other stoneflies. 
It is apparent, though, that these thoraces are constructed 
on the same general plan, and show remarkably similar 
capabilities for structural modification. 
