BY R. J. TILLYARD. 



591 



which is correlated with the growth of the scutelluni. We are 

 bound to assume that, in the original Hemiptera, the scutellum 

 was of more normal size than it now is; and, consequenth% the 

 wings v/ere not so completely folded over one another distally as 

 they now are. In fact, we have to start from the normal type 

 of scutellum and the roof-like position of holding the wings, still 

 to be found in many Homoptera of the present dav. From this, 

 a gradual enlargement of the scutellum, together with a close 

 folding-down of the wings upon one another, leads us to the 

 condition found in recent Heteroptera. Here the dividino- line 

 marks the limit of the portions of the wing that are folded upon 

 one another; while the anal area, or clavus, borders the two pos- 

 terior sides of the large triangular scutellum. Thus we have to 

 correlate the length of the clavus with the size of the scutellum, 

 the tendency being towards enlargement as evolution progresses. 

 ►Since, in Du7istaniopsis, there is no trace of a clavus in the pre- 

 served part of the wing, and the same is 

 true of Prosbole, we have to conclude that 

 both these forms had only a small or moder- 

 ate-sized scutellum, bordered by a corres- 

 pondingly short clavus, or even, perhaps, 

 by scarcely any claval area at all. 



As soon as the overlapping of the two 

 distal parts of the wing, or membranes, has 

 been brought about, it will be clear that 

 these two parts will in future act (while 

 the insect is at rest) as one only; whereas 

 the coria of the two wings remain separate. 

 There will, therefore, be little tendency 

 towards the thickening of the membrane Text-fag. 2i.. 



at all, and the " heteroptery" might be expected to advance 

 more quickly than ever. The line of evolution would then 

 culminate in forms having a thick corium, from which all traces 



* Diagram of a Shield-bug, family Pentatomidce, to show the five parts 

 of the shield, viz., 1, the pronotum, pn; 2, the scutellum, .sc; 'A and 4, the 

 right and left coria, co; and 5, the two completely' overlapping membranes, 

 7nh, The clavus, cl, borders the scutellum on either side. 



