1924] 
How a Beetle Folds its Wings 
255 
Represent the principal lines of stiffness in the wing by 
heavy solid bars, as in figure 3: A, B, C, D, E. As A is drawn 
away from E the part of the wing lying between them will first 
unfold like a fan, and soon become tense. I have represented 
the lines along which the tension is most important by chain- 
lines on the diagram: Al-Bl, A2-B2, B2-C, C-D, D-E. The 
flattening of the basal part of the wing is obvious enough; but 
the reason why the apex opens is not so plain. Now let us rep- 
resent the principal lines of stiffness in each folded area of the 
apex of the wing in the same way as the basal ones. There are 
four of these areas, but one is so supported by the surrounding 
ones that it will be seen to need no support, and a glance at the 
chitinizations in the wing-substance shows that in fact it has 
none. The other three may be represented by the bars K, N 
and S. Take K. The inner half of it lies between the bars A and 
B, in such a way that it lies in taut, flat membrane. So any 
tendency to bend it up or down will draw on the lines of tension 
Al-Bl and A2-B2, which will elastically support it. A working 
model can be easily made by looping two rubber bands on two 
fingers, and then running a match through the two bands (fig. 8). 
N is supported in exactly the same way. Then the tips of K 
and N in turn develop a third line of tension (K-N), which, with 
the help of A2-B2 supports S in a similar manner. It can be 
seen that the small folds in the middle of the wing are supported 
by the tension between A and B, and so have no structural 
value. In fact the arrangement of these folds varies a good deal 
in detail among forms that have essentially this manner of fold- 
ing, being considerably more complex in the Elateridse, and 
especially the Hydrophilida3, and somewhat simplified in such 
forms as Cebrio and many Buprestid*. These variations are of 
considerable value in throwing light on relationships, but do not 
affect the mechanics. In the same way the fourth of the outer 
areas, below S, (which we may call T) is supported by the tension 
between S and the apex of N, and needs no structural support of 
its own. In sum the support of the unfolded wing is due first 
to the fanlike arrangement of the supports which run from the 
base, and the tension which muscles in the base put on the first 
and last of them (A and the body behind E), and secondly that 
