116 



SOME POINTS IN INSECT MECHANICS. 



folded can then be completely covered by the elytra. The muscle 

 operating the folding part is contained in an enlarged nervure at 

 the base of the wing. 



Some of the beetles with short elytra and some of the Ortho- 

 ptera (notably earwigs) have two such joints on the length of the 

 wing so that the latter when folded is only one third as long as 

 when extended. 



The folds in such wings are extremely complex and can be 

 more or less represented by a fan at the end of a handle, half the 

 length of the fan itself. The wing-membrane must be supposed 

 to be continued from the fan proper to the base of handle. Each 

 rib of the fan is jointed at its half length, but the axis of these 

 joints is in the plane of the fan and at right angles to the rib. 

 See text-fig. 6. 



Text -figure 7. 

 A 



Text-figure 8. 

 ^ O 



Text-figs. 7 & 8.— To illustrate the triple folding of wing of Earwig. 

 The letters mark identical points in the expanded and folded conditions. 



In folding, the fan is first shut and the leading edge turned 

 through nearly 180°, but before this motion is completed the 

 half-way joint comes into action so that the outer edge of the 

 membrane is now under the joint of the fan. See text-fig. 7. 



In the case of earwigs, at any rate of the species foiind in 

 England, it seems more than doubtful whether the wings are 

 ever used. The thorax only contains traces of flight muscles, and 

 I have not been able to distinguish any folding muscles in the 

 wings themselves. 



The ancestors of the present species probably had working 

 wings which for some reason have now fallen into disuse, j 



