224 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



trachea? of the trunk, some have regarded them as being connected with the respiratory 

 system, whilst others have looked upon them as the receptacles of a subtle fluid, which the 

 insect can introduce and withdraw at pleasure to obtain the requisite degree of expansion 

 and tension in the wing. Neither hypothesis is satisfactory, as respiration and flight can be 

 performed in their absence. They appear to me, when present, rather to act as 





m 



stays or stretchers, in virtue of their rigidity and elasticity alone 



their 



ment being such that they admit of the wing being folded in various directions, if 

 necessary, during flexion, and give it the requisite degree of firmness during extension. 

 They are, therefore, in every respect analogous to the skeleton of the wing in the bat and 

 bird. In those wings which, during the period of repose, are folded up beneath the 



elytra, the mere extension of the wing in the dead insect, where no injection of fluid can 

 occur, causes the nervures to fall into position, and the membranous portions of the wing 

 to unfurl or roll out precisely as in the living insect, and as happens in the bat and bird. 

 This result is obtained by the arrangement of the neurge at the axis of the wing, the an- 

 terior one occupying a higher position than that further back, as in the leaves of a fan. 

 The spiral arrangement occurring at the axis extends also to the margins, so that wings 

 which fold up or close, as well as those which do not, are twisted upon themselves, 

 and present a certain degree of convexity in the one direction and concavity in the other, 

 their free edges supplying those fine curves which act with such efficacy upon the 

 air in obtaining the maximum of resistance and the minimum of displacement. As 

 illustrative examples of the form of wing alluded to, those of the Sphinx Moth (Plate XIII. 

 fig. 19), Beetle (Plate XIII. fig. We), and Ply (Plate XIII. fig. 20) may be cited,— the 

 pinions in those insects acting as helices or twisted levers (Plate XV. fig. 68), and ele- 

 vating weights much greater than the area of the wing would seem to warrant. The in- 

 sects adverted to fly, as a rule, with great accuracy and speed, and frequently in a straight 

 line. The twisting of the wing upon itself before and during its action, to which I have 

 directed attention, occurs also in the wing of the bat (Plate XV. fig. 69) and bird 

 (Plate XV. fig. 70), and has not, so far as I am aware, been adverted to in any of the 

 existing treatises on flight. It is occasioned in the bat and bird by the spiral configura- 

 tion of the articular surfaces of the bones of the wing, and by the rotation of the bones of 

 the arm, forearm, and hand upon their long axes. In the insect it is due to the conforma- 

 tion of the shoulder-joint, this being furnished with a system of check-ligaments, and with 

 horny prominences or stops, set, as nearly as may be, at right angles to each other, and 

 fashioned so as to necessitate the wing acting in the manner specified. The view here pro- 

 mulgated is discussed at length, when describing the wing of the bird, at pages 240-251 

 inclusive, and is calculated, it appears to me, to throw additional light on the theory and 

 practice of flight. The twisting is least marked in those insects and birds whose wings are 

 large as compared with their bodies. The butterfly may be cited as an example. Here the 

 wings are unusually ample and, as a consequence, unusually flattened (Plate XIII. fig. 

 ~~. The flat form of wing, however, is not calculated to act with velocity and precision 

 or to impress the air favourably. In other words, it does not respond to the rotation 



which occurs in 



2 



all wings during flexion and extension to the same extent as the mor 

 twisted form, and as a result it smites the air clumsily— a circumstance which, taken i 



