IN RELATION TO AERONAUTICS. 329 



a duplicatnre of intcgnment, or investing membrane, and are strengthened in 

 various directions by a system of hollow, horny tubes, known to entomologists 

 as the nenrae or nervures. These nervures taper towards the extremity of the 

 wing, and are strongest towards its root and anterior margin, where they supply 

 the place of the arm in bats and birds. 



The neurae are arranged at the axis of the wing after the manner of a fan or 

 spiral stair ; the anterior one occupying a higher position than that further back, 

 and so of the others. As this arrangement extends also to the margins, the 

 wings are more or less twisted upon themselves, and present a certain degree of 

 convexity upon their superior or upper surface, and a corresponding concavity on 

 their inferior or under surface ; their free edges supplying these fine curves 

 which act with such efficacy upon the air in ol)taining the maximum of resist- 

 ance and the minimum of displacement. As illustrative examples of the form 

 of wing alluded to, that of the beetle, bee, and fiy may be cited ; the pinion in 

 those insects acting as helices, or twisted levers and elevating weights, much 

 greater than the area of the wing would seem to warrant. The insects adverted 

 to fly, as a rule, with great accuracy and speed, and frequently in a straight line. 



From the foregoing account it is evident that the wings of insects vary as 

 regards their number, size, and shape. They also differ as regards their sur- 

 faces, margins, venation, degree of consistence and position, so that it might 

 naturally be asked, Do the several orders of wings act according to a common 

 principle, or does each wing act according to a principle of its own ? There 

 can, 1 think, he but one answer to this question. All wings obtain their lever- 

 age by presenting oblique surfaces to the air, the degree of ol)liquity gradually 

 increasing in a direction from behind, forwards and downwards, during extension 

 when the sudden or effective stroke is being given, and gradually decreasing in 

 an opposite direction during flexion, or when the wing is being more slowly 

 recovered preparatory to making a second stroke. The ettective stnjke in insects — 

 and this holds true also of birds — is therefore delivered doiciucards and fortcards, 

 and not, as the majority of writers believe, vertically, or even slightly back- 

 wards. This arises from the curious circumstance, that insects and birds when 

 flying actually fall through the medium which elevates them, thein' course being 

 indicated by the resultant of two forces, viz : that of gravity, pulling vertically 

 downwards, and that of the wing, acting at a given angle in an upward direc- 

 tion. The wing of the bird acts after the manner of a boy's kite, the only dif- 

 ference being that the kite is pidlcd forward upon the wind by the string and 

 the hand, whereas in the bird the wing is pushed forward on the wind by the 

 weight of the body and the life residing in the pinion itself. The angle at 

 which the wing acts most efficaciously as an elevator, as proved by an examina- 

 tion of the pinion of the living insect, bat, and bird, when fully extended and 

 ready to give the effective stroke, is an angle of 45° with the horizon. As, 

 however, this angle could not be uniformly maintained without a rotary motion 

 which would wrench the wings from their fixings, a compromise is adopted, the 

 wing being made to rotate on its axis to the extent of a quarter of a turn in one 

 directi(Ui during extension, and the same amount in an opposite direction during 

 flexion. That the wing rotates upon its axis as explained may be readily ascer- 

 tained by watching the movement in the larger domestic fly. If the insect be 

 contemplated either from above or beneath, the blur presented by the rapidly 

 oscillating wing will be found to be concave, the depressed portion representing 

 the wing when its plane of least resistance is parallel with the plane of progres- 

 sion. Of tins I have had the most convincing proof, particularly in semi-torpid 

 insects where the wing was plied with less vigor than usual. To confer on the 

 wing the multiplicity of movement which it requires, it is supplied with a double 

 hinge or compound joint which enables it to move not only in an upward, down- 

 ward, forward, and backward direction, but also at various intermediate degrees 

 of obliquity. An insect furnished with wings thus hinged may, as far as steadi- 



