236 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



transversely upon the back beneath the elytra. When flight is contemplated, the elytra, 

 or first pair of wings, are raised and carried forward (Plate XIII. fig. 16 r). When they 

 make a right angle with the long axis of the body, they present their plane of least re- 

 sistance ; but by the time they travel other 45° or so (Plate XV. fig. 56), which they do 

 to get beyond the reach of the membranous or second pair of wings (Plate XIII. 

 fig. 16 e), they acquire the degree of obliquity necessary for flight, and act mechanically 

 as gliders. When the membranous or second pair of wings are extended until they 

 make a right angle with the body, they do not, as in the case of the elytra, present 

 their plane of least resistance, but an angle of 30° or so with the horizon, so that they 

 are then in a proper position for making the effective or down stroke. In some instances, 

 as in the Cockroaches, where the elytra or first pair of wings are semimembranous and 

 slightly twisted upon themselves (Plate XIII. fig. 21r), they are geared to the second pair 

 (a), so that the upper and lower wings act together to obtain a common result. The range 

 of the wing (when the angle of flight is attained) varies greatly in different insects— in 

 some equaling the quarter of a circle, in others three-eighths of a circle, and in others 

 quite a half (Plate XV. fig. 58 a a'). The wings, moreover, I have s^ood reason to believe, 



can be made to oscillate within given areas anteriorly, posteriorly, or centrally with regard 

 to the plane of the body, or in intermediate positions with regard to it and a perpendicular 

 line. The wing or wings of the one side can likewise be made to move independently of 

 those of the other, so that the centre of gravity, which, in insects, bats, and Jbirds, is sus- 

 pended, is not disturbed in the endless evolutions involved in ascending, descending, and 

 wheeling. The centre of gravity varies in insects according to the shape of the body, the 

 length and shape of the limbs and antennae, and the position, shape, and size of the pinions. 

 It is corrected in some by curving the body, in others by bending or straightening the 



limbs and antennae, but principally in all by the judicious play of the wings themseb 



To confer on the pinion the multiplicity of movement which it requires, it is supplied 

 with a double hinge or compound joint, which enables it not only to move in an upward, 

 downward, forward and backward direction, but also at various intermediate degrees of 

 obliquity. An msect furnished with wings thus hinged may, as far as steadiness of body 

 is concerned, be not inaptly compared to a compass set upon gimbals, the universality of 

 the wmg-movements rendering any elaborate attempt at balancing quite unnecessary. 

 The rapidity with which the wing oscillates is, with few exceptions, very great, the 

 velocity, as a rule, increasing as the area of the wing (as compared with that of the body) 



decreases. The wing, during its passage throuo* tho »* ;„ ™„ • * -i i a 



, . -. & i a »»*fee uuougn tne air, in many instances emits a loud 



drummmg sound or note which is high or low as the vibrations are rapid or the reverse. 

 These sounds hare been thought by some to afford an infallible indication of the number 

 of the vibrations made by the wing during any given period. Unfortunately however, 



for tins theory, the sounds m question, although frequently, are not always produced by 



wing-s truetures ; so that the pitch of the note cannot be explained on the theory of 

 vibrations as applied to acoustics. 



I have endeavoured to show that flight is secured in the insect, not because its body, 

 as compared Wl th the atmosphere, is comparatively light, but because its power, as com- 

 pared w lt h its S1 ze, 1S very great,-thi S power enabling it to apply its wings to, and with- 





