PETTIGREW ON THE MECHANISM OF FLIGHT. 237 



draw them from, the air with astonishing velocity at various' degrees of obliquity, to 

 obtain the maximum of resistance in a downward direction, and the minimum of dis- 

 placement in an upward one — to convert them, in fact, into spiral inclined planes, with 

 which to tread the air and rise upon it, as a kite upon the wind or a swimmer upon the 

 water. I use the phrase "various degrees of obliquity" because, although Ihe effective 

 or down stroke is delivered at an angle of 30° or thereabouts, the wings act more or less 

 perfectly as elevators and propellers from the moment they leave their position of rest or 

 plane of least resistance until they make the angle referred to, and likewise during the 

 up or back stroke, when the wings are being recovered. The power of the insect is con- 

 sequently conserved and utilized to an astonishing degree. 



It only remains for me to allude briefly to the manner in which the wings are moved. 



Arrangement for moving the Wings of Insects Sfc— In all insects, with the exception 

 of the Dragonflies, the muscles which play the wings are confined within the barrel- 

 shaped thorax, to which the wings are articulated. They consist of a vert ical and a trans- 

 verse set, — the transverse set, by their contraction, compressing the cylinder laterally, 

 and causing its mesial portion to ascend, and the wings to descend ; the vertical set, by 

 their contraction, compressing the cylinder from above downwards, and causing the 

 wings to ascend, according as the vertical measurement of the thorax is reduced by 

 lateral bulging*. While the wings are ascending and descending, they are obliged to 



rotate on their long axes— the spiral configuration of the joints, and the arrangement of 

 the elastic and other structures which bind them to the body, conferring on 1 hem the various 

 degrees of obliquity which characterize the down and up strokes (vide pages 22G and 227). 

 Since the two sets of muscles act alternately, as in the auricles and ventricles of the heart, 

 the one set rests while the other is active; and it is just possible that in this and in the 

 action of the elastic ligament which recovers or flexes the wing we have an explanation, not 

 only of the prodigious power wielded by insects, but also of their endurance. In the IAbeU 

 lute or Dragonflies, the muscles are inserted into the root of the wing as in the bat and 

 bird, the only difference being that in the latter the muscles creep along t he wing to its ex- 

 tremity. In all the wings which I have examined, whether in the insect, bat, or bird, 

 the wing is recovered, flexed, or drawn towards the body by the action o\ an elastic 

 ligament, this structure, by its mere contraction, causing the wing, when fully extended 



* The following is the account given by Chabrier :_« It is generally through the intervention of the proper motions 

 of the dorsum, which are very considerable during flight, that the wings or the elytra are moved equally and simu 

 taneously. Thus, when it is elevated, it carries with it the internal side of the base of the wmgs «k winch lt . 

 articulated, from which ensues the depression of the external side of the wing; and when .t approach, the ^ 

 portion of the trunk, the contrary takes place and the wings are raised. Darmg the *~^T»* 

 dorsum is curved from before backwards, or in such a manner that its anterior extremity . *"+^£f£> 

 terior, that its middle is elevated, and its lateral portions removed further from each other. ^^^^ 

 in the elevation of the wings ; the anterior extremity of the dorsum bemg removed to a greate Stance from the 



posterior Tts mid die being d pressed, and its sides brought nearer to each other. T us its bending m one dnection 

 posterior, it middle oe g p ^ ^ ^ by ^ alternatl0n8 of this motlon , 



produces a diminution ot its curve in uie uuauuu j ti i. nr ._ vP i i 



Listed by other means, the body is alternately compressed and ddated, and the „,ngs are nosed and depr .-> 1 bj 



assistea Dy otner means, i Thorax in Insects and on Us unctions during I light, by 



turns." -General Observations on the Anatomy of the Thorax m Insects, an, •_ , 



E. T. Bennett, F.L.S., *c (Exacted chiefly from the - Essat snr le vol des Inseetes, par J. Lhabner, Mem. do 



Museum 



2 



VOL. XX VI. 



