DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 363 



Consideration of the Forces which Propel the Wings of Insects. — Proceeding 

 now to a consideration of the forces which propel and regulate the wings of 

 insects, I find that in the thorax of the insect the muscles are arranged in two 

 principal sets in the form of a cross — i.e., there is a powerful vertical set which 

 runs from above downwards, and a powerful antero-posterior set which runs 

 from before backwards. There are likewise a few slender muscles which proceed 

 in a more or less oblique direction. The antero-posterior and verti'^al sets of 

 muscles are quite distinct, as are likewise the oblique muscles. Portions, how- 

 ever, of the vertical and oblique muscles terminate at the root of the wing in 

 jelly -looking points which greatly resemble rudimentary tendons, so that I am 

 inclined to believe that the vertical and oblique muscles exercise a direct 

 influence on the movements of the wing. The contraction of the antero-pos- 

 terior set of muscles (indirectly assisted by the oblique ones) elevates the 

 dorsum of the thorax by causing its anterior extremity to approach its posterior 

 extremity, and by causing the thorax to bulge out or expand laterally. This 

 change in the thorax necessitates the descent of the wing. The contraction of 

 the vertical set (aided by the oblique ones) has a precisely opposite effect, and 

 necessitates its ascent. While the wing is ascending and descending the oblique 

 muscles cause it to rotate on its long axis, the bipartite division of the wing 

 at its root, the spiral configuration of the joint, and the arrangement of the 

 elastic and other structures which connect the pinion with the body, together 

 with the resistance it experiences from the air, conferring on it the various 

 angles which characterise the down and up strokes. The wing may therefore 

 be said to be depressed by the contraction of the antero-posterior set of 

 muscles, aided by the oblique muscles, and elevated by the contraction of 

 the vertical and oblique muscles, aided by the elastic ligaments, and the reac- 

 tion of the air. If we adopt this view we have a perfect physiological expla- 

 nation of the phenomenon, as we have a complete circle or cycle of motion, 

 the antero-posterior set of muscles contracting when the vertical set of muscles 

 are relaxing, and vice versa, an arrangement which gives an equal period of 

 activity and repose to both sets. This, I may add, is in conformity with all 

 other muscular arrangements, where we have what are usually denominated 

 extensors and flexors, but which, as I have shown elsewhere," are simply the two 

 halves of a circle of muscle and of motion, an arrangement for securing diametri- 

 cally opposite results in limbs and the condition of activity and rest in muscles. 

 Chabrier's account, which I subjoin, virtually supports this hypothesis : — 

 " 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 simultaneously. Thus, when it is elevated, it carries 



On the Mechanical Ajjpliances by Avhich Flight is attained in the Animal Kingdom, Trans. 

 Linn. Society, vol. xxvi. pages 200, 201, and 262. 



VOL. XXVI. PART II. 5 B 



