_Consideration of the Forces which propel the Wings of Insects._--In the thorax of insects 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 vertical sets of muscles are quite distinct, as are likewise the oblique muscles. Portions, however, 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 shortening of the antero-posterior 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 shortening 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 characterize the down and up strokes. The wing may therefore be said to be depressed by the shortening of the antero-posterior set of muscles, aided by the oblique muscles, and elevated by the shortening of the vertical and oblique muscles, aided by the elastic ligaments, and the reaction of the air. If we adopt this view we have a perfect physiological explanation of the phenomenon, as we have a complete circle or cycle of motion, the antero-posterior set of muscles shortening when the vertical set of muscles are elongating, and _vice versâ_. This, I may add, is in conformity with all other muscular arrangements, where we have what are usually denominated extensors and flexors, pronators and supinators, abductors and adductors, etc., but which, as I have already explained (pp. 24 to 34), are simply the two halves of a circle of muscle and of motion, an arrangement for securing diametrically opposite movements in the travelling surfaces of all animals. Chabrier’s account, which I subjoin, virtually supports this hypothesis:--