_Straus-Durckheim’s Views._--Durckheim believes the insect abstracts from the air by means of _the inclined plane_ a component force (composant) which it employs _to support_ and _direct_ itself. In his Theology of Nature he describes a schematic wing as follows:--It consists of a _rigid ribbing_ in front, and _a flexible sail_ behind. A membrane so constructed will, according to him, be fit for flight. It will suffice if such a sail _elevates_ and _lowers_ itself successively. It will, of its own accord, dispose itself as an inclined plane, _and receiving obliquely the reaction of the air_, it transfers _into tractile force_ a part of the _vertical impulsion it has received_. These two parts of the wing are, moreover, equally indispensable to each other. If we compare the schematic wing of Durckheim with that of Borelli they will be found to be identical, both as regards their construction and the manner of their application. Professor Marey, so late as 1869, repeats the arguments and views of Borelli and Durckheim, with very trifling alterations. Marey describes two artificial wings, the one composed of a _rigid rod_ and _sail_--the rod representing _the stiff anterior margin_ of the wing; the sail, which is made of paper bordered with card-board, _the flexible posterior portion_. The other wing consists of a _rigid nervure_ in front and behind of thin parchment which supports _fine rods of steel_. He states, that if the wing only elevates and depresses itself, “_the resistance of the air_ is sufficient to produce all the other movements. In effect the wing of an insect has not the power of equal resistance in every part. On the anterior margin the extended nervures make it _rigid_, while behind it is fine and _flexible_. During the vigorous depression of the wing the nervure has the power of _remaining rigid_, whereas the _flexible portion_, being pushed in _an upward direction_ on account of the resistance it experiences from the air, _assumes an oblique position_, which causes the upper surface of the wing _to look forwards_.” ... “At first the plane of the wing is parallel with the body of the animal. It lowers itself--the _front part_ of the wing _strongly resists_, the sail which follows it _being flexible yields_. Carried by the ribbing (the anterior margin of the wing) which lowers itself, the sail or posterior margin of the wing being raised meanwhile by the air, which sets it straight again, the sail will take an intermediate position, and _incline itself about 45° plus_ or _minus_ according to circumstances. The wing continues its movements of depression inclined to the horizon, but the impulse of the air which continues its effect, and naturally acts upon the surface which it strikes, has the power of resolving itself into two forces, _a vertical_ and _a horizontal force_, the first suffices _to raise_ the animal, the second to _move it along_.”[117] The reverse of this, Marey