460 



PHYSIOLOGY. 



by the irregularity of the nervures, for upon the anterior margin 

 stiff, firm and inflexible nervures are found, but upon the posterior 

 margin there are none, and in its vicinity there are only soft, thin and 

 flexible ones. Also the oblique position of the wing to the direct plane 

 of its motion effects the entire progression in the air ; so that by the 

 pressure of the air going obliquely against the surfaces of the wing, it 

 acts like any other power upon an oblique plane which admits of being 

 divided into two so called parallelograms of force, one of which is lost, 

 but the other acts perpendicularly, yet somewhat less effectually than 

 the original force. An equal force, which, like that of the stroke of 

 the wing, presses downwards, presses also upwards in the stroke, which, 

 likewise, may be divided into two forces, one of which is lost. We 

 thereby acquire, therefore, two moving forces, both of which, it is true, 

 stand perpendicularly to the wings, but yet cut each other in their 

 direction, as the posture of the wing is different in its rising and sinking 

 These two moving forces consequently form, when we add to them 

 their parallels, a third parallelogram of forces, and the diagonal of 

 this parallelogram, drawn through the angles where both forces meet 

 in the horizontal plane fixed by the centre of gravity, describes the 

 line of flight. 



Had not nature concurred in all these adaptations, had, for instance, 

 the surfaces of the wings stood at right angles to the plane of motion of 

 the wings, progression in the air could not have taken place, but the 

 insect must necessarily have stopped short in the air upon the very first 

 stroke of its wings, as the pressure from above and beneath would have 

 been opposed in a linear direction, and, in consequence of the rapidity 

 of the motion of the wings, would have neutralised each other. But 

 this is actually the case in some volatile motions, namely, in hovering, 

 or the stopping at one spot in the air. The insect can give voluntarily 

 such a posture to the wings, that the propelling forces oppose each 

 other in a linear direction, and the consequence of which is, that it 

 remains hovering at one spot in the air. But it requires much exertion, 

 whence it is that the strokes of the wings follow each other more 

 rapidly, and the buzz during it is shriller and louder. We particularly 

 observe this capacity of hovering in the Diptera, which, in consequence 

 of the narrowness of the base of their wings, possess the power of moving 

 the wings on all sides, and among them again we observe it most perfect 

 in the Hombylii, Anthracodea, and Syrphodea, likewise in many true 

 genera of flies, viz. Miftogramma. This order also is distinguished 



