Oct, 29, 1874] 



NATURE 



wing being different in the up and down strokes ; and, as is 

 found to be the case, the thick limb is reversed by turning 

 the insect round so that it presents its other side to the 

 observer. The same conckision is arrived at by the 

 employment of the method to be now described. 

 Without sensibly interfering with the movement of the 



517 



wing, its tip may be made to come in contact with a 

 revolving cylinder, in which the surface is covered with 

 smoked paper. " Although the figures thus produced are 

 for the most part incomplete, we are able, by means of 

 their scattered elements, to reconstruct the figure which 

 has been shown by the optical method." Fig. 2 is one 



of the results obtained, in which several of the loops are a Wheatstone's kaleidophone rod, tuned to the octave, 



distinctly seen, as is generally the case when, as here, the or, in other words, vibrating twice transversely for each 



insect is so held as to rub the cylinder with the hinder edge longitudinal vibration. 



of the tip of the wing. That a figure of 8 movement of a Still we are not able to say in which direction the wing 



point, when made to record on a revolving drum, produces is moving in the different branches of the 8 figure ; the 



a similar curve, is seen from Fig. 3, which is a tracing of following simple experiment determines this completely. 



A slender glass rod is smoked at the tip in the flame of a 

 candle, and held at right angles to the direction in which 

 the wing moves, in the different parts of the wing-tip 

 tract, as in Fig. 4. It is evident that if the wing hits 

 the rod whilst it is descending, it will rub off the smoke 

 film from its upper, and whilst ascending, from its lower 



surface. Supposing that, in the figure, the head of the 

 insect is directed to the right : when the glass rod enters 

 the loops at b' and a it is found to strike the upper sur- 

 face ; when at b and a', the lower; consequently the arrows 

 indicate the true direction of the wing's motion. 



The foregoing facts, when taken in connection with 



their known anatomical arrangements, place us in a 

 position to discuss the mechanism of the flight of insects. 

 These animals possess muscles, &c,, which produce direct 

 downward and upward movements of the wings, and these 

 movements only ; therefore the expansion of this vertical 

 line into a figure of 8 must be caused by forces acting 

 external to the thoracic or wing-moving mechanism ; in 



other words, by peculiarities in the structure of the wings 

 themselves. Simple inspection of the wing of a fly shoivs 

 it to be formed of a rigid, or comparatively rigid, anterior 

 nervure, which supports a thin more yielding membrane 

 behind it. In its descent, the resistance of the air retards 

 the movement of the more yielding posterior portion of 

 the wing sufficiently to cause the lower surface of its 



