PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 2-11 

 Fijr. 7. 



This figure shows from the tracing of the wing of a wasp the upper 

 loop and tlie whole extent of one of the branches of the figure of eight. 

 The median portion of this branch is only dotted on account of the 

 feeble friction of the wing. We may, therefore, be permitted to con- 

 clude that if the trace of an insect's wing could be obtained entire at 

 one operation, the same figure would be presented which we have seen 

 described in space by the gilded spot on the wing of the wasp, namely, 

 a figure of eight, which our ingenious acoustician, Koenig, was the first 

 to obtain with a spiral Wheatstone's rod, making two horizontal to one 

 vertical oscillation. 



It now appears to me sufficiently established that in the more ex- 

 tended motions of flight the wings of insects describe a figure of eight 

 in space. Furthermore, that the luminous figure which a speck of gold 

 on a wing i^resents in its motions has shown us that the periods of 

 nscent and descent of the wing are accompanied by a change of plane 

 in that organ. It is this fact which will shortly enable us to exi)lain the 

 mechanism of flight in insects. 



III. — Mechanism of the flight of insects — how they peopel 



THEMSELVES. 



The preceding lessons have been devoted to the study of the frequency 

 and the form of the strokes of the wings of insects. You have seen 

 that the frequency varied in diftercnt species, and in passing from the 

 butterfly, for example, to the house fly, or the gnat, the variations may be 

 considerable. The flight of the butterfly is slow, the strokes of its wings 

 succeed each other at considerable intervals, propelling it by bounds 

 and jerks, and producing an irregular and capricious flight. The gnat 

 darts with rapidity straight at its object, emitting along its path a clear, 

 sharp, strident sound. Between these two extremes we find all inter- 

 mediate stages. Furthermore, the same insect, under different condi- 

 tions, varies the rapidity of its motions within extensive limits ; when 

 free from all restraint its movements are rapid and precipitous, but 

 when captured they are immediately relaxed, and although the fre- 

 quency of the movements of the wing varies, the form of the motion 

 does not change. It is in all cases the same, always a double loop, a 

 figure of eight. Whether this figure be more or less apparent, whether 

 its branches be more or less equal, matters little ', it exists, and an 

 attentive examination does not fail to reveal it. 



Before drawing from this fact the conclusions which it warrants ; before 

 extracting from it the solution of the problem with which we are occu- 

 pied — that is to say, the mechanism of flight — let us rapidly review the 

 history of the question, and see how far previous authors have advanced 

 in its solution. Without going further back, we find in the work of 

 Borelli a chapter devoted to this subject, in which he considers the force 

 which the bird or insect must employ to sustain or move itself in space. 

 IG s 



