190 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



The wing, then, encounters at least 10 times the resistance in fan- 

 nin<T that it does in gliding through the air. It should be said that 

 this last experiment is somewhat crude, for the wing necessarily 

 could not be made to cut the air with that delicate precision Avhich 

 is probably realized by the insect in flight. I should not be 

 surprised, if in nature the insects encountered at least 20 times 

 the resistance in beating the air, that they do in merely gliding 

 through it. 



Concerning Mr. Kellogg's supposition, that the scales were devel- 

 oped to "protect and to strengthen the wing-membranes," I will 

 admit that they may serve in some slight degree to protect the wing- 

 membranes from scratches, etc. ; but I am unable to accept his con- 

 clusion, that they strengthen the wing-membranes, any more than 

 that tlie shingles upon a roof serve to add strength to it. The 

 wing-membranes themselves are tough, elastic, and not easily torn or 

 scratched, and the scaleless wings of the Xeuroptera and Hjine- 

 noptera are very rarely found torn or scratched in nature. 



In 1858 Mr. Alexander Agassiz called attention ('59) to the fact, 

 that " the nervures of the wings of butterflies are so arranged as to 

 give the greatest lightness and strength ; they are hollow, with theu- 

 greatest diameter at the base of the wing, the point of greatest 

 strain, their diameter gradually diminishing to the edge of the 

 membrane. If a section be made across such a wing parallel to the 

 axis of the body, we find very much the arrangement which has 

 been experimentally proved by Fairbain and Stephenson as giving 

 the greatest strength of beams, as exemplified in the tubular bridge. 

 We find the strongest nervure placed either on or near the anterior 

 edge of the upper wing ; there is no such nervure on the lower 

 wing, all being of nearly the same size, as such a one would have 

 prevented the elasticity of the wing from assisting the flight to 

 any considerable extent." Mr. Agassiz has informed me that he 

 carried out an extensive series of experiments upon the rigidity of 

 the Avings of various species of Lepidoptera. He placed little 

 platinum strips upon the wings and observed the extent of the 

 bending produced. His results demonstrated that the Sphinx moths 

 possess by far the strongest wings, and that the Danaoid and 

 Acraeoid Heliconidae have very weak wings. The reason for this 

 probably lies in the fact, that the Sphinx moths move their wings 

 with great rapidity, while, according to Bates ('62) and all sub- 

 sequent observers, the Heliconidae have a slow flight. 



