196 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The wing, then, encounters at least 10 times the resistance in fan- 
ning 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 which 
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, ete.; but I am unable to accept his con- 
clusion, that they strengthen the wing-membranes, any more than 
that the 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 Neuroptera and Ifyme- 
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 their 
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 
sarried out an extensive series of experiments upon the rigidity of 
the wings of various species of Lepidoptera. Ile 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. 
