FLIGHT OF INSECTS. 357 



add considerably to their weight, and impede 

 the velocity of their action. This inconvenience 

 appears to have been in a great measure com- 

 pensated by the greater size of the wings, and by 

 the extent of the surface with which they strike 

 the air. Still, however, it is sufficiently obvious 

 that insects of this order fly with less rapidity 

 and steadiness than most others. But this un- 

 steadiness, again, is turned to good account ; for 

 the butterfly, by its irregular and apparently 

 capricious movements, alternately dipping and 

 rising in the air, so as to describe a series of zig- 

 zag lines, more easily eludes capture when pur- 

 sued, not only by naturalists, but also by birds 

 that are eagerly seeking to secure them. It is 

 astonishing to what a distance the silk worm 

 moths will fly : some have been known to travel 

 more than a hundred miles in a short time. The 

 Papilio Iris often rises to so great a height in 

 the air as to be quite invisible. 



A mechanical contrivance is adopted in many 

 of the Lepidoptera for keeping their wings 

 steady during flight, consisting of a hook 

 covered with hair and scales, attached to the 

 under side of the upper wings near their base, 

 and connected also by means of bristles to the 

 base of the lower wing: by this attachment all 

 the wings are locked together and brought into 

 action at the same time. Insects of the Sphinx 

 tribe are also provided with a kind of rudder 



