Mar., '07] ENTOMOLOGICAL NEWS. 8l 



with its head toward the wind, or at a very slight angle to it. 

 The reason for this is perfectly obvious. It assumes the posi- 

 tion in which it offers the least resistance to the wind, in which 

 it needs to exert no muscular effort to keep its wings folded, 

 and in which the muscular effort of flight is exerted symmetri- 

 cally. Occasionally a butterfly would attempt to turn away 

 from the wind while resting upon the clusters of flowers ; but 

 it was unable to maintain this position. The wind would either 

 shift it back again, or blow the wings forward over its head, 

 thus rendering the insect helpless. 



On May 12th I was able to observe their flight in the 

 face of a gale. It was a bright, hot afternoon and the wind 

 was blowing at the rate of from 45 to 50 miles per hour. Since 

 A. olympia is a weak flier, it could not face a blast like this. 

 The wind swept directly across the crests of the low sand 

 ridges, and the only place that specimens could be found at 

 all was on the leeward sides of the dunes, or immediately 

 behind protecting groups of willows and pines which are often 

 found in the moist depressions between them. 



Protected thus from the direct force of the wind, the butter- 

 flies were seen pursuing an irregular course, dancing from side 

 to side, but always headed toward the wind, and making slow 

 progress toward the crests of the dunes. This advance was 

 continued until the crest was reached; but no sooner had one 

 reached this point than it was caught in the wind and hurled 

 backward and downward by the eddying blasts, to begin anew 

 the ascent of the leeward slope. This process was observed 

 again and again. 



It is not hard to find in this mechanically determined 

 flight a great advantage to the insect. Flight toward the wind 

 tends to keep it within the limits of its habitat. The enforced 

 retrograde movement is counterbalanced by the progress of the 

 butterfly toward the wind. If it flew with the wind, either 

 it would be blown inland, far away from the habitat of its 

 food plant, or it would perish in the waters of Lake Michigan. 



The habits described above depend, no doubt, upon struc- 

 tural characters, and do not owe their origin to natural selec- 

 tion ; for it is recognized that natural selection can be exer- 



