BUTTERFLIES OF MONTANA. 47 



The Seasonal Migration of Anosia Plexippus. 



The range of Anosia plexippus is from Patagmia to Hudson Bay. 

 It has not been found in hibernation and, considering its size and con- 

 spicuous coloring, it may well be said that it does not pass the winter 

 North of latitude 31 degrees. A careful study of its range, time of 

 appearance, and habits, shows unquestionably that there is a seasonal 

 migration of this very interesting form. 



Riley first suggested these movements, when state entomologist of 

 Missouri, in the following words: "There is a southward migration late 

 in the growing season, in congregated masses, and a northward disper- 

 sion early in the season, through isolated individuals." 



The more evident movement is in the fall, when in the Mississippi 

 valley and along the Atlantic Coast, the September air seems to be full 

 of brown butterflies, usually ap:3arently taking advantage of the cooler 

 winds from the North, though many times bravely beating into the winds 

 with a very strong flight for a butterfly. The Aster patches growing 

 along the roadside and in neglected fields seem to be the favorite places 

 of assembly. At such times a zoology class has no difficulty in provid- 

 ing themselves with material. In fact an active boy with a net can 

 catch them by the hundred in one afternoon. 



The collecting together of such numbers was formerly accounted for 

 by the presence of plenty of food. This alone seems inadequate, sinc^. 

 many fields, equally inviting, will contain a normal population. It seems 

 likely that a swarm may busy themselves in this way in the fields, await- 

 ing favorable winds. We must pause to consider the probable absence of 

 initiative on the part of the insect. It is more likely influenced by the 

 temperature, the North wind being the cooler, and, for some reason not 

 explained, awakening in the insect the dormant instinct of migration. 



An instance of the gathering of thousands of Anosia in one tree 

 and the fact that they remained there with occasional short excursions, 

 until a change of wind and temperature, was noted in Bu lington, lova, 

 the last week of September, 1905. The fict that th?se insects alight 

 on trees in such swarms as to give ther color to the tree, h^s be3n often 

 recorded. Tnis is not the only evidence of their flocking. There is the 

 best of authority for stories of southward migration. Some have ro- 

 ported clouds so dense as to at times cas!: a shadow. Dr. Scudder le- 

 lates a flight on September 2, in New Hampshire, when the southward 

 flight of a swarm was observed by himself. He states that they 

 stretched out in a thin stream that occupied hours in passing. He esti- 

 mated that in a limited field in front of him, they were passing at the 

 rate of 1,-500 per hour. It is little wonder that they are said to fill a 

 tree, bending the twigs to breaking with their weight. Granting that 

 these accounts of large numbers of Anosia in one swarm are exceptional, 

 we can see that spread over this broad land there must be countless mil- 

 lions moving southward every autumn. What advantage, if any, is 

 gained in the association of these insects in swarms, is not easily dis- 

 cerned. 



