64 Bulletin of Wisconsin Natural History Society. Yol. 1, No. 1 
had been suffering losses for a month or more, Reynard had no 
doubt been at large in that district during all that time. 
A mink, Putoriiis z'isoii, was lately captured behind a refrig- 
erator in a saloon on Ninth Street, having been discovered there 
by a dog that entered the saloon along with a customer. 
W. J. Bennetts. 
Swarming of the Milkweed Butterfly, Danais Arcfiippus* 
During the summer months — June, July and August — these 
butterflies are present in large numbers, though I have never seen 
them flying in companies of more than two or three. During the 
early part of September, however, I have noticed for the past six 
or seven years that they gathered in large migratory swarms. 
The motive of these migrations, I believe, is the impossibility of 
finding food and to avoid the cold of the winter months. The 
insects are naturally forced to seek milder climes and localities 
where plant life is more abundant. The question arises in my 
mind, however, what then might induce these insects to subject 
themselves to the dangers and trials of a northward journey in the 
spring of the year. The milkweeds are well represented in the 
south and plant life in general is more abundant than in our lati- 
tude, so that we cannot attribute their northward migration to be 
due to the lack 6i food or to cold. I believe that these annual 
migrations can be likened to those of birds of passage, and that the 
same instinct which governs these birds in their migration is also 
the cause of the same habit in Danais ar chip pus. 
In September 9 to 11, 1898, I noticed small swarms of D. A. 
in different places in the city, containing some forty or fifty in- 
dividuals, some flying, others busily engaged in sipping the horse 
brine in the street ; this they seemed particularly to relish. It is 
interesting to note how^ closely the given dates correspond with the 
following: From Aug. 29 to Sept. 6, 1899, their gatherings came 
to my notice in various parts of the city. A daily increase in their 
numbers was noticeable, particularly so at Lake Park, which it 
appeared was chosen as their place of rendezvous. Toward Sept. 
6, I was unable to note any of the smaller swarms throughout the 
city, but found immense swarms at Lake Park, where they were 
present in thousands. The females outnumbered the males con- 
siderably. Several pairs were found in copulation. A strange 
fact which came to my notice was that these insects would sit very 
closelv together, one upon each adjoining leaf, and thus several 
branches on one tree were covered with them, while the rest of the 
tree would be deserted. 
