136 NATURE STUDY REVIEW [10:4— April, 1914 



end, to the cover; they then drop, hanging only by the button 

 of silk and remain hanging with the anterior part curved upward 

 for, usually about another twelve hours when the caterpillar skin 

 is shed and we have the beautiful green chrysalis. The classes 

 keep a record of each caterpillar raised from the egg from the date 

 it hatches until it becomes a butterfly and know just how long it 

 remains in each stage. 



Until the summer of 1910 I seldom came across a monarch 

 caterpillar which had been stung by parasitic flies but that sum- 

 mer the greater part of those which I did not raise from the egg 

 were stung even though they were small when I found them; 

 the following summer the proportion was also large ; a caterpillar 

 would seem all right and make its button of silk preparatory 

 to becoming a chrysalis ; at the right time it would drop and hang 

 from its support as normal caterpillars do but in a few hours it 

 would straighten out, hanging head downward while soon from 

 its body white maggots about three-eighths of an inch in length 

 would drop. The usual nimiber of these was three but seven 

 issued from one caterpillar in our class room ; gradually the white 

 skin of the maggot would darken and harden imtil in about one- 

 half hour it had become the brown pupa case from which in a 

 few days a fly would emerge which a casual observer would think 

 was a house fly; the caterpillar may live several hours after the 

 parasites leave its body. 



Every one who knows anything of monarch butterflies knows 

 that they migrate southward in immense flocks in the fall. One 

 Slimmer evening as I was walking about the campus of the Min- 

 nesota College of Agriculture I came across two trees which looked 

 as if every leaf upon them had suddenly turned brown but in a 

 moment I realized that I was looking upon a flock of migrating 

 monarchs rather than upon dead-brown leaves. There was noth- 

 ing surprising in this ; I had seen such flocks before but now comes 

 something very mysterious; the next year I saw those same 

 two trees covered with another flock of migrating monarchs. 

 Why was that? Why were those same two trees chosen the 

 second year and by another flock? It must have been, "just a 

 happen so," but is it not probable that monarchs follow a certain 

 path of migration as birds do ? By what instinct they are guided 

 we know not. 



St. Anthony Park, Minn. 



