15 
way into well-formed, compact heads. When the heads are forming, 
and they commence work on the outer clasping leaf, this appears to 
have the effect to cause it to partially open or withdraw from the 
head, which enables the worm to work more freely. This also has a 
tendency to prevent the ^cabbage from heading. 
Notwithstanding the butterfly appears to be aggressive in its nature, 
in this country at least, driving away by its simple presence or in 
some other way, our native congeneric species, the larva on the other 
hand is of a quiet and peaceful disposition, avoiding intrusion upon 
each other and upon other insects. It is with much hesitancy it passes 
from its leaf to another; and Dr. Fitch noticed that if an aphis was 
located on a leaf near where the worm was feeding, the latter would 
without encroaching upon or in any way molesting it leave untouched a 
portion of the leaf a half an inch or more in extent around the aphis. 
I have observed an aphis walking freely over one of them without it 
making any effort to disturb or cast it off. 
Although apparently so tender and delicate in its organization it is 
exceedingly tenacious of life; acrid substances which will readily destroy 
many other caterpillars appears to have but little or no effect upon 
it. I have noticed them eating away though covered with pulverized 
lime; Dr. Fitch has observed the same thing with reference to white 
hellebore, and yet this will readily destroy the currant worm or saw- 
fly larva. The same author also states that on one occasion he found 
one frozen in a cake of ice which after being thawed out revived and 
completed its transformations. How it is possible for an insect appa¬ 
rently so tender and composed almost entirely of fluids, to survive 
under such a degree of cold is a difficult problem to decide. Cab¬ 
bage is capable of surviving very severe cold, and hence we presume 
is a native of a northern climate, it is therefore possible that it com¬ 
municates this property to the worm which feeds upon it. 
When it has completed its growth, admonished doubtless by some 
peculiar feelipg that it is about to undergo a change, it becomes un¬ 
easy and commences running to and fro seeking some place of retire¬ 
ment where it may safely pass the pupa or chrysalis state. For this 
purpose it selects, if it be found, some place above ground, that 
is somewhat protected, as the underside or margin of a rail, board or 
projecting substance, apparently preferring dry woody substances. I 
have observed them climbing the sides of a two-story house to the 
eaves before content with the position. Having found a place that 
is satisfactory, the first act of the catterpillar is to spin a little mat on 
the surface, in which it can fix the hooklets of its feet as a means of 
support. It next spins a strong silken cord across the middle of its 
body fastening it firmly on each side to the plank or rail on which' it 
is placed. Soon after this operation is through, the worm commences 
contracting in length, especially its anterior portion; the color also 
changes from the bright green to a dull or dirty yellowish-green ap¬ 
proaching more and more to the dull gray of the pupa. The outer 
skin during this time, is loosened from the pupal skin within which 
by this time it is completely formed; it now gives way before the 
pressure of the insect within and splits open along the head and front 
part of the back. The pupa proper now appears and having relieved 
itself of the old larval skin, assumes the chrysalid form of the 
species. 
