174 NORTH AMERICAN INSECTS. 
they may alight. They particularly infest these places, be- 
cause there are so few of the birds there which feed par- 
ticularly upon them. 
But the ravages of these insects are not confined to our 
shade trees, for they feed also upon the leaves of our fruit 
trees, as well as of elms, poplar, lime, and other trees. 
When fully grown they are forced by nature to go to the 
ground, in order there to undergo their metamorphosis into 
a cocoon; and as they are not provided with sixteen feet, 
like other caterpillars, they are very poor pedestrians, and 
find it much easier to let themselves down to the ground by 
means of the silken thread which issues from their mouth 
as they need it. They sometimes descend from a height 
of more than fifty feet in a few moments, while, if they were 
obliged to depend upon their ten, or, at most, twelve legs, 
the journey would occupy them several days. 
Caterpillars generally have sixteen legs, or feet, placed at 
equal distances along the under part of the body; but these - 
have only five or six feet at each extremity, and none under 
the middle, so that when they walk they stand on the hind 
feet, and throw their fore feet and body as far ahead as its 
" length will allow ; then, standing on their fore feet, they draw 
up the hind ones to them, making an arch of the footless 
centre of the body. This process, it is evident, must be 
slow; and it is probably on account of this singular method 
of locomotion, which resembles somewhat that of spanning 
or measuring, that they received from Linneus the name 
Geometre, and from other authors the names “ Measurers,” 
*¢Span-worms,” and ‘ Tailors.” 
In the United States we find a great variety of these 
caterpillars, all of which are in their season metamorphosed 
into small moths, the most conspicuous of which is: 
The CanKkER-worM (Anisopteryx pometaria). The cater- 
pillars of this moth are usually hatched from their eggs in 
the spring, and when very yeung are of a dark-brown color, 
