284 NATURAL HISTORY. 
fifty grubs or caterpillars to their nests as food for them- 
selves and their young. Now, if there were only one 
million of these birds, of which each one devours 6000 
caterpillars in the months of April, May, June, and July 
(by no means a large computation), the number of cat- 
erpillars and grubs thus destroyed will amount to 
6,000,000,000 annually.” 
482. Caterpillars are all spinners, the thread coming 
from a fleshy point in the under lip. Besides the em- 
ployment of this spinning machine in making the cocoon 
for the pupa state, many of them also use it as a means 
of escape from their enemies, letting themselves sudden- 
ly down by the thread they spin to a place of safety. If 
a bird espies one in a rolled-up leaf, he may not secure it, 
for, as he puts in his bill at one end, the caterpillar may 
escape at the other, dropping itself down quickly as far 
as it pleases. 
483. Most of the caterpillars are solitary in their hab- 
its, but some live in Societies. This is the case with the 
Tent-caterpillars. These spin large tents of silk in the 
branches of trees, which are water-proof, although they 
are so slight in their appearance. They increase very 
fast, and, if let alone, colonies from the original commu- 
nity will spread their web-like tents in all parts of the 
tree. 
484. Of the caterpillars called Spanworms there are 
many species. The most conspicuous is what is common- 
ly called the Canker-worm, so destructive to many fruit 
and shade trees from devouring their leaves. These cat- 
erpillars finish their work of devastation in June, when 
they are only four weeks old, and descend by their silk- 
en cords to the ground, which they enter to the depth of 
several inches. Here they pass into the pupa state. In 
the autumn they issue from the ground in the imago 
state. The female is wingless, and therefore must climb 
up the trunk of the tree to lay her eggs on the branches, 
which she does in clusters of a hundred or more. There 
