GEOMETERS. 
459 
any height, by means of a silken thread, ■which they spin 
from their mouths ■svhile falling. Whenever they are dis- 
turbed they make use of this faculty, drop suddenly, and. 
hang suspended till the danger is past, after which they 
climb up again by the same thread. In order to do this, 
the span-worm bends back its head and catches hold of the 
thread above its head with one of the legs of the third 
segment, then, raising its head, it seizes the thread with its 
jaws and fore legs, and, by repeating the same operations 
with tolerable rapidity, it soon reaches its former station 
on the tree. These span-worms are naked, or only thinly 
covered with very short down; they are mostly smooth, 
but sometimes have warts or irregular projections on their 
backs. They change their color usually as they grow older, 
are sometimes striped, and sometimes of one uniform color, 
nearly resembling the bark of the plants on which they are 
found. When not eating, many of them rest on the two 
hindmost pairs of legs against the side of a branch, with the 
body extended from the branch, so that they might be mis- 
taken for a twig of the tree; and in this position they will 
often remain for hours together. 
When about to transfonn, most of these insects descend 
from the plants on which they live, and either bury them¬ 
selves in the ground, or conceal themselves on the surface 
under a slight covering of leaves fastened together with 
O O O 
silken threads. Some make more regular cocoons, which, 
however, are ver}’ thin, and generally more or less covered 
on the outside with leaves. 
i 
The cocoons of the European, 
tailed Geometer (Ourapteryx 
mmhucana)^ which lives on 
the elder, and of our chain- 
dotted Geometer (Greometra 
catenaria)^ (Fig. 225, Fig. 226 
cocoon, Fig. 227 larva,) which 
is found on the wood-wax, are made with regular meshes, like 
Fig. 225. 
