GEOMETERS. 459 



any height, by means of a silken thread, which they spin 

 from their mouths while 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 transform, 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 

 silken threads. Some make more regular cocoons, which, 



o 7 ' 



however, are very thin, and generally more or less covered 

 on the outside with leaves. rig. 225 



The cocoons of the European, 

 tailed Geometer {Ourapteryx 

 sambucaria), 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 

 58* 



