LEPIDOPTERA. 357 



over the ground, step by step, as they proceed. Most of these 

 caterpilkirs have only ten legs; namely, six, whicli are jointed 

 and taperhig, under the fore part of the body, and four fleshy 

 proplegs, at the hhider extremity ; the three intermediate pairs 

 of proplegs being wanting. Consequently, in creeping, they 

 arch up the back while they bring forward the hinder part of 

 the body, and then resting on their hind legs, stretch out to 

 their full length, in a straight line, before taking another step 

 with their hind legs. Some of the Geometers have twelve or 

 fourteen legs; but the additional proplegs are bo short that the 

 caterpillars cannot use them in creeping, and their motions are 

 the same as those that have only ten legs. Some caterpillars 

 with fourteen legs, and wanting only the terminal pair of 

 proplegs, are placed in this tribe on account of the resemblance 

 of their moths to those of the true Geometers. The latter live 

 on trees and bushes, and most of them undergo their transfor- 

 mations upon or in the ground, to reach which, by travelling 

 along the branches and down the stem, would be a long and 

 tedious journey to them, on account of the deficiency of their 

 legs, and the slowness of their gait. But they are not reduced 

 to this necessity; for they have the power of letting themselves 

 down from any height, by means of a silken thread, which 

 they spin from their mouths while falling. Whenever they 

 are disturbed 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, 



