GEOMETRIC CATERPILLARS. 321 



placed far back, and a pair of anal prolegs. In a few 

 instances, the rudiments of one or two pairs of addi- 

 tional prolegs are to be seen in front of the ordinary 

 single pair, but these are never of any use to the 

 creature in walking, and its movements are always 

 effected in the way above described, by alternately 

 attaching the limbs of the fore and hind parts of its 

 body, extending the intervening portion and drawing 

 it up into a loop. From this peculiarity the cater- 

 pillars have received the common English name of 

 Loopers, and for the same reason the term Geometra 

 was long ago applied by Entomologists to the Moths 

 of this group, from their caterpillars apparently mea- 

 suring the ground they passed over ; hence the tribe 

 is denominated GEOMETRINA. 



This peculiar mode of walking is not, however, the 

 only singular habit to which the position of the pro- 

 legs in these caterpillars gives rise. Their attitude 

 in repose is very remarkable ; they attach themselves 

 to a branch by their four prolegs, and then extending 

 the body in a straight line, remain so perfectly stiff 

 and motionless, that even in a careful examination 

 they may often be mistaken for dead twigs ; and this 

 deception is greatly facilitated in many species by the 

 colour and texture of their bodies, which, exhibiting 

 various protuberances and warts, present the closest 

 possible resemblance to the twigs of trees. It is ge- 

 nerally supposed that this position is assumed by 

 these larvae for the purpose of deceiving their enemies, 

 and perhaps with birds it might be successful, but I 

 suspect that none but a very green Ichneumon would 

 be misled by the simulation. Many of the larvae also, 

 when alarmed, immediately drop from their position, 

 suspending themselves, like spiders, by a silken thread, 



