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GEOMETRIC. 



This family admits of a more explicit definition 

 than most of those previously considered. Although 

 it is not in the perfect insect that the most marked 

 peculiarities appear, we can seldom fail at once to 

 recognise them, even in that state, by their general 

 weakness of appearance, attenuated form of body, 

 and comparatively ample expanse of wing. But 

 the singular mode in which the caterpillar advances 

 over a surface, as if measuring it by successive 

 spans, is the most distinguishing peculiarity, and 

 has suggested the name. In typical examples they 

 have three pair of pectoral legs, a single pair of 

 abdominal prolegs, and a pair of anal ones. With 

 the two latter of these they fix themselves to the 

 plane of position, then move the body forwards as 

 far as possible and rest on their pectoral legs, when 

 the hinder part is drawn forwards to the head and 

 the body curved upwards into a kind of loop ; the 

 same process is again repeated, and a rapid progres- 

 sion thus effected. " Their muscular power is very 

 great, and hence their positions during repose are 

 striking. Fixing themselves by their anal feet 

 alone, they extend their bodies in a straight line, 

 holding them in that position for a great length 



