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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 
