How insects move 



cylindrical abdomen clearly divided into segments, each of which has 

 horizontal and vertical muscles. Several of the segments have fleshy 

 prolegs, which in Lepidoptera are equipped with hooks or crochets to 

 help them to grip. 



By pulling alternately on the upper (dorsal) and lower (ventral) 

 horizontal muscles, and with suitably timed contractions of the vertical 

 muscles, each pair of prolegs is hfted up, moved forward, and planted 

 down again (Fig. 48). The abdomen is thus moved forward with that 

 rippling movement that we have all seen caterpillars make. The 'looper' 

 caterpillars of the family Geometridae generally have prolegs only on 

 the sixth and tenth abdominal segments (Fig. 39), and so the second 

 pair have to reach a long way forward at each step. This causes the body 

 of the caterpillar to be arched upwards in the 'loop' that gives them 

 their name. 



Fig. 48. Diagram of the action of the muscles when a cater- 

 pillar is crawling towards the right : in segment a the upper, 

 or dorsal longitudinal muscles have contracted; in segment b 

 the vertical (dorso-ventral) muscles have contracted, raising 

 the false leg from the ground; in segment c the lower, or 

 ventral longitudinal muscles have contracted, pulling segment 

 b forward. From Wigglesworth, 1953 



Larvae of flies (maggots and grubs) have a rather similar way of 

 crawUng, but their pseudopods extend in a ring all round each segment, 

 and the larva can wriggle in any direction, not merely up and down. 

 They also make use of their mouth-hooks to grip and pull themselves 

 along (see also below, under 'jumping'). 



Just before the emergence of an adult insect of the Holometabola from 

 its pupa, the latter generally works its way up to the surface of soil or 

 wood in which it has been buried, by similar crawling movements of the 

 abdomen. Instead of prolegs, the pupa often has rows of backwardly 



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