V. HOW INSECTS MOVE 



{Crawling; walking and running; jumping; swimming; 



flight) 



Muscular movement 

 Since insects have an external skeleton, made up of plates of hardened 

 skin, all the muscles have to be accommodated inside this. There is 

 therefore a network of muscles criss-crossing from one plate, or sclerite, 

 to another, to control all the possible movements at the various joints. 

 This is especially so in the thorax, which is almost entirely filled with 

 the muscles needed to operate the six legs and one or two pairs of wings. 

 The muscles may be attached to the inner surface of a sclerite, or to an 

 internal ridge called a phragma or an apodeme. 



If a muscle is attached to a phragma, or to a small sclerite, or to the 

 inside of a tubular sclerite, such as a segment of a leg, then it has a 

 rigid attachment against which to pull. The bigger, flatter sclerites of 

 the thorax and abdomen have a certain flexibility, and may be distorted 

 when a muscle pulls them. This, which might seem to be a defect, is 

 used to advantage in two of the most important muscular activities of 

 insects. We have already seen how compression of the abdominal seg- 

 ments helps to force air in and out of the body during breathing; below, 

 under the heading of flight, we shall see how distortion of the thorax 

 by two opposing sets of muscles operates the wings. 



The ways in which insects move from one place to another may be 

 classified as follows : 



Crawling 

 When we think of 'crawling' we think of moving along on the belly, as 

 opposed to being supported by legs. Except for a few degenerate forms, 

 which do not move, the nymphs of Hemimetabolous insects and the 

 adults of nearly all groups have thoracic legs, and either walk or run; 

 crawling is mainly an activity of the larvae of Holometabola. Many of 

 these have thoracic legs, but those larvae in which the abdomen rests 

 on the ground have to progress by a crawling motion, helped sometimes 

 by the fleshy pseudopods, or prolegs that are often found on some of the 

 abdominal segments. 



Crawling is most highly developed in caterpillars, which have a 



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