How insects move 



Fig. 50. A magnificent robber-fly. Order Diptera, with a wing-span of 

 more than two inches 



the first set give support while the other three move. Since the thorax 

 does not twist, the effect is to swing the whole body from side to side, 

 and the insect moves with a seaman-like roll. Studying the legs by slow- 

 motion film shows that, Hke the legs of horses, they do not move exactly 

 together, but have an off-beat rhythm. 



The insea uses this first type of motion at all speeds, and does not 

 change abruptly, as a horse does when it begins to canter. The hind-legs 

 are used together, however, in jumping^ and may be powerfully developed 

 for this purpose, as in grasshoppers and fleas (Figs. 14, 55). Other insects 

 jump strongly without using their legs. 



' Springtails ' (Collembola) have a special two-pronged organ, called 

 a furcula, attached to the fourth abdominal segment, and extending 

 forwards under the abdomen, where its tip is held by a catch {retina- 

 culum). When the furcula is released it presses down on the surface 

 beneath the insea, and throws the springtail up and forward, just Hke 

 those jumping frogs that children have. 



* Chck-beetles ' (Frontispiece : family Elateridae, the larvae of which 

 are 'wireworms') have a jumping mechanism of a rather diff"erent kind. 

 Underneath the thorax a stiff projection from the prothorax presses 

 against the edge of a pit in the mesothorax. When it slips over the rim 

 the thorax is given a violent twitch, in a sort of tiddleywink action. This 

 mechanism is used when the insect falls over on to its back, and the 



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