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TEXT-BOOK OF ZOOLOGY 



support (many inanimate objects, e.g., lamps, tables, etc., are supported 

 in a similar fashion). In running the insect always places three of its 

 legs (two of one and one of the opposite side) upon the ground in the form 

 of a triangle, while the three others are lifted up from the ground. Con- 

 sequently the animal remains, even whilst moving along the ground, in 

 a condition of stable equilibrium. (Compare with kangaroo and beaver. 

 How far are vertebrates while running in a condition of unstable 

 equilibrium ?) 



"We have already seen (p. 309) why the legs of insects (like those of 

 all arthropods) require to be jointed. In the adult insect they consist, 

 as a rule, of five divisions. A short proximal joint, the coxa, connects 

 the leg to the body skeleton, and in conjunction with the second joint, or 



trochanter, consider- 

 '"' ably increases the 



mobility of the leg. 

 The next joint, or fe- 

 mur, is always the 

 strongest part of the 

 leg, containing the 

 muscles which move 

 the two following 

 joints, the elongated 

 tibia and the tarsus 

 or foot, which itself 

 usually consists of 

 several joints. (Why 

 do not these names 

 have the same mean- 

 ing as in the legs of a 

 mammal, from which 

 they are borrowed ?) The structure of the different portions of the leg 

 varies considerably, in accordance with the various modes of locomotion 

 of the insects, and according as the legs subserve additional functions 

 other than those of simple locomotion. In addition to such legs as are 

 adapted for running and climbing (cursorial legs), such as are found 

 in most insects, we further distinguish swimming or natatorial legs (see 

 water-beetle), gressorial legs (see stick insects), saltatorial legs (see 

 grasshoppers), fossorial legs (see mole-cricket), and raptorial legs (see 

 mantis). 



(b) As has been already mentioned, the second and third thoracic 

 somites each carry a pair of wings upon their dorsal surface. They 

 are therefore situated above the centre of gravity of the body (just 



Diagrammatic Section through the Thorax of an Insect. 



H., Heart; D., intestine; N., the two nerve cords; Al., stig- 

 mata, with which are connected the tracheae L. (The shaded 

 portions indicate the muscles which move the wings, legs, and 

 the different divisions of the legs. ) H. , coxa ; Sr. , trochanter ; 

 S., femur ; Sch., tibia ; F., foot ; K., terminal claws. 



