4 o ANIMAL LIFE 



the less complex worms in their simple muscular 

 bodies, in their wriggling movements that serve for 

 swimming, and the looping or stretching evolutions 

 by which they creep. Their bodies, too, are sub- 

 divided into rings or segments, which bear a certain 

 number of appendages arranged segmentally. These 

 limbs, as we pass from the lower forms, such as Scolo- 

 pendrella, to the higher insects (fig. 5), become de- 

 finitely limited to three pairs for progression, and thus 

 entail a correspondingly greater nicety of balance. 

 The body becomes divided into three portions and is 

 balanced about the middle one of these ; carrying, 

 in fact, to a further degree the division of the body into 

 head, thorax, and abdomen, already seen in Crustacea. 

 It is from this middle section of the body that the 

 wings arise. Vertically over the second and third legs, 

 pouch-like outgrowths of the body-wall form two 

 pairs of flattened lamella:, strengthened by ribs and 

 veinings like leaves. The arrangement of the muscles 

 that raise and depress the wings is a most complex 

 one, but consists essentially of a double lever, both 

 elevator and depressor acting vertically but on 

 opposite sides of the fulcrum. These muscles are of 

 great size and power, and where the wing meets most 

 resistance and does most work, namely, at its front 

 edge, it is stiffened by a selvage of veins. 



To solve the problem of flight the economy and 

 distribution of weight, the intensity and application of 

 the muscular power have to be completely studied. 



The body requires to balance about the middle, 



