54 THE BIOLOGY OF INSECTS 



When both sets of the longitudinal muscles of the abdomen 

 contract, that region is shortened by a partial telescoping of 

 the segments ; contraction of the abdominal tergals tends to 

 straighten the abdomen or to bend the tail upwards, while 

 contraction of the sternals flexes the abdomen ventralwards. 

 There are also short oblique tergal and sternal muscles inserted 

 laterally, which when they contract bend the abdomen to one 

 side or the other. 



In many insects during the early (larval) stages of their 

 life-history, when the limbs are relatively small or even 

 wanting, these muscles connecting adjacent segments are 

 the main agents in locomotion. By stretching out the 

 head region and then drawing the rest of the body after it 

 by waves of contraction, the characteristic creeping move- 

 ment of a grub or maggot (Plate II, D) is brought about. 



A moderately full account of the muscular system of any 

 insect would be far beyond the scope of this book ; one of 

 the famous past-time students of insect anatomy, Pierre 

 Lyonet (1762), dissected out and described sixteen hundred 

 and forty-six muscles in the caterpillar of the Goat Moth. 

 These muscles, like the other organs of immature insects 

 which have to pass through a transformation before attaining 

 maturity, are arranged so as to be specially adapted for the 

 insect's activities during the larval period of its life, and these 

 are commonly widely different from the activities of the 

 creature when adult. 



Most insects when fully grown have the power of flight, 

 and the action of the wings is therefore among the most 

 characteristic and remarkable of all the movements of insects. 

 The wing of an insect of powerful flight, such as a dragon-fly, 

 appears to be a sheet of firm, transparent membrane 

 stiffened by tubular " veins " or nervures. Study of the 

 method of wing-growth, however, shows that a wing 

 arises as a pouch or outgrowth from the dorso-lateral 

 region of the thoracic segment that bears it. This pouch 

 becomes flattened and in the developed wing the two folds — 

 '' roof" and '' floor" as they might be called — come into close 

 apposition over the greater part of the surface, which is 



