40 INTERNAL ANATOMY. 



nature of the external skeleton enable the insect to 

 enlarge the abdomen both vertically and laterally. It 

 contracts and dilates very regularly. When contract- 

 ing, the air is driven out of the tracheae, much as it is 

 from a pierced rubber ball when a boy squeezes it. 

 As the muscles cease to operate, and the walls resume 

 their former shape, the air rushes into the interior of 

 the air-vessels just as it does into the boy's ball when 

 the pressure of the hand is relaxed. 



Plateau, in an article on the Respiratory Movements 

 of Insects,^ gives figures showing the size of the body 

 of several insects after inspiration and expiration. In 

 the locust and other Acridian Orthoptera the dorsal 

 and ventral parts of the terga and sterna approach and 

 recede alternately, the sterna being usually the more 

 yielding. This is not always the case, however, as 

 will be seen by the description of the mode of breath- 

 ing of the cockroach (see p. 102). 



It is evident the system of tubes and sacs must 

 make the body lighter than if the same spaces were 

 occupied by solid matter ; and when the air contained 

 in them is heated by the normal warmth of the ani- 

 mal, they add, probably, still more to the buoyancy of 

 the body. 



The habit of using an organ is known to possess the 

 power of producing modifications or variations. This 

 law of use may be applied to the explanation of the 

 changes that have taken place in the primitive appen- 

 dages of the vertebrate which have become the arms 

 and legs of the mammal suitable for walking, wings 



^ See The Cockroach, Miall and Denny, p. 159. 



