n] GROWTH AND CHANGE 11 



air-tube system are formed by inpushings of the 

 outer skin (ectoderm) and are consequently lined 

 with an extension of the chitinous cuticle which is 

 shed and renewed at every moult. 



In all insects these successive moults tend to be 

 associated with change of form, sometimes slight, 

 sometimes very great. The new cuticle is rarely an 

 exact reproduction of the old one, it exhibits some 

 new features, which are often indications of the 

 insect's approach towards maturity. Even in some 

 of those interesting and primitive insects the Bristle- 

 tails (Thysanura) and Spring-tails (Collembola), in 

 which wings are never developed, perceptible differ- 

 ences in the form and arrangement of the abdominal 

 limbs can be traced through the successive stages, as 

 R. Heymons (1906) and K. W. Verhoeff (1911) have 

 shown for Machilis. But the changes undergone by 

 such insects are comparatively so slight, that the 

 creatures are often known as 'Ametabola' or insects 

 without transformation in the life-history. Now 

 there are a considerable number of winged insects 

 cockroaches and grasshoppers for example in which 

 the observable changes are also comparatively slight. 

 We will sketch briefly the main features of the life- 

 story of such an insect. 



The young creature is hatched from the egg in 

 a form closely resembling, on the whole, that of its 

 parent, so that the term 'miniature adult' sometimes 



