STRUCTURE AND CLASSIFICATION. 



49 



which may or may not resemble the parent that laid it. Assum- 

 ing that it does, it is without trace of wings. It grows apace, 

 moults as often as necessary, and at last attains very distinct wing 

 pads. It is now in the pupal stage, and with the next moult the 

 wings become fully developed. It is then adult and fitted to 

 reproduce its kind. This sort of metamorphosis is said to be 

 incomplete, because there is no inactive stage, and because there 

 is always a close resemblance in all stages. The larvae are quite 

 usually termed nymphs, and differ from the adults only in size, in 

 the absence of wings, and in the undeveloped sexual characters. 

 A peculiarity of this method of growth or moulting is worthy of 

 incidental mention. When the insect moults, it is not the outer 



Fig. 26. 



Moulting of a grasshopper. — a, nymph ready to change ; b, the skin split along the 

 back and the adult emerging ; c, continues the process, and at d, the adult insect 

 drying out. 



skin alone that is affected : all the most delicate mouth parts with 

 their hairs and processes are cast and replaced by a new cover- 

 ing ; the lining of a large proportion of the digestive system 

 is shed, and the inner coat or surface of the tracheal tubes is 

 renewed, so that practically the insect gets, in part, a new diges- 

 tive system and a new breathing apparatus, every time it increases 

 in size. The number of these moults varies in the different orders, 

 and even in species of the same order. There may be as few as 

 three or four, and there may be twenty or more. We can some- 

 times modify the normal number by changing the circumstances 

 under which the insects live. Thus, the larva of the common 

 Dermestid found in museum cases normally moults four times 



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