ANATOMY AND MORPHOLOGY OF INSECTS. 19 



Many larvae are, however, vermiform when they leave the 

 egg, and either possess or are without rudimentary legs. Such 

 larvae are characteristic of the highest forms of insect life, and 

 always pass through a nymph stage, in which there are usually 

 no active manifestations of vitality. 



Putting all these facts together, it is evident that the vermi- 

 form, or caterpillar form, of the larva is an acquired one. Those 

 insects which exhibit the least differentiation of structure re- 

 semble a campodea larva even in their sexually mature con- 

 dition, and are gradually developed from larvae like themselves, 

 so that there is ecdysis, but not metamorphosis. 



Where wings are developed in the last stages, they are 

 acquired gradually, becoming larger at each moult. Such 

 insects have been classed as Ametabola. The development in 

 these insects takes place chiefly in the egg, and the eggs are 

 always comparatively large in proportion to the adult insect, and 

 comparatively few in number. This is especially the case 

 where the perfect insect exhibits special peculiarities, as, for 

 example amongst the grasshoppers and Phasmidae, and the 

 larva is produced from the egg, not as a non-differentiated cam- 

 podea larva, but having the general characters of the perfect 

 insect. 



Those insects which are most specialized, or, in other words, 

 most highly organised, do not exhibit the campodea stage, but 

 the larva is already vermiform when it leaves the egg. Deve- 

 lopment in the egg is arrested at an early period, as in the 

 fly, and does not recommence until the larva has attained its 

 full growth. Harvey had an intuition of this ; he says : ' The 

 eggs of insects do not contain a sufficiency of nourishment for 

 the production of the young, which is born as a larva, and, 

 after feeding and collecting a sufficiency of nutriment, returns 

 to the condition of an egg in the pupa, from which the perfect 

 insect is developed.' Such insects produce comparatively 

 small, but usually very numerous, eggs, and are known as 

 Metabola. The larvae are chiefly characterised by the great 

 activity of the organs of vegetative growth, and by the compara- 



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