AMETABOLOUS AND METABOLOUS STAGES 595 



an adaptation induced in the hexapod type after the first winged 

 insects appeared, and which became more marked in the more 

 specialized insects and at a period comparatively late in geological 

 history, i.e. perhaps at or soon after the beginning of the Car- 

 boniferous period. 1 



The transformations of the pterygote insects vary greatly in 

 degree, and it is difficult to draw the line between the grades. 

 Those in which the adults differ from the freshly hatched young 

 only oi 1 mainly in having wings are generally said to have an incom- 

 plete or gradual metamorphosis. There is no inactive, resting, or 

 pupal stage, and the wings are acquired only after successive moults. 

 Insects with an incomplete metamorphosis are the Urthoptera, 

 Dermaptera, Flatyptera (Mallophaga, Plecoptera, Corrodentia, 

 Embidae), Ephemeridae, Odonata, Thysanoptera, and Hemiptera, 

 with the exception of the male Coccidae, in which there is a resting 

 or subnymph stage. As regards the number of moults in the 

 Synaptera, Grassi states that in Campodea there is a single frag- 

 mentary ecdysis, while Sommers tells us that Macrotoma plumbed 

 sheds its skin throughout life, even after attaining its full size. 



As an example of the partial metamorphosis of the hemimetabo- 

 lous insects we may select that of the locust, in which there are five 

 moults and six stages (instars), as seen in Fig. 558, five of which 

 are nymphal. In the first two stages there are no rudiments of 

 wings, these appearing after the second moult. Besides the acqui- 

 sition of wings there are slight differences after each moult, both in 

 structure and color, besides size, so that we may always recognize 

 the comparative age and the particular stage of growth of any 

 individual. 2 



We have watched the development of Melanoplus spretus from the egg to the 

 imago, and examined thousands of specimens which show the six stages. On 

 the other hand, European authors differ as to whether there are three, four, or 

 five moults in the migratory locust. 3 It is not improbable that, as is the case 

 with many other insects, the number of moults may vary according to the tem- 

 perature and food, variation in these agencies causing either retardation or 

 rapidity in development. 



Those with a complete metamorphosis are said to be metaboloas 

 or holometabolous. (Lang.) 



1 In his Fur Darwin (l$l>3), Fritz Miiller gives his reasons for the opinion that the 

 so-called " complete metamorphosis " of insects was not inherited from the primitive 

 ancestor of all insects, but acquired at a later period. 



2 For further details see the 1st Report of the U. S. Entomological Commission, 

 1878, pp. 27H-281. 



3 See Kiippeu ueber die Heuschreckeu in Siidrussland, 1862, pp. 22, 23. 



