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The difference between the first stage (/. e. the one following the 

 egg) of the Heterometabola and Holometabola is so great that one 

 feels that the term " larva," generally applied to this stage of the 

 Holometabola, is quite unsuitable; similarly the term "pupa" applied 

 to the resting or quiescent stage of the Holometabola is equally 

 inapplicable. We still often speak of the "larva" of heterome- 

 tabolous insects, although we generally apply the term "nymph" to 

 the pre-imaginal stage. The larva and nymph, however, are quite 

 undifferentiated in the Orthoptera, Odonata, Ephemeridae, and Hemip- 

 tera, and hence many of our best authorities use the term " nymph " 

 to designate all the stages between the egg and imago in these 

 insects. In this sense, nymphs are the young of insects which lead 

 an active life, and have the mouth-parts formed on the same general 

 lines as those of the adult insect. 



During the larval period of holometabolic and the nymph period 

 of heterometabolic insects, the animal undergoes a certain number 

 of moults or ecdyses of its skin. The intervals between two succes- 

 sive moults are called stadia, and since the various species moult a 

 varying number of times (even the sexes of the same species some- 

 times vary in this respect), the number of stadia varies not only 

 with the species, but sometimes even with the sexes of the same 

 species. It is well known that even in the larval and nymph stages 

 there is sometimes considerable difference in the appearance of the 

 insect, the newly hatched larva looking a very different creature com- 

 pared with its appearance after the first moult, and this again from its 

 appearance after the second moult. The appearance of the insect at 

 various stages of its larval existence is designated by the term 

 "instar," the period from egg to the first moult being called the first 

 " instar," and so on. 



The metamorphosis, however, even of the heterometabolous 

 insects, varies in degree, that of the Orthoptera, Dermaptera, 

 Platyptera, etc., being very slight and gradual, without any resting- 

 stage, the chief external difference between the adult and the 

 newly hatched nymph being the presence of wings, the chief internal 

 difference being the complete development of the sexual glands. 

 In the Odonata, Plecoptera, etc., the changes are more marked, 

 although there is no truly inactive pre-imaginal stage. 



The Holometabolous insects comprise the Neuroptera, Coleoptera, 

 Mecoptera, Trichoptera, Lepidoptera, Siphonaptera, i)iptera, and 

 Hymenoptera. These present a true larval and a distinct pupal 

 stage. In these orders, too, the species present in each of the three 

 later stages essential differences in form, structure, habits, and mode 

 of life. In the heterometabolous insects the nymph obtains at each 

 subsequent moult a greater likeness to the adult, whilst in the holo- 

 metabolous insects the larva often differs much in each succeeding 

 instar from its previous appearance, then suddenly assumes a quiescent, 

 or pupal form (often protected by being enclosed within a cocoon), 

 which always differs widely from the larval, and finally assumes the 



