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 or mainly in having wings are generally said to have an incomplete 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 Orthoptera, Dermaptera, Platyptera (Mallophaga, Plecoptera, Corrodentia, Embidæ), Ephemeridæ, Odonata, Thysanoptera, and Hemiptera, with the exception of the male Coccidæ, in which there is a resting or sub-nymph stage. As regards the number of moults in the Synaptera, Grassi states that in Campodea there is a single fragmentary ecdysis, while Sommers tells us that Macrotoma plumbea sheds its skin throughout life, even after attaining its full size.
As an example of the partial metamorphosis of the hemimetabolous 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 acquisition 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.[[88]]
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.[[89]] It is not improbable that, as is the case with many other insects, the number of moults may vary according to the temperature and food, variation in these agencies causing either retardation or rapidity in development.
Those with a complete metamorphosis are said to be metabolous or holometabolous. (Lang.)
Leach[[90]] in 1815 gave the name of Ametabola to insects without, and Metabola to insects with a metamorphosis.
Fig. 558.—Partial metamorphosis of Melanoplus femur-rubrum, showing the five nymph stages, and the gradual growth of the wings, which are first visible externally in 3, 3b, 3c.—Emerton del.
Latreille (1831) called insects with an incomplete metamorphosis homotenous (which means similar to the end of life), and those with a complete metamorphosis, polymorphous. For the different degrees of metamorphosis of insects he employed two terms: for the incomplete degree, metamorphosis dimidia, and for the total or pupal, metamorphosis perfecta.
Westwood in his Introduction to the Modern Classification of Insects (1839), taking into account the relation of the larva with the imago, divided insects into two divisions: the Heteromorpha, or those in which there is no resemblance between the parent and its offspring, and Homomorpha, in which the larva resembles the imago, except in the absence of wings.
