ABSTRACT
THE GREAT group of insects contains an enormous range of different forms, and any treatment of insect development which can fit into the framework of this book must be an extremely summary and simplified one. There is not only a very wide range of different types of embryonic development to be covered, but our knowledge of the phenomena of insect metamorphosis is considerable, and, although we have neglected this problem in the annelids and echinoderms, it seems desirable to give some account of it in insects. As is well known, there are all gradations in the intensity of the changes involved in metamorphosis. In some primitive insects (the Ametabola), the larval form develops gradually into the adult, with no sudden or marked change. There are other more complex types, in which the wing buds may be exposed in the larval stages (Exopterygota) or concealed beneath the surface until the time of metamorphosis (Endopterygota); and again the larva (in this case often known as a 'nymph') may be directly transformed into the adult (Hemimetabola) or there may be a pupal stage intercalated (Holometabola). In the extreme type, the Holometabola, the adult organism may have almost no immediately obvious similarity to the larval; the life-history comprises two very distinct developmental systems. Since there will be no space to treat all the intermediate conditions, we shall fmd it convenient to deal first with the embryonic development, by which the larva is produced, and then to pass on to the processes of metamorphosis, particularly in those types in which they are most intense and far-reaching.
