The inquiry as to what are the determining causes of the metamorphic changes of the blowfly and other Insects has as yet but little advanced. Why does the larva grow up to a certain period with great rapidity, then cease its appropriating power and break up the parts that have been so rapidly and recently formed? And why do the imaginal buds remain quiescent till the other tissues are being disintegrated, and then, instead of sharing the general condition of disintegration, commence a career of development? To these questions no satisfactory answer has yet been given, though the remarkable studies, already referred to, of Bataillon on the later larval life {169}of the silkworm suggest the direction in which knowledge may be found, for they show that the physiological conditions of the later larval life are different from those of the earlier life, possibly as the direct result of the mere aggregation of matter, and the consequent different relations of the parts of the organism to atmospheric and aqueous conditions. If we wish to understand metamorphosis, we must supplement the old opinion that ecdysis is merely an occurrence to facilitate expansion, by the more modern conception that it is also an important physiological process. That shedding the skin is done solely to permit of enlargement of size is a view rendered untenable by many considerations. The integument can increase and stretch to an enormous extent without the aid of moulting; witness the queen-termite, and the honey-bearers of the _Myrmecocystus_ ants. Many moults are made when increase of size does not demand them, and the shedding of the skin at the time of pupation is accompanied by a decrease in size. And if moulting be merely connected with increase of size, it is impossible to see why _Cloëon_ should require two dozen moults, while _Campodea_ can do with one, or why a collembolon should go on moulting during the period of life subsequent to the cessation of growth. The attention of entomologists has been chiefly directed to the ecdyses connected with the disclosure of the pupal and imaginal instars. Various important transformations may, however, occur previous to this, and when they do so it is always in connexion with ecdyses. Caterpillars frequently assume a different appearance and change their habits or character at a particular ecdysis; and in Orthoptera each ecdysis is accompanied by a change of form of the thoracic segments; this change is very considerable at one of the intermediate ecdyses.