THE THEORETICAL ASPECT OF INSECT METAMORPHOSIS
Authors: HENSON H. 
Published: January 2008

Summary.(i) The general theory is developed that metamorphosis is not the activation of imaginal rudiments but a repetition of the developmental processes occurring during embryogenesis. Furthermore, the developmental phenomena accompanying ordinary ecdysis are comparable to metamorphosis and hence also to embryogenesis. Insect life histories consist therefore of a series of repeated developmental cycles all similar in essence to embryogenesis. In insects without metamorphosis these cycles are all alike and no abrupt change takes place. In metaboious forms the earlier cycles are modified to varying degrees. In the Hemimetabola the earlier cycles are subject to the influence of inhibitory hormones and the adult features appear only at the last ecdysis. In the Holometabola the larval cycles are subject to a much more powerful ‘suppression’ influence which checks development and is maintained almost at full level throughout larval life. At metamorphosis this influence is removed and a full development can then take place. Hence degree of metamorphosis depends on the degree of influence of the suppression factor.(2) In the embryonic development of Calliphora the mid‐gut develops in three sections: (a) the pro‐enteron, formed by direct transformation of the inner end of the ‘stomodaeum’ and lying between the stomodaeal membrane and the anterior imaginal ring, (b) the mes‐enteron, formed from polar rudiments (or in other types from the proliferations from the inner end of the stomodaeum and proctodaeum), and (f) the met‐enteron, formed by direct transformation of the inner end of the ‘proctodaeum’, carrying the Malpighian tubules as anterior appendages, and situated between the proctodaeal membrane and the posterior imaginal ring. It is believed that many widely divergent types, including Aptera and Orthoptera, as well as Holometabola, conform to this mode of development. In Calliphora, at metamorphosis, the anterior imaginal ring forms a new ‘pupal stomodaeum’ which produces an adult pro‐enteron in the same way as the corresponding larval structure arose in the embryo. Similarly, the posterior imaginal ring recreates the pupal proctodaeum and eventually forms an adult met‐enteron. In spite of serious gaps in our knowledge it is believed that most of the Holometabola conduct their metamorphoses in this way. It is also probable that in many cases new mid‐gut cells are budded off from the imaginal ring to form a new adult mes‐enteron.(3) The basic similarity between the developmental cycles associated with metamorphosis and those associated with ordinary ecdysis is shown by the experimental results derived from Rhodnius. Here any ecdysis can produce nymphal or imaginal characteristics according to the presence or absence of inhibitor hormone. ICpithelial renovation in the mid‐gut, such as occurs at metamorphosis, takes place at all ecdyses, except in the higher Diptera and Hymenoptera. The formation of the imaginal Malpighian tubules at metamorphosis in the Hymenoptera is similar to the mode of origin of new tubules in ordinary instars in Blatta.(4) The suppression of development in early instars is shown in several ways. Cell hypertrophy, characteristic of many larvae, is due to a high degree of polyploidy. Thus cell division, but not chromosome division, is suppressed during larval stages. At metamorphosis, when the suppression influence is removed, rapid mitosis occurs, and the correct chromosome numbers are restored. Epithelial renovation in the mid‐gut usually occurs at each ecdysis (as well as at metamorphosis), but in the higher Diptera and Hymenoptera it is suppressed during larval stages. Imaginal cells are not imaginal rudiments but merely suppressed renovation cells, which resume their normal activity at metamorphosis because the suppression influence is then removed. If a new adult mid‐gut is produced it probably always arises from the imaginal rings as in Calandra. The imaginal Malpighian tubules of the higher Hymenoptera are homologous with the secondary tubules which arise in every instar in Blatta. Their non‐appearance prior to metamorphosis is thus an indication of suppression of their development during larval stages. In the vast majority of Coleoptera, Lepidoptera, and Diptera secondary tubules are permanently suppressed and the larval tubules become the adult organs. The lifting of the influence of the suppression factor at metamorphosis shows that it has much in common with the inhibitory hormone in Rhodnius. However, the latter has not been shown actually to suppress development, although at present only its action on the hypodermis is at all well known.(5) Variations in larval forms can be explained by assuming that they are due to varying degrees of intensity of action of the suppression influence. They may be arranged in a series such as nymph, campodeiform larva, various degrees of reduction of the campodeiform type, caterpillars, grubs and maggots. Such a series then represents successive reduction due to progressively more severe suppression in embryonic and larval stages. Hypermetamorphosis is the result of varying degrees of suppression at the various ecdyses of an individual life history. The protopod and polypod instars of certain of the parasitic Hymenoptera are precociously hatched embryos and not definitive larval forms like the rest.