On the Problem of the Principles and the Morphological Expression of the Ontogeny in Arihropoda

摘要

A hypothetical initial pattern of the arthropod ontogeny, the origin and role of the larval phase, and significance of the metamorphosis are analyzed and discussed. The Arthropoda are characterized by a unique chiti-nous-segmented exoskeleton, which is changed during the molt processes, and by the multi-phase ontogeny; both these features should be treated in close interrelations. The existence of several developmental phases is defined by the two oppositely directed evolutionary processes: embryonization and de-embryonization. These two processes compete within a certain arthropod group. The result of the competition depends on starting conditions as well as on ecological and physiological features of the realization of the entire ontogeny, in particular, on the preconditions to the enrichment of the egg with a yolk. In its extreme expression, the latter factor may potentially result in the non-larval mode of development. Nevertheless, even when eggs are rich in yolk, hatching occurs only in the larva with accelerated tissue differentiation and, correspondingly, morphological differences from the adult organism. Consequently, the yolk richness of eggs provides the larval life owing to the remaining embryonic yolk rather than the processes of the"embryonalization" as defined Schmalhausen (1982). However, another point of view on the nature of the arthropod ontogeny may be suggested. Initially deeply embryonized animals with the non-larval type of development, i.e., directly capable of reproduction, and with a single protophase, have undergone in the course of evolution, first, the superstruction of ontogeny and, second, deembryonization that have both resulted in the appearance of a number of successive postembryonic stages with certain preadaptations and, in addition, lead to a wide ecological radiation in different groups. Thus, molts and changes of stages could appear in the ontogeny of various groups of arthropods not at once but gradually in the course of progressive de embryonization. The divergence of developmental ways of both the larva and the adult animal leads to such a metamorphosis, which, in the most general sense, expresses the transition between the larval and adult developmental programs (Cohen and Massey, 1983), particularly if the larval organization possesses not only phyletic features like those of the poly-chaetan trochophore but also adaptive ones like in maggots.