Hormonal control of the molting process in arthropods

Abstract

The main function of the repeated sheddings of the coherent, unexpansible surface layer of the integument in arthropods is to enable the morphogenesis and also a more extensive growth of the body (over the limits given by elasticity of the intersegmental membranes). The role of hormones in this process is to induce molting, and to do this in correlation with both extra- and intraenvironmental conditions and to synchronize the process over the entire body surface. By repeated molting the postembryonic ontogeny of arthropods is subdivided into a series of instars. Each instar has a degree of morphogenesis and other features including the time delimitation within the postembryonic period that are characteristic for a given species. It is known that two hormones are involved in inducing each molting cycle in insects: (1) the activation hormone, produced by the neurosecretory cells of the pars intercerebralis protocerebri—this has been identified as a polypeptide and has been found to be different from other neurohomones of the brain: (2) the actual molting hormone, or ecdysone, produced by the prothoracic glands or related structures (the ventral head glands, pericardial glands, peritracheal glands, or the corresponding parts of the ring gland). Ecdysone is the first invertebrate hormone to have been identified chemically; it has been synthesized recently. It has also been found to occur in other arthropods, first in crustaceans, and recently, in an especially active modification, also in some plants (e.g., several fern species). Data are available on its changes in concentration in the insect body during an intermolting period. A theory on its mode of action has been formulated based on observation of its effects on the so-called puffing patterns in the polytene chromosomes of some Diptera. It has been claimed that ecdysone works as a DNA derepressor, most probably conditioning the synthesis of specific proteins in epidermal cells and in various other cells of ectodermal origin. A number of ecdysone inhibitors have been found recently, and their effect on the molting process is being analyzed. The endocrine molting mechanism in crustaceans in closely related to that in insects. The equivalent of the activation hormone here is produced by a group of neurosecretory cells in either of the eye lobes (the so-called X-organ) which reaches the hemolymph via the sinus glands, a storage organ analogous to the corpora cardiaca of insects. In many crustaceans, as in most of the Malacostraca, the neurohormone of the X-organs differs, from the activation of hormones of insects in that it produces an inhibitory effect on molting—when its source has been removed, the animals molt more often and more regularly. The actual molting hormone seems to be produced by the rostral glands (Y-organ), structures related to the prothoracic glands of insects. The rostral glands in Malacostraca are supposed to produce crustecdysone, a steroid differing from endysome only by one extra hydroxyl group. Although the knowledge of the endocrine system in other groups of arthropods is much less complete than that of insects and crustaceans, the supposition is well based that similar mechanisms occur here also: a neurohormone produced by neurosecretory cells of the brain influencing the secretion of the other, glandular hormone which is produced by special glands placed in the ventral portion of thorax. The main function of the neurohormone seems to be a correlation of the production of the glandular hormone with environmental conditions, be it by activating or inhibiting its secretion by the glands. Recently, the possibility of an intervention of a molting hormone (identical with the ecdysone most probably) in the process of dissolving the internal egg membrane (the white cuticle of locust eggs) has been suggested. It seems to be produced by the pleuropods and to act by inducing chitinase production by the serosa cells. In recent years, additional hormones engaged in the molting process of insects or in some of its phases has been suggested, as, e.g., bursicon (in Diptera), and proctodon in Lepidoptera. In either case, however, the relation of the assumed hormone to the activation hormone and other brain neurohormones and their interaction with the ecdysone remains to be clarified. Nevertheless, doubts as to the exclusive role of ecdysone in inducing the molting process have been expressed also from other sources. The investigation of the endocrine basis of the molting process in arthropods is thus still far from complete, and much remains to be discovered and proved.