Nouveaux aspects du contrôle hormonal du cycle biologique des Odonates: recherches sur la larve d’Aeshna cyanea (Müller) (Anisoptera: Aeshnidae)

The life history of this insect consists of successive events, among which development and diapause occupy the longest duration. These processes are controlled by a series of physiological mechanisms of hormonal nature in answer to external factors (photoperiod, temperature, feeding). Although the hormonal control of the main biological processes, such as moulting, metamorphosis and diapause is known as a whole, the recent use of chemically pure hormones, or of substances which mimic their action, makes it possible to clarify numerous problems, as well as to contest some others. (I) Moulting. – Following the classical scheme, ventral glands secrete ecdysone or moulting hormone (a steroid compound) in response to a hormonal stimulus from the brain. This hormone acts on the epidermis and on other target organs involved in moulting. However, it is noteworthy that an important time interval separates the functional activity of ventral glands from the moulting hormone activity itself. Indeed, ventral glands must be extirpated within 24 hours after ecdysis if a permanent larva is to be obtained which cannot undergo moulting anymore. Only the implantation of these glands to such a larva, even after several months, allows the resumption of development, i.e. moulting and metamorphosis. On the other hand, in a normal last larval instar (duration: 25 days), moulting hormone activity can be demonstrated in the Calliphora-bioassay over an 8 day period, starting on day 16 and reaching a maximum on day 20. The requirements of this hormone are the highest just on day 20, as is shown by studies on the metabolic fate of injected radiolabelled ecdysone. Indeed, at this time the radioactivity which can be recovered from whole insects is very high after the injection, whereas at beginning of the intermoult (on day 6 for instance) injected ecdysone will be excreted rapidly. No moulting hormone activity can be detected in permanent larvae. Such larvae do not contain the factor elaborated by the ventral glands, which is perhaps of polypeptidic nature, and are unable to secrete ecdysone. The injection of chemically pure ecdysone into permanent larvae induces apolysis and secretion of a new cuticle within a very short period. Penultimate instar larvae complete a normal morphogenesis, in contrast to last instar larvae in which the growth and differentiation processes of metamorphosis cannot take place in consequence of the rapidity of both apolysis and secretion of the new cuticle. Therefore it seems that two hormonal factors are necessary to ensure a normal moulting. (2) Metamorphosis. – This phenomenon occurs only if moulting hormone is acting alone. In fact, if juvenile hormone (secreted by the corpora allata) is present in addition to moulting hormone the insects perform a typical larval moult. The removal of corpora allata being surgically impossible, their role was investigated by implanting active corpora allata into last instar larvae where these glands are normally inactive. This operation leads to a supernumerary larval moult or an intermediate form between larva and adult called adultoid. The use of a substance which mimics juvenile hormone effects, farnesyl methyl ether, makes it possible to dose accurately the juvenilizing effect and to obtain as desired (by injecting this compound to last instar larvae) adultoids which are more or less close either to a larva or to an adult. The response of larvae depends both on the dose of mimicking substance and on the time of injection. The 6th day of the last instar, after which the juvenilizing factor is unable to affect metamorphosis, corresponds to a critical period coinciding with the definitive programming of adult development. (3) Diapause. – Induction and termination of larval diapause which are subjected to photoperiod, are also under the same hormonal control as moulting. However the brain, acting as the integration site of the photoperiodic stimuli, has a considerable importance in the control of diapause. Indeed, whereas implanting active ventral glands into diapauzing larvae has no effect, an active brain is able to terminate diapause. Finally, the injection of chemically pure ecdysone into diapauzing larvae makes it possible for them to resume development. It seems, therefore, that the brain hormone is able not only to stimulate ventral glands, but also to maintain their activity until ecdysone is secreted in sufficient amounts in order to ensure the resumption of development.