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Published online by Cambridge University Press: 19 September 2011
Laboratory studies were carried out in Egypt to examine the effects of treatment of the nymphs of Schistocerca gregaria during the later period of third instar with different dosages of gamma radiation ranging from 1000 to 3000 rad. The data indicate that treatment with a dose of 3000 rad induced complete mortality for all treated nymphs. However, irradiation with a dose of 2000 rad produced fourth instar nymphs which enter a state of suspended development for about 14 days. Exposure to a dose of 1000 rad induced three types of response associated with the endocrine activities. Some of the treated nymphs developed to the fifth instar with soft and thin cuticle without tanning or darkening, these nymphs did not survive more than a few hours after ecdysis. Other treated nymphs became fifth instar with anomalous wings and normal cuticle which survived about 18 days as permanent fifth instar nymphs before dying, while the rest of the treated nymphs moulted to permanent fourth instar which survived more than 1 month and eventually died. Corpora allata (CA) volumes in the permanent nymphs were estimated by planimeter method, revealing a pronounced reduction in their size in comparison with the untreated controls. It reflects a sharp inhibition in their synthetic activity of juvenile hormone (JH).
We deduce that the permanent nymphs may be produced as a result of a direct inhibition on the cerebral neurosecretory cells of the brain, the source of stimulating factors for JH and ecdysone production. Gamma rays prevented the production of the tanning and darkening factor (Bursicon Hormone) in these cells.
Etudes de laboratoire ont été effectue en Egypte afin d'evaluer l'activité neurosecretrice chez les nymphs permanents de S. gregaria proviennent des larves traitées au 3e stade par des doses variées de rayons de Gamma (entre 1000 et 3000 rad). Une mortalité totale a été obtenue chez les nymphs lors d'un traitement par une dose de 3000 rad. Par contre, l'irradiation par 2000 rad. Resulte le passage au 4e stade nymphale qui rentre ensuite dans un stade de devélopement arrêté durant 14 jours. Trois types de reactions; associées avec l'activité endocrine; ont été observé lorsqu'on traite avec 1000 rad: devéloppement des nymphs jusqu'au 5e stade, caracterisé par un cuticle de'epaisseur varié et non colore, ces nymphs ne peuvent survivre que quelques heures aprés la mue, autres nymphs traités passent au stade cinq caracterisé par des anomalies aux ailes et cuticle normale, ces individus survivent 18 jours comme nymphs permanents avant mourir, le reste des nymphs survivent plus qu'un mois comme nymphs permanents au 4e stade avant de mourir.
Une reduction remarquable de volume des Corps allates a été observe ches les insectes traités parraport au temoin. C'est probablement due à l'inhibition de la synthèse de l'Hormone Juvenile.
Nous pensons que l'apparition des nymphs permanents est attribuée à l'inhibition directe des cellules cérébrales neurosecretrices. Les rayons de Gamma arrêtent la production du facteur responsable de la coloration foncé (Bursicon Hormone) chez ces insectes.