Disturbance of larval orientation in Spodoptera frugiperda (Lepidoptera : Noctuidae) by microsurgical removal of endocrine centres

Abstract

The larvae of S. frugiperda showed differences in orientation depending largely upon their physiological states. A preference for wet conditions manifested itself in the orientation of the penultimate instar larvae, whereas the larvae of the final stage exhibited preference neither for wet nor for dry conditions. However, the ‘light level’ proved to be of prime importance in the final stage, such that it drove the larvae almost exclusively to the dark, while it appeared to be of secondary importance in the preceding stage where the larvae only responded photopositively if deprived of the humidity-preference. Microsurgical removal of the medial neurosecretory cells in the pars intercerebralis resulted in a preference for dry conditions in both larval instars concomitant with an appreciable increase in the total water content of the larvae. The normal situation could be easily restored by implantation of some active cells. Allatectomy failed to alter the normal preferences of the final instar larvae, whereas it induced a humidity-preference in the larvae of the penultimate stage, a response exhibited normally by intact larvae of the final stage. This result might suggest the implication of the morphogenetic hormones in some way or another in the control of larval response to the ‘humidity level’. However, photopreference in both larval instars appeared to be genetically controlled.