Parasite Redirection of Neurohormonally Driven Developmental Pathways that are Associated with Size Thresholds
The metamorphosis of insects provides an interesting and useful model with which to study how neural and neurohormonal systems regulate the transition from the immature to the adult stage. One level of regulation involves or is closely correlated with size thresholds. Classical and recent work with hemipterans has shown that the stimulus for molting is derived from stretch receptors which are sensitive to gut distension (Nijhout 1981). Research on lepidopteran growth and development has identified several size thresholds which regulate expression of certain developmental programs. The cycle of larval-larval molts continues until the insect reaches a body or exoskeletal size which signals attainment of the final larval instar. The size of the sclerotized head capsule is an index of this critical size, and a critical head capsule width has been determined for several species. During the final feeding phase, size thresholds have been correlated with commitment of certain tissues toward metamorphosis. The feeding stage ends when a final size threshold is reached, which is a function of the size of the insect at ecdysis to the final instar (Nijhout 1981; Jones et al. 1981).
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