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Insect Growth and Metamorphosis
Insect growth includes a mixture of continuous increase of body weight and discontinuous increase of exoskeleton size. Like other poikilothermic animals, insect growth varies with ambient temperatures, which determine the rates of various biochemical reactions of internal body metabolism. In addition, nutrient supply from food diet is necessary for insects to grow. The nutrient signal in insects is delivered to all internal tissues via insulin-like peptide (ILP). ILP signal transduction pathway in the target cells is highly similar to that of vertebrates. Insulin receptor (InR) specifically binds to ILP and activates PI3K to increase PIP3 intracellular level. The secondary messenger activates a specific serine-threonine kinase (Akt). Akt phosphorylates a nuclear receptor, FOXO, to prevent its translocation into nucleus and activates R6 kinase to upregulate protein synthesis.
Insect exoskeleton is a physical barrier for growth and should be replaced with a new and greater size cuticle by molting at every growth. Upon reaching a critical body size for molting, the brain releases PTTH to stimulate 20-hydroxyecdysone (20E) biosynthesis in the PTG. The secreted 20E binds to EcR in the epidermal cells and forms a heterodimer receptor complex with USP. The active receptor enters the nucleus and activates molting-associated genes by inducing a specific upstream transcription factors. Decrease of 20E levels triggers apolysis to detach the old cuticle from the newly synthesized cuticle. With a series of neuropeptides, such as corazonin, ETH, EH, and CCAP, a sterotyped ecdysis behavior is released. The newly molted cuticle is then sclerotized by quinone molecules via bursicon signal.
Metamorphosis is one of the most remarkable and characteristic physiological phenomena in insects among animals. Amphibians like frogs also exhibit the metamorphic development from an immature tadpole to a matured adult via two antagonistic developmental hormones, prolactin and thyroxine. In insects, juvenile hormone and 20E regulate the metamorphic developments, such as nymph-to-adult, larva-to-pupa, and pupa-to-adult.
Insects are useful to monitor climate change by analyzing annual variation in their season phenology. Day-degree model has been widely used to predict insect occurrence based on climate change scenarios. However, insect growth depends on nutrient availability and quality as well as ambient temperature. Furthermore, insects also evolve in their growth and metamorphic developmental patterns according to climate change probably via epigenetic mode followed by genetic differentiation. Thus, we need to refine the prediction model of insect occurrence based on day-degree model with a fixed developmental threshold temperature.
