Insect life cycles Ontogeny – developmental history from egg to adult Eclosion – egg hatching and adult emergence Apolysis – epidermis separates from cuticle Ecdysis – shedding of old cuticle (molting) Instar – immature stage between two molts Stadia – time spent in a life stage Insect life cycles Insect life cycle patterns Ametaboly – ancestral pattern, eggs hatch immatures that resemble adults except lack reproductive organs. No wings Hemimetaboly – eggs hatch immatures that develop external wind pads as they develop (exopterygotes) Holometaboly – egg hatch immatures with no trace of wings until pupal stage (endopterygotes) Larval and nymphal stages Ametabolous and hemimetabolous immatures are nymphs Holometabolous immatures are larvae Nymphs often eat same food as adults (high competition) Three order level exceptions Larvae eat different food than adult (low competition) Endopterygotes have three functional larval types Polypod, Oligopod and Apod larvae Only in Holometabolous orders Polypod – thoracic legs and abdominal prolegs Lepidoptera, most Mecoptera, symphytan Hymenoptera Oligopod – thoracic legs only (lack abdominal prolegs) Most orders except above and Diptera, Siphonaptera, Strepsiptera Apod – Lack true legs All Diptera, Aculeata Hymenoptera, Coleoptera, Siphonaptera Polypod, Oligopod and Apod larvae Hypermetamorphosis A major change in larval form Often in parasitic insects. First larva is called a triungulin. An active stage that finds the host Later instars become inactive and eventually feed on host Meloidae (Coleoptera) Strepsiptera Parasitic Hymenoptera https://scrubmuncher.wordpress.com/2011/08/01/521/ Pupae Exarate pupa – legs, wings, mouthparts, antennae free Articulated/non-articulated mandibles to escape Ptilinum – some Diptera Obtect pupa – appendages tightly appressed to body Exarate pupa https://en.wikipedia.org/wiki/Pupa Obtect pupa https://www.britannica.com/science/pupa Ptilinum Used to break out of puparium Imagos (adults) The reproductive and dispersive stage No molting once the imaginal stage is reached Last hours to years, depending on species A series of neuropeptides and hormones initiate molting and behaviors associated with adult emergence Newly emerged adults are teneral (soft bodies). Swallowing air, filling trachea with air and hemolymph pressure expands adult body and wings before hardening of the cuticle https://www.stevegreerphotography.com/2011/03/09/monarch-butterfly-metamorphosis/ Endrocrine centers Neurosecretory cells - Modified neurons found throughout the nervous system that produce most of the insect hormones Corpora cardiaca – stores and releases neurohormones, controls prothoracic glands Prothoracic glands – Ecdysone (molting hormone), disappears in adults except Archaeognatha (bristletails) and Zygentoma (silverfish) Corpora allata – secretes juvenile hormone which regulates metamorphosis and reproduction Inka cells – located near spiracles, produce pre-ecdysis and ecdysis triggering hormones Ecdysteroid – any steroid hormone associated with molting Ecdysone and 20-hydroxyecdysone are the most common Juvenile hormones – control metamorphosis and sexual development. Juvenile hormones maintain larval characters and inhibits metamorphosis (i.e. adult development) Voltinism Voltinism is the number of generations a species has in a year Univoltine – one generation per year (Dobsonflies) Bivoltine – two generations per year (many insects in Texas) Multivoltine – > 2 generations per year (Diptera, Ephemeroptera) Semivoltine – > than a year to complete a generation (Homoptera - cicadas, Coleoptera) Diapause* A period of dormancy affecting some life stage (egg – adult) Initiated by high or low temperature and drought If it occurs in summer – Aestivation If it occurs in winter – Hibernation Quiescence – slowing/halting of development due to unfavorable conditions, with development resuming with favorable conditions Diapause - slowing/halting of development COMBINED with physiological changes, and development resuming is linked to physiological stimuli, not just favorable conditions. Can be obligatory or facultative Dealing with extreme cold conditions Freeze tolerance – freezing happens in non-cellular areas (haemolymph, gut, malpighian tubules) antifreeze compounds allow supercooling, controlled cell dehydration Freeze avoidance – supercooling and removal of ice nucleating compounds, death due to exceeding supercooling temperature Chill tolerance – supercooling, death due to duration of low temperature exposure Chill susceptibility – supercools, but cannot handle extended low temperatures Opportunistic survival – behavior, migrate Dealing with extreme hot conditions Death is caused by dehydration and protein denaturing Acclimation – gradual exposure to rising (and falling) temps Behaviors – time of activity, seek shelter, stilting Polypedilum vanderplanki Cryptobiosis – dehydration to survive drought and high temperatures. Larva is alive, but all metabolism has ceased. Aridity Fog basking behavior http://www.animalsanimals.com/results.asp?image=INS%20030BAA015%200 1 Aridity Reduce cuticular water loss Reduce water loss via respiration Lower metabolic rate Water conservation by dehydrating feces Migration Diapause tracks resources in time Migration tracks resources in space Unidirectional movement away from initial location Pre- and post-movement behaviors Energy reallocation in body (flight vs reproduction) Monarch butterflies Polymorphism and polyphenism Morphological variants (within a population or across generations) Genetic polymorphism Sexual dimorphism Brachyptery in water striders (Heteroptera: Gerridae) Butterfly mimicry Environmental polymorphism (polyphenism) Wing color in butterflies (photoperiod & temperature) Caterpillar color (food or plant color) Age Grading (immature age determination) What age or instar are immature stage at? Dyar’s law or rule post molt/pre molt head width distance (1.3-1.7) Determinant vs indeterminate # instars Length-mass regression linear size Predicting mass from measurement Full size range Sclerotized structure Age Grading (adult age determination) Difficult because no growth so sclerotized structure don’t change size 4 strategies 1) physiology or reproductive morphology 2) somatic structure changes 3) wear and tear on body (unreliable) 4) changes in gene expression Environmental effects on development Temperature – Degree days (hours) Insects require an accumulation of ‘heat’ to complete their life cycle. Insects are poikilothermic, metabolism driven by environmental temperature Can be used to predict emergences of insects Determines diapause in most cases Environmental effects on development Photoperiod (light/dark cycle) Very predictable, changes with latitude Humidity (water vapor in the air) Desiccation susceptibility increases at low humidity Mutagens and toxins (human derived) Fluctuating asymmetry (measurement of how left/right side of insect differ) Environmental effects on development Biotic effects Quantity and quality of food If both are low, either take longer to develop or smaller adult size in poor food situations Crowding – migratory locusts change from solitary to gregarious Linkages to phenology of host plants (cicadas)
