FSE 04 Insect Structure

FSE04fo r en sic ento m o lo gyInsect StructureEntomology: the scientific study of insects.Insects are a class of arthropods that have beenincredibly successful.Possible reasons for their success include:The ability to fly.• Escape predators.• Change habitats.Find food.Find mating partners.Small size.• Can occupy and exist in a wide range of habitats.•The exoskeleton provides great protection andprevents water loss.Diverse body structures.• Body structures enable insects to live in a widerange of habitats. Eg, aquatic insects can extractoxygen from water.•ööThe process of metamorphosis enables insects toexploit different food sources at different stages ofdevelopment.Food supply.•Insects feed on anything ranging from nectar todead flesh.Reproduction.• They reproduce very quickly and in large numbers.Insects also contribute significantly to the ecosystem.Even though it would be great to be outdoors withoutmosquitoes, a world without insects would be vastlydifferent.For example, insects are:• pollinators,• a food source for many other animals,• decomposers and recyclers of natural materials,• producers of products such as silk and honey,•key members of medical and scientific research,and• prime evidence in forensic science.Structure of insectsKey structures of insects include:•three main body regions: head, thorax andabdomen (these regions may be furthersegmented),• an exoskeleton,•no more than three pairs of segmented legsattached to the thorax,• one pair of antennae,• compound eyes,• one or two pairs of wings, and•small holes or spiracles along the side of theirbody.Insects are the only arthropod class to achieve flight.Figure 1: A diagram of an ant showing basic insectstructure.USDA Forest Service Archives, USDA Forest Service, www.insectimages.org

FSE04fo r en sic ento m o lo gyInsect StructureFigure 2: A diagram clearly showing the segmented legs and wings originating from the thorax of an insect.USDA ARS Archives, USDA Agricultural Research Service, www.insectimages.orgWebsitesThe following websites have some useful information.The CSIRO site is extensive but does required some searching – a lot of information about insects is available.http://www.ento.csiro.au/about_insects/index.htmlWithin the CSIRO site is the “Anatomical Atlas of Flies”. This is a fantastic online tool where students can view andget information about the external structure and morphology of flies.http://www.ento.csiro.au/biology/fly/fly.htmlThe Manduca Project is from the University of Arizona and is definitely worth a look. It has a substantial number ofresources including videos showing events such as moths emerging from their pupal case. The website is basedaround students raising the tobacco hornworm Manduca sexta.http://insected.arizona.edu/manduca/default.htmlThe Australian Museum site.http://www.amonline.net.au/insects/insects/index.htmA more “kid” orientated site but has lots of hidden information.http://www.earthlife.net/insects/six.htmlA useful site with general information.http://www.kendall-bioresearch.co.uk/morph.htm

FSE04fo r en sic ento m o lo gyInsect StructureInsect Life cycleThe typical life cycle of an insect is as shown in Figure 3.Figure 3: The life cycle of a cottonwood leaf beetle(Chrysomela scripta).Adapted from James Solomon, USDA Forest Service, www.insectimages.orgFor example, in Figure 4, various stages of the lifecycle of a cottonwood leaf beetle (Chrysomela scripta)are shown on one leaf!Figure 4: The life cycle of a cotton leaf beetleJames Solomon, USDA Forest Service, www.insectimages.org

FSE04fo r en sic ento m o lo gyInsect StructureLength of the life cycle.The progression through the life cycle is dependenton a host of variables with the main determinantbeing temperature. This is discussed in detail in FSE07– Forensic Entomology.MetamorphosisMetamorphosis refers to a major change of form orstructure during development. Most insects have aform of metamorphosis. This strategy enables insectsto exploit different food sources at different stages oftheir development. It is one of the key reasons for theirgreat success.The Australian Museum:http://www.amonline.net.au/insects/insects/metamorphosis.htmAn example of the absolute difference betweenhabitat and food source at different stages in the lifecycle is a mosquito. The larvae live in water and eatplankton, the adult fly around and sucks blood! The‘role’ of the larvae is to eat while the ‘role’ of the adultis to reproduce.Complete metamorphosisThe adult insect looks completely different to theimmature insect. As shown in Figure 3 and 4, theinsects have a life cycle where they completely changefrom an egg → larvae → inactive pupae→ adult.These insects are often called ‘Holometabolous’,meaning they undergo a complete (Holo = total)change (metabolous = metamorphosis or change).For example, the pecan leaf casebearer (LepidopteraAcrobasis juglandis) undergoes completemetamorphosis.Figure 5: Complete metamorphosis in the pecan leaf casebearer.Citation: Jerry A. Payne, USDA Agricultural Research Service, www.insectimages.org

FSE04fo r en sic ento m o lo gyInsect StructureTable 4: Major orders of insects with complete metamorphosisOrder Example CharacteristicsColeoptera Beetles The forewings are hardened into “elytra” which protect the delicatehindwings. They have biting and chewing mouthparts.Lepidoptera Moths, butterflies Two pairs of membranous wings and sucking mouthparts. At rest mothshold their wings flat while butterflies hold wings vertically (and haveclubbed antennae).Hymenoptera Wasps, ants and bees Membranous wings although sometimes wingless. Chewing mouthpartsand a constriction between abdomen segments: wasp waist.Diptera Flies The hind wings are reduced to balancing organs called halters. Lapping,sucking or piercing mouthparts.Incomplete metamorphosisSome insects however undergo incompletemetamorphosis in that they gradually change to anadult. Immature insects in these orders are callednymphs and they gradually grow in size. This is calledHemimetabolous or incomplete metamorphosis.Figure 6: Incomplete metatmorphosis life cycle of thebrown marmorated stink bug (Halyomorpha halys).Adapted from Citation: Gary Bernon, USDA APHIS, www.insectimages.org

FSE04fo r en sic ento m o lo gyInsect StructureFigure 7: Incomplete metamorphosis in the brownmarmorated stink bug (Halyomorpha halys).Citation: Gary Bernon, USDA APHIS, www.insectimages.orgFigure 7 shows several early instar nymphs in threedifferent stages on an egg mass: the one dark rednymph is a first stage or instar nymph, the light rednymph has just molted and is therefore a new secondstage or second instar nymph. The other 4 blacknymphs are second stage or second instar nymphs. Itis at this stage when aggregating nymphs may have atick-like appearance. Note also that all the eggs havehatched.NB: the CSIRO site has lots of good information aboutvarious orders of insects.http://www.ento.csiro.au/education/insects/diptera.htmlhttp://www.ento.csiro.au/education/insects/lepidoptera.htmlSubstitute the name of the order of insect (just beforethe html) in the above URL.

FSE04fo r en sic ento m o lo gyInsect StructureTable 3: Major orders of insects with incomplete metamorphosis.Order Example CharacteristicsCollembola Springtails Primitive wingless insects, largely in soilsThysanura Silverfish Primitive wingless insects, in caves and damp housesEphemeroptera Mayflies Larvae in fresh water, adults live only daysOdonataDragonfliesGenerally large insects found worldwide, carnivorous and herbivores, Larvae predators infresh waterPlecoptera Stoneflies Adults usually live near river banks, Larvae in fresh waterBlattodea Cockroaches Omnivorous, scavengers, found worldwideIsoptera Termites Social insects living in vast colonies with one queen to lay all the eggs, most feed on woodMantodea Mantids Predatory insects with large eyes and grasping front legs, mostly in the tropicsDermaptera Earwigs Omnivorous insects with fan-shaped hind wings and pincers on tailOrthoptera Grasshoppers Grass-feeding insects with jumping hind legsPhasmatodeaLeaf-insectsLeaf-feeding insects with camouflaged, flattened or slender bodies, look like leaves or sticks ofplantsPsocoptera Book lice Small chewing insects, feed on tree bark, book bindings, also found in foodPhthiraptera Parasitic lice Parasites of birds and mammals, live on skin, feed on feathers, skin or blood, winglessHemiptera True bugs Piercing or sucking mouthparts, feed on plants, insects or mammalsThysanoptera Thrips Tiny insects with fringed wings, herbivorous with sucking mouthpartsMegaloptera Alderflies Larvae are aquatic and carnivorous, adults have long antennae, two pairs of large wingsNeuroptera Lacewings Predators as larvae, adults are carnivorous or herbivoresNB – not all insect orders are listed.

FSE04fo r en sic ento m o lo gyInsect StructureGlossaryChrysalis:Cocoon:Compound eye:Frass:Elytron:Haltere:Hemimetabolism:Holometabolism:Instar:Larvae:Moult:Nymph:Pupae:Tagmata:Ovipositor:The pupa of a butterfly or moth.A silk casing within which larvae (particularly of some Lepidoptera) passes into and throughthe pupal stage.A large grouping of light gathering organs that allow the insect to detect color and motion.For most insects, resolution is not very good.Another term for larvae insect droppings or excretion.The hardened protective forewings of a beetle.Club shaped organ that take the place of the hind wings in true flies (diptera). They are usedfor balance and stability in flight.An incomplete metamorphosis where an insects development does not include a pupalstage. The larvae are known as nymphs and resemble the adults.A complete metamorphosis where an insects development passed through fours stages- egg, larvae, pupae, & adult.The stage between each moult of a larvae. For example a first instar larvae would refer to alarvae between hatching and it’s first moult. Final instar would refer to the last stage prior tobecoming a pupae.The immature form of an insect that follows the egg stage.To shed periodically part or all of the outer covering which is then replaced by new growth.A larvae of an exopterygote (insect with incomplete metamorphosis)A non-feeding stage of development between the last larvae and adult forms, characterizedby many anatomical changes and, often, by enclosure in a cell or cocoon.The 3 main regions of the insect body: head, thorax and abdomen.The organ with which many insects deposit their eggs. Some ichneumon flies have a longovipositor fitted to pierce the eggs or larvae of other insects, in order to lay their own eggswithin the same.10