AN INTRODUCTION TO INSECT STRUCTURE - · PDF fileAN INTRODUCTION TO INSECT STRUCTURE ......

AN INTRODUCTION TOAN INTRODUCTION TOINSECT STRUCTUREINSECT STRUCTURE

B.K.Mitchell and J.S.ScottB.K.Mitchell and J.S.ScottDepartment of Biological SciencesDepartment of Biological SciencesUniversity of AlbertaUniversity of Alberta

Supported in part bySupported in part byAcademic Technologies for LearningAcademic Technologies for Learningand Faculty of Science, University of Albertaand Faculty of Science, University of Alberta

These modules are designed primarily for use in introductory entomology courses at theUniversity of Alberta. Not all courses require the same level of detail - please consult yourinstructor for advice on study strategies. In general, it will be best to study each module as a unit,reading the slides within a module in the order in which they are presented. For quicker navigationand review, use the thumbnalis (click on the view thumbnails button near the top left corner of thereader).

THE INSECT HEADTHE INSECT HEADTHORAX AND ABDOMENTHORAX AND ABDOMEN

MOUTHPARTSMOUTHPARTSCOCKROACH DISSECTIONCOCKROACH DISSECTION

The Insect HeadThe Insect HeadInsects are strongly cephalized animals, that is, many of the importantfunctions are moved anteriorly with a high degree of merging orcondensing of segments, sensory structures and neural ganglia. Thismodule illustrates the preceding statement. Additional information onthe insect head can be found in the mouthpart module.Six or seven segments are condensed to form the head capsule. Thisstrong structure provides protection for the brain, support for eyes,ocelli, antennae and mouthparts. The strongest muscles in the headserve the mandibles in chewing insects and the sucking pump inpiercing-sucking insects.The hard exoskeleton that is a common feature of arthropods isparticularly well illustrated in the head. The lesser appreciated internalcuticular support structures are also well represented by the cross-bracing tentorium.



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Grasshoppers and cockroaches(drawings) are often used to illustrate themost basic form of the insect head. Theeye is often prominent and themouthparts can seem strange to us. Theevolutionary flexibility of the fourmouthpart segmental appendages is thetheme of the mouthpart segment of thisseries.

In cockroaches, thehead is very flexiblyattached to the thoraxvia an extensive neckmembrane (arrow). Theoesophagus, heart,tracheal system, andnervous system all passthrough the foramen atthe back of the head( ).

*

*

Back View

Insect heads take many shapes reflecting the characteristic feeding habitsof the species concerned. For example, predaceous insects have forwarddirected mouthparts with a corresponding forward protrusion of the head.The predators in this collage of photos are probably obvious.

scorpionfly

antlion larva

waterbeetlelarva

tsetse fly cicada

stalkfly

The three most prominent arrangements of the head have been given thenames Prognathous , Hypognathous and Opisthorhynchous .

Tigerbeetle

Leafbeetle

Plant bug

Prognathous

Hypognathous

Opisthorhynchous

The compound eyes are often the mostprominent structures on the insect head.Adult holometabolous insects,as well asimmatures and adults of hemimetabolousinsects have them. Insect compound eyeshave thousands of more or less equivalentsensory cartridges called ommatidia. Eachommatidium has a hexagonal lens(hundreds in focus in this picture) and sixto eight light-sensitive cells. Singlehomologous sensory cells from numerousadjacent ommatidia respond to light intheir limited field of view and send theinformation to the same place in the opticlobe of the brain. The image formed in theinsect brain is not as detailed as ours butinsects can be exquisitely sensitive tomovement in the visual field. They alsocan see in colour.

opticlobe

retinularlayer

This section through a compoundeye shows some of the features.The lens layer is in red, the retinularlayer in black and gray and the firstof three parts of the optic lobe inblue-gray.

The retinular layer contains thesensory cells (six to eight perommatidium). The dark layer justunder the lens layer containspigments that help reduce lightscattering during the day.

This is a dark adapted eye so the pigments are all at one end. Thisincreases light scattering (decreases resolution) but it increases sensitivity.

lenslayer

Jake Remple, a Canadianentomologist, summarizedthis interpretation of insecthead segmentation basedon a careful examinationof developing embryos ofthe blister beetle Lytta.

The last three segmentsare easy to see at allstages of development(mandibular, maxillary andlabial) because they bearobvious bi-lateralappendages.

The three anterior segments required developmental analysis to confirmtheir presence. The acron is homologous with the front part of the body ofbi-laterally symmetrical animals such as the earthworm and is notconsidered a true segment because it lacks a pair of appendages.

HEAD OF INSECTAcron + 6 segments

1 - labral 2 - antennal 3 - intercalary 4 - mandibular 5 - maxillary 6 - labial

12

3

45

6

acron

corpus pedunculatumarchicerebrumprosocerebrumdeutocerebrumtritocerebrumcommisurelabral nervenervus connectivusnervus recurrensfrontal ganglionnervus procurrens

The largest structure in the head-neck region is the brain and sub-oesophageal ganglion complex. The oesophagus passes between these twomajor masses of nervous tissue. These are dorsal and lateral viewsrespectively. The numbers correspond to the segmentation numbers of theprevious slide.

1

23

commisure4

5

6

12

3

45 6

frontal ganglion

recurrent nerve

antennal nerveoptic lobe

corpus cardiacum

corpus allatum

oesophagusoesophagus

hypocerebralganglion

COCKROACH BRAIN - dorsal aspect Drawing of dissected cockroachhead showing brain and relatednerves. Frontal and hypocerebralganglia are part of thestomatogastric nervous system,while the corpora cardiaca andcorpora allata are part of thehormonal system.

This is a scanningelectron micrograph of theright side of a bugembryo.

Can you find the followingsegments?

• labial• labral• mandibular• maxillary

The green arrow marks the boundary between thehead and thorax.The eye is the bulge on the side of the head.Ventral is down and anterior is to the right.

eye

A typical ganglion (functional concentration of nerve cells) is composed ofa peripheral region of cell bodies (nerve and glial cells - red) and a regionof nerve to nerve connections called the neuropil.

The sub-oesophagealganglion (SOG) ofall insects servesthe majormouthparts andit’s therefore muchinvolved withfeeding. Thispicture is of afrontal sectionthrough the SOGof a fly.

neuropil

labellarnerve

nervetract

oesophagus

oesophagus

cellbody

brain

suboesophagealganglion

This unusual view of thesuboesophageal ganglion (SOG)of a fleshfly shows the brain andSOG in the same orientation asthe previous picture. Use theoesophagus as a referencepoint. Here a single neuron hasbeen filled with a fluorescent dyeto reveal its extensive branchingpattern in the SOG and in thebrain. The cell body of thisneuron is the bright dot justabove the cell body label. Thisneuron is one of many thatprocess taste input from thelabellar sensilla.

The inside of the headcontains more than thebrain, of course. There aremany muscles thatoperate the variousappendages - themouthpart muscles beingparticularly complex.

This cleared whole mountreveals another aspect ofthe head that is oftenoverlooked - the tentorium(red arrows).

The tentorium is the “internal skeleton” which gives strength to the headcapsule and provides a place for muscle attachment. It is formed by tube-shaped inward extensions of the exoskeleton. The mandibles, maxillaryand labial palpi are all visible in this frontal view.

END OF INSECT HEAD MODULEEND OF INSECT HEAD MODULE

The Insect Thorax and AbdomenThe Insect Thorax and AbdomenIn the simplest terms, the thorax is the locomotory centre of the insectsince all six legs and the wings are found there. The largest musclesare also found in the thorax. The thorax is a box-like structure withextensive internal cuticular cross bracing. It also sports numerouscuticular plates (sclerites) that are intimately involved in locomotion.To conserve mass, some of the thoracic muscles are involved in bothwalking and flying. This works because these are mutually exclusivebehaviours and the motor nervous system plays the appropriate motorprogram at the appropriate time. The abdomen is simple in its anteriorregion getting quite complex in the last three segments where thesclerites of the external reproductive system are found. Both male andfemale insects have extensive cuticular modifications for reproduction,females particularly for oviposition (egg laying) and males for sperminsertion. Many insects can be identified at the species level only bylooking carefully at the male genitalia.



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Ventral View of Periplaneta Thorax

pronotumneck region

prothoracicsternum

mesothoracicpleuron

tergum

metathoraciccoxa

mesothoracictrochanter

mesothoracicfemur

membranouscuticle

Meso- and Metathoracic Segments of Periplaneta - ventral view

mesosternum

metasternum

coxalcavity

coxalcavity

mesopleuron

metapleuron

Grasshoppers are much strongerflyers than cockroaches and thisis clearly reflected in the structureof the thorax. Note the greaterfusion of the ventral thoracicsclerites to form a strongpterothoracic “box” to supportflight.

mesothorax

spinasternum

metathorax

Mesothorax of spruce budworm moth - lateral view

basalar subalar

dorsoventralflight muscle

muscles to alary sclerites

subalar muscle(wing supination)

basalar muscle(wing pronation) trochanter muscle

coxa trochanter

tergum

prothoracicpleuron

flight musclepart of cuticularcross bracing

leg muscleventral nerve cord

gut

haemocoel

Cross section through the thorax of a cockroach showing some majorinternal structures. Note the size of the muscles and that some are cut inlongitudinal section while others are cut in cross section.

As with most insects, the cockroach abdomen is composed of a series ofsimilar segments until the terminal end when sex complicates things. Matingbehaviour, pheromone production, egg production and release are allhandled largely by the abdomen and particularly by modifications of the lastabdominal segments. The male and female cockroach abdominal tipsshown here clearly reveal some obvious differences.

female

maleseg ix

anal style

anal cercus

seg vii

suranalplate

( )

( )

Insect legs have numerousadaptations that depend onthe insect’s lifestyle. Here aretwo examples. At top, a legfrom an aquatic insect (notethe numerous fine hairs thatspread for swimming) it isalso used to name the basicleg parts. At bottom, raptorialforelegs from a mantispid(mantidfly) used for seizingprey then clamping duringfeeding.

coxatrochanter

femur tibia

tarsus

The wing must move with greatdexterity if the insect is to fly properly.The axillary sclerites (red arrows)provide a major part of this complexattachment and the muscles thatinsert on them are responsible forsmall but crucial movements of thewing in flight. This sclerite/musclesystem is also involved in wingfolding.

The Wing Base

Most insects have four wingswhich either beat quiteindependently as indragonflies or with varyingdegrees of unity. The frontand hind wings of moths arestrongly linked together andthe hind wing has a strongstructure called the frenulum(red arrow) that is part of thelinking apparatus. It hooksinto a process on the frontwing called a jugum (injugate moths in any case).

In the honeybee (not shown), the hind wing has little hooks that look verymuch like the hooks in a Velcro strip and the fore wing has a correspondingsurface for attachment.

Diptera, as you can see from the name, have two wings instead of four.In flies, the hind wings take the form of dumbell shaped structures thatact as balancing organs during flight. These highly modified andspecialized hind wings are called halteres

haltere

Wing veins provide support for the thin, delicate membranous cuticlethat make up the rest of the wing, as shown here by these two wingsfrom a fly. Many veins also house tracheae and provide a passage forhaemolymph (blood). Numerous sense organs are found on the wings,especially wind sensitive hairs, and the cells in these organs mustrespire and receive nutrients. Accessory hearts at the base of the wingprovide a sometimes complex flow of haemolymph in the veins. Wingveins can also be important taxonomic characters at the Family level.

Respiration in insects is mediated by a complex, multi-branched trachealsystem. The tracheal tubes (tracheae) are epidermal in origin and so arelined with thin cuticle. They communicate with the outside via segmentalopenings called spiracles found on the thorax and abdomen. Whenpresent, there are two spiracles per segment. Not all segments havespiracles, for example, compare the spiracles in an adult cockroach andan adult fly. These photos show examples of spiracles and associatedtracheae. The spiracle on the right has a filter system to prevent entry ofdust. Many spiracles have complex opening/closing mechanisms.

Mating behaviour ininsects is often acomplex businessthat is a critical part ofthe species isolatingmechanism. Thus,intricate behaviourscorrectly performedsignal speciesidentity.

In addition to behaviour, the structure of the last few abdominal segmentsplays a major role in species recognition. Here two craneflies are locked incopula. Specialized sclerites of the last three abdominal sclerites (terminalia)allow only members of the same species to proceed this far.

In the Odonata the male actually grabs the female by the back of theneck during mating and as they fly or perch, the female reachesaround with her abdomen and takes sperm from a specialized holdingpouch near the junction of the male thorax and abdomen.

maleabdomen

Insects often have ratherastonishing reproductivestructures, especially themale intromittent organ oraedeagus. In this flourbeetle the aedeagus iswithdrawn into the abdomenwhen not in use.

aedeagus

testis

accessorygland

This drawing shows, in generalized form, the way female abdominalsegments 8, 9 and 10 are modified for egg laying. Along with mating,egg laying requires extensive modification of sclerites.

tergite8

2 gonapophysis

gonangium

1 gonapophysis

tergite9

tergite10stylus

1coxopodium

2 coxopodium

In these views of actual specimensyou can identify some of thecuticular parts shown in thedrawing of the generalized femaleabdomen. This specimen is aleafhopper.

Parasitic wasps take the businessof ovipositor modification to anextreme but all of the basic partsare there. 50% of all wasps areparasitic on other insects andneed to lay their eggs with greatprecision since many are quitehost specific.

Flies, such as this housefly, have a different version of the ovipositor.The last few abdominal segments telescope into the larger anteriorabdominal segments except when the fly is laying eggs. Then thesesegments are extended as shown here.

In bees, the ovipositor functionis subsumed by the sting whichis served by a poison gland.Oviposition is through a simpleopening.

sting

tergite9

2 coxopodium

bulb

gland

poisonsac

sheath

gonangium

2 gonapophysis

1 gonapophysis

Male and female abdomens are alsovariously modified, mostly for, in thecase of the male, holding the femalein position during copulation and inthe female accepting the male duringcopulation and also for oviposition(egg laying).waterbug

cranefly

o

snakeflyo o

The entire abdomen can also serve important functions in arthropods. Thetail flick that is part of the escape response of the crayfish is perhaps thebest known (it’s part of the reason that lobster tails are such good eating).Here is another use of the whole abdomen displayed by blackfly larvae.They twist their abdomen 180 degrees so that their feeding fans can beproperly oriented in the mainstream water flow.

mainstream flow

boundary layer

vortex

laminar sublayersubstrate

Requirements of internalorgans also influence theshape of the abdomen. Thecrop of this mosquito is so fullof blood that the abdomen isswollen to near maximum size(cuticle does not stretch). Alarge egg load can also swellthe abdomen like this.

Many aquatic insects usespecial adaptations of thelast abdominal segments fororienting in the stream or forbreathing. Notice how thismayfly nymph uses itscaudal filaments fororientation.

The head, thorax and abdomen are covered with cuticle as are the foregut,hindgut and tracheal system. These are the basic components of insectcuticle that you should be familiar with. The epicuticle provides a continuouswax covering to prevent desiccation. The exocuticle is a mix of chitin (likecellulose) and tanned protein. You can see why it won’t stretch. Theendocuticle, including mesocuticle is flexible and elastic.

epicuticle

trichoid sensillum

dermal gland duct

epicuticle

basementmembrane

trichogencell

epidermalcell

oenocyte dermal gland

exocuticle

exocuticle(hard)

endocuticle(soft)

epidermis

This is a section of bending cuticle.As you can see, it is not exactlylike the previous drawing. Not allcuticle looks alike. Some is verythick, others thin. Some cuticle haswell developed mesocuticle, asshown here. The blue endocuticleis elastic, so this cuticle is highlybendable.

As the functional skeleton of arthropods,the cuticle is the primary place for muscleattachment. This section shows howintimate is the connection betweenmuscle and cuticle. Muscle extensionscalled tonofibrillae make the actual link.At moulting, these connections mustremain functional until the last momentwhen new connections are establishedwith the new cuticle developing beneaththe old.

musclescuticle

exocuticle

endocuticle

mesocuticle

tonofibrillae

The spectacular picture of a wing base of a moth, shows a top view ofbending cuticle. The yellow ovals are the cones of mesocuticle protrudingdown into the blue endocuticle, so this particular sclerite is very flexible.The large areas of yellow cuticle are exocuticle and are relatively hard.

bending cuticle

exocuticle

endocuticle

END OF THORAX ANDEND OF THORAX ANDABDOMEN MODULEABDOMEN MODULE

Insects owe their great success to their abilities to adapt to awide variety of habitats. Adaptations have occurred, indeedcontinue to occur, at every level of organization from themolecular to the ecological. The comparative study of insectmouthparts is first and foremost a study of adaptation. Wesingle out mouthparts and the gross morphological level oforganization in introductory courses because the variety ofadaptations is so great that we can address vast landscapesof insect evolution by looking at mouthparts alone. Also, theyare relatively easy to see and appreciate and, perhaps mostimportant, the mouthparts are entral to insect trophicbehaviour cfood gathering behaviour).

Mouthparts of InsectsMouthparts of Insects



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The American cockroach(Periplaneta americana) has“generalized” mouthparts. Thisinsect is omnivorous and itsmouthparts are well suited tochewing on a wide variety offood items.

The mouthparts are welldeveloped in cockroaches. Eachmouthpart pair comprise the twobi-lateral appendages of theirsegment.

The mouthpart-bearingsegments are the labral,mandibular, maxillary and labial.Two of the segments (mx andlab) have sensory palpi as wellas other functional parts.

antenna

emaxilla

galea

labium

mandible labialpalplabrum

maxillarypalp

foramenmagnum

antenna

eye

cardo

stipes

maxilla

prementum

labial palp

paraglossagalea

prestomen

labium

maxillary palp

This posterior view of thecockroach head showsadditional features as well asthe various mouthpartappendages.Note the complexity of themaxilla and labium.See next slide for additionaldetails.

Hidden among the maxillaeand labium is the “tongue”or hypopharynx. It has verythin (membranous) cuticleand contains the opening ofthe salivary gland duct.

The labium is the mostcomplex of the cockroachmouthparts and exists as asingle piece rather than astwo distinct, mirror imageparts as in the maxillae. Thissingle labium is clearlyformed from a fusion of bi-lateral appendages as thepaired palpi, glossae andparaglossae reveal.

labial palp

glossa paraglossa

prementummentum

submentum

mandiblelabrum

hypopharynx

labium lacinia

maxillarypalp

stipes

cardomaxilla

mandible

maxilla

This frontal view of a cockroachhead gives a better idea of whatthe mouthparts look like in situ.The tentorium is also visible (seemodule on the head in thisseries).

Note the well sclerotizedmandibular “teeth” (black ridgesin lower center) and theprotruding galeae/lacinia andparaglossa. The mandiblesappear to be at the front since thelabrum which largely covers themis transparent in this clearedspecimen.

antenna

eye

mandible

labialpalp

maxillarypalp

hypopharynx

This detailed view of acockroach maxilla clearly showsthe sclerites that make up thiscomplex appendage. Thedarker the cuticle the heavierthe tanning and usually theheavier the sclerotization. Thus,the lightest cuticle ismembranous-like and semi-transparent while the darkestcuticle is hard.

The blue arrow shows wherethe maxilla attaches to thehead.

stipes

cardo

maxillary palp

galealacinia

sub-mentum

mentumpre-

mentum

glossaparaglossa

labialpalp

This labium was taken from thesame specimen as the maxilla inthe previous slide. Note that muchof the cuticle is thin, so much sothat the muscles operating thelabial palpi are visible (bluearrowheads ).

Compare the tips of the galea andlacinia in the previous slide with theglossae and paraglossae in thisphoto. All are dark indicatinghardened cuticle. These structuresare used to manipulate food.

By contrast, the tips of the palpi inboth appendages are covered withmembranous cuticle and, as we willsee later, with many sensory pegs.

labrum

mandible

hypopharynx

labium

maxilla

You may be asked todissect the mouthparts of acockroach in the laboratoryand this is what your finaldissection should look like.The parts are arranged inthe order of segmentationfrom anterior to posterior.The hypopharynx is the onlypart that is not a truesegmental appendage butrather a modified part of theprostomium (foregut).

maxilla

mandible

REVIEWREVIEW

Can you recognize the following?

• antennae• clypeus• eye• frons• labial palpi• labrum• mandible• maxillary palpi

REVIEWREVIEWHere’s a better challenge. This isa lateral view of a cockroachhead with the mouthpartsextruded. Which of the followingcan you recognize?

• antennal base• clypeus• eye• galea• hypopharynx• labial palp• labium• labrum• mandible• maxillary palp

This completes our review of basic insectmouthparts as found in the American cockroach(Periplaneta americana). Make sure that youunderstand this basic plan before proceeding.

The rest of this module on mouthparts shows howvarious insect groups have used the basicmandibulate ground plan to produce highlymodified mouthparts exquisitely adapted to theirown way of obtaining food.

Dipteran mouthpart modifications

Blackflies (Simulidae) are small insects buttheir mouthparts are relatively large. All ofthe parts are present in adults but they aregreatly modified from the basic plan. Thelabellum is used for nectar feeding but thelabrum, lacinia, and mandibles aremodified as piercing structures for bloodfeeding. The relatively large wound and theantigenic properties of the salivary proteinslead to ugly skin reactions.

labrum

maxillary palplabellum

eye

antenna


Horseflies (Tabanidae) have the same kinds of mouthpart modifications asblackflies, but since they are larger, are easier for us to study (their bitealso causes more physical damage). All of the basic parts can be seenhere and don’t resemble the mouthparts of the cockroach. Neverthelessthese are the appendages of the same segments - the result of the samebasic developmental plan.

maxillarypalp

maxillarypalp

labellum

labrum

lacinia

mandible hypopharynx mandible

lacinia

horsefly adult -horsefly adult -dissected mouthpartsdissected mouthparts

((exex. micro-slide). micro-slide)

antennaemandibles

labrummaxillary

palpi

laciniae

hypopharynx

labium

e

pharyngealpump

salivaryduct

tentorium

Mosquitoes have carried the business of piercingand sucking to a fine art. The stylets are modifiedlabrum, maxillae and mandibles, just as in horsefliesand blackflies, but the cross section size is markedlyreduced making for a much stealthier approach tofeeding.

Only femalemosquitoes takeblood. Males donot possess thenecessary m.pts.The mosquitoshown has allm.pts hence is afemale. Bothsexes suck nectar,and possess thelarge labellum.


Maxillary stylets- greenLabral stylet - blueMandibular stylets - yellowFood canal - redSalivary canal - pinkHypopharynx - blackLabium - tan

Mosquito head and mouthparts - frontal view


x.s.

labellum( not seen in X.S.)

rostrum

maxillarypalpi

labrum

labellum pseudotracheae

haustellum

The higher flies (houseflies and relatives)have lost the mandibles and maxillae,except for the maxillary palpi. Theyspecialize in modifications of the labialappendages only to producesophisticated lapping, probing, scrapingand piercing mouthparts. Most of themcannot pierce host skin but some, likethe tsetse fly (later slide) can.

The fly labellum is a complexsensory organ and feedingstructure looking something likea sock. It’s dexterous beingcontrolled by numerous smallmuscles.


This SEM (Scanning ElectronMicroscope) micrograph of afleshfly labellum shows the sensoryhairs that cover the surface of theoral surface of the lobes. Thesesensory hairs taste sugars, salts,amino acids, water and otherchemicals as well as conferring adelicate sense of touch on thewhole organ.


labellum

front

haustellum

antenna

maxillarypalpi

labellum haustellum

Tsetse flies have carried labellarmodification to an extremeunmatched by any other insect.The fine labellum with its toothbearing tip makes a needle-likepenetration of the host skin.


eyeantenna

maxillarypalp

labrum

haustellum

labellum {

Lepidopteran mouthpart modifications

In moths and butterflies the sucking mouthpart is largely the modifiedmaxillary galea. The area along the food canal is lined with taste pegs.The maxillary palpi are much reduced and mandibles are absent.

foodcanal

maxillarypalp

stipes

cardo

galea

ocellusantenna

eye

labialpalp

galea

epipharynx

labrum

pilifer

antenna

eye

labialpalp

galeae

Lepidopteran mouthpart modifications

The real thing looks different fromthe previous drawing of course. Forone thing, moths and butterflies arecovered with scales (modifiedcuticular hairs), as seen in thiscleared lepidopteran head. Thelarge labial palpi are sensory, infact, in some specialized nocturnalmoths the organs of hearing arealso on the palpi. The galeae areseparated in this preparation.During feeding, they are held closetogether to form a sucking tube.

maxillarypalp

mandible

labialpalp

flabellumglossa

Hymenopteran mouthpart modifications

The sawflies, bees and wasps havevarious arrangements of mouthparts,the most complex being those of bees(photo at right). Like lepidopterans,bees use a tube to suck nectar but it isa modified glossa (part of the labium).Mandibles are retained primarily forhandling wax - in fact, the structure ofthe mandibles gives rise to the precisehexagonal arrangement of the cells inthe honeycomb.

dilating muscles

salivaryduct

hypopharynx

maxilla mandible labium

Hemipteran mouthpart modifications

Insects with incomplete metamorphosis alsouse highly modified mouthparts as shown in thetrue bugs and their relatives. These are used forpiercing and sucking and are similar in someways to the modifications seen in mosquitoes.Note the large dilating muscles of the suckingpump - the largest muscle in the bug head.

mandible maxilla

This concludes our survey of mouthpart modifications. Inseveral places we mentioned that sensory structures werepresent on certain mouthparts, such as palp tips. In fact,one of the greatest concentrations of chemosensilla isfound on the mouthparts - the other major site being theantennae.

In the next section, we look in detail at some sensorystructures and comment briefly on their function. The studyof sense organs has major implications for pheromonebiology, insect-plant interactions and the chemicalmodification of feeding by the use of antifeedants.

mandiblemaxillary

palp

labialpalp

If you have been studying this module carefully, you will likelybe able to quickly make sense of this ventral view of the headand mouthparts of a leaf beetle (Chrysomelidae). Leaf beetleschew leaves hence have strong mandibles. The palpi are welldeveloped for sensory purposes as are parts of the labrum andthe galea.

maxillarypalp

Galeae of leaf beetles have specialized taste hairs with roundedtips (red arrowheads). These are used to taste leaves before adecision is made to take a meal. Leaves of poor host plants don’ttaste right to the beetle so small meals are taken. Leaves of hostplants are assessed quickly and extended feeding occurs.

1 2

34

These two examples of tastehairs were taken from leafbeetle galea. The pore in thetip is where the leaf juiceenters in order to be tasted.The sensillum with the littlefingers in the right handpicture does not have muchof a pore and is probablyused to sense temperatureand humidity.

Inside the large taste hairs we find thesensory cells, as this electronmicroscope cross section through thebase of the larger sensillum at top rightshows. The four numbered cells arechemosensory while the one markedwith a yellow arrowhead ismechanosensory. Most taste hairs ininsects can also sense movement.

This series of increasingmagnifications are takenfrom the inside of thelabrum (epipharynx) of aleaf beetle. The epipharynxforms the anterior surfaceof the buccal cavity (roof ofthe mouth so to speak) andit also bears taste-sensitivestructures. These sensillaeare not hair-like but takethe shape of short pegs-in-pits. They too have pores.

The most obvious of the mouthpart sensory structures arethe palpi (maxillary and labial). In leaf beetles, the palp tipsprovide four surfaces that are covered with small sensorypegs - some for taste and some for smell.

1

10

100

These three drawingssummarize the structure of atypical insect taste hair orsensillum. There are usuallyfour cells projecting to thepore tip and these tastedifferent chemicals with someoverlap in sensitivity. Amechanosensory cell (notshown) usually terminates atthe base of the sensillum.Supporting cells (brown, blueand green) are important tothe physiology of the entiresystem but they are notsensory. The numbersindicate the length of thescale bar in micrometers.

µµ

µµ

µµ

End of Mouthpart ModuleEnd of Mouthpart Module

While it is impossible to chose an insect species that truly representsthe morphology of all, the cockroach has gained a commandingposition as the insect of choice for illustrating the ‘basic’ insect bodyplan. Another strong contender for this role is the grasshopper. Eventerms like ‘the cockroach’ and ‘the grasshopper’ are terriblymisleading, since there are many species of each with sometimesmarked differences in various structures. Nevertheless, cockroachesand grasshoppers are good representatives of insects with chewingmandibles and lacking extreme morphological specializations of themajor body parts.Here we address the major external and internal parts of theAmerican cockroach, Periplaneta americana. Some of the terms willbe new but many will be strikingly familiar for they are named aftertheir, sometimes very approximate, counterparts in vertebrates.



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Cockroach DissectionCockroach Dissection

DISSECTION OF COCKROACH - MOUTHPARTSDISSECTION OF COCKROACH - MOUTHPARTS

Lateral view of Periplaneta americana.

frontal viewof entire head

showing positionof mouthparts

frontal viewof entire head,

stained and cleared toshow underlying

mouthparts

clypeus

mandibles

labrum

hypopharynx

maxilla

DISSECTION OF COCKROACH - MOUTHPARTSDISSECTION OF COCKROACH - MOUTHPARTSDISSECTION OF COCKROACH - MOUTHPARTS

labium

clypeus(6)

mandibles(5)

hypopharynx(2)

mandibles (5)

labium (4)

labium (4)

hypopharynx (2)

(1) - (6) order of removal from head

DISSECTION OF COCKROACH - MOUTHPARTSDISSECTION OF COCKROACH - MOUTHPARTSDISSECTION OF COCKROACH - MOUTHPARTS

maxilla (3)

maxilla (3)

labrum (1)

labrum (1)

Recently killed male cockroach,with legs removed and partiallyimbedded (dorsal side up) in blackcomposite wax, in a dissecting tray.Distilled water or Ringer’s solutionis added to the dish before thedissection begins.

DISSECTION OF COCKROACH - GENERALDISSECTION OF COCKROACH - GENERALDISSECTION OF COCKROACH - GENERAL

cockroach mountedin black composite wax

Abdomen of male cockroach.Wings and abdominal tergitesremoved exposing fat body,tracheae, dorsal vesseland underlying alimentary organs.

fat body

dorsalvessel

crop

tracheae

stomach

testis lieswithin thisfat body

DISSECTION OF COCKROACH - GENERALDISSECTION OF COCKROACH - GENERALDISSECTION OF COCKROACH - GENERAL

dorsalvessel

fat body

tracheae

spiracle

insect pinplaced through

spiracle opening intoconnecting trachea

tracheae

tracheae

tracheal supply around stomach

DISSECTION OF COCKROACH - RESPIRATORY SYSTEMDISSECTION OF COCKROACH - RESPIRATORY SYSTEMDISSECTION OF COCKROACH - RESPIRATORY SYSTEM

apertureoperculum}

DISSECTION OF COCKROACH - ALIMENTARY SYSTEMDISSECTION OF COCKROACH - ALIMENTARY SYSTEMDISSECTION OF COCKROACH - ALIMENTARY SYSTEM

oesophagus

enteric caeca

colon

vasdeferens

testis

rectum

ileum Malpighiantubules

stomach

gizzard

crop

DISSECTION OF COCKROACH - NERVOUS SYSTEMDISSECTION OF COCKROACH - NERVOUS SYSTEMDISSECTION OF COCKROACH - NERVOUS SYSTEM

fat body

HEAD

THORAX

ABDOMEN

mesothoracicganglion

abdominalganglion

interganglionicconnective

metathoracicganglion



suboesophagealganglion

HEADantenna

cercus

abdominalmuscles

DISSECTION OF COCKROACH - REPRODUCTIVE SYSTEMDISSECTION OF COCKROACH - REPRODUCTIVE SYSTEMDISSECTION OF COCKROACH - REPRODUCTIVE SYSTEMO

testis

vas deferens

vesiculaseminalis

anal styles

anal cercus

external featuresO

fat

testis

ovarioles

terminalegg

analcercus

DISSECTION OF COCKROACH - REPRODUCTIVE SYSTEMDISSECTION OF COCKROACH - REPRODUCTIVE SYSTEMDISSECTION OF COCKROACH - REPRODUCTIVE SYSTEMO

cellularsheath

surroundingovarioles

colleterialglandovarioles separated

from cellular sheath

external featuresO

END OF COCKROACHEND OF COCKROACHDISSECTION MODULEDISSECTION MODULE

AN INTRODUCTION TO INSECT STRUCTURE - · PDF fileAN INTRODUCTION TO INSECT STRUCTURE ......

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