an introduction to insect structure -
TRANSCRIPT
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.
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
B.K.Mitchell and J.S.ScottB.K.Mitchell and J.S.ScottDepartment of Biological SciencesDepartment of Biological SciencesUniversity of AlbertaUniversity of Alberta
©
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.
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
B.K.Mitchell and J.S.ScottB.K.Mitchell and J.S.ScottDepartment of Biological SciencesDepartment of Biological SciencesUniversity of AlbertaUniversity of Alberta
©
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
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
B.K.Mitchell and J.S.ScottB.K.Mitchell and J.S.ScottDepartment of Biological SciencesDepartment of Biological SciencesUniversity of AlbertaUniversity of Alberta
©
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
Dipteran mouthpart modifications
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.
Dipteran mouthpart modifications
Maxillary stylets- greenLabral stylet - blueMandibular stylets - yellowFood canal - redSalivary canal - pinkHypopharynx - blackLabium - tan
Mosquito head and mouthparts - frontal view
Dipteran mouthpart modifications
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.
Dipteran mouthpart modifications
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.
Dipteran mouthpart modifications
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.
Dipteran mouthpart modifications
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.
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
B.K.Mitchell and J.S.ScottB.K.Mitchell and J.S.ScottDepartment of Biological SciencesDepartment of Biological SciencesUniversity of AlbertaUniversity of Alberta
©
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
interganglionicconnective
interganglionicconnective
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