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Chapter 2

Structure and Function

10 2. Structure and Function

This chapter introduces the basics of anatomyand physiology of insects that are necessary forthe classification of insects and development ofsuccessful pest control strategies. An under-standing of external structures of insects is aprerequisite for their taxonomy and identifica-tion. Fundamental life processes such assensing, reproduction and locomotion are out-lined for the comprehension of insect behav-iour, population dynamics and dispersal.

2.1 External AnatomyInsects show great diversity of shape and form.A generalised model as shown in fig. 2-1 istherefore used to study structure and function.Comparative anatomy studies adaptations ofbasic structures like legs or mouthparts tovarious ways of feeding or locomotion. Despitethe diversity of shapes and forms, moststructures are built according to the same planand are then called homologous. Fig. 2-2indicates orientation and anatomical planesthat describe the location of various body parts.Adult insects are generally made up of three

major tagmata or body parts, head, thoraxand abdomen, as summarised below. The bodyparts, their appendages and respectivefunctions are further discussed in this chapter.

Body Part Main Function No. of SegmentsHead: feeding, sensing and

nervous coordination 6Thorax: locomotion (legs, wings) 3Abdomen: digestion, reproduction 11 or fewer

and excretion

Fig. 2-1: Generalised model of an insect.S 1-8: sternum 1-8, T 1-10: tergum 1-10(reproduced from CSIRO, 1991)

Fig. 2-2: Orientation and anatomical planes(reproduced from Gullan, P.J. and Cranston, P.S., 1994)

2. Structure and Function 11

2.1.1 Integument and Exoskeleton

The integument is the outer protectivecovering of insects and other Arthropoda,forming the exoskeleton or body wall. Unlikevertebrates that have an internal skeleton(endoskeleton), insects possess a capsule-likeexoskeleton. The exoskeleton provides sup-port for soft tissues, attachment for muscles,protection against physical stress like me-chanical impact and evaporation, protectionagainst germs and other parasites. Moreover,the exoskeleton bears sense organs. Theintegument is composed of the basement, theepidermis and the inert (dead) cuticle asshown in fig. 2-3. The cuticle consists ofstructural proteins, pigments and chitin.Chitin is a tough and rigid protein thatundergoes a tanning process directly after themoult. Thus the initially soft cuticle hardensor sclerotizes. Once hardened, the cuticle isinflexible and can not grow any more.Therefore the growth of an insect is restrictedand makes moults necessary for gaining bodysize, as described in chapter 2.2.9. Theoutmost layer of the cuticle is wax-coveredand sometimes has an additional cementlayer.The exoskeleton is composed of variousplates joined together with softer connectingmembranes, thus allowing movement espe-cially in the regions of the joints as shown infigs. 2-18 and 2-22.

The body coloration of insects iseither made from physical orpigmentary colours. Physicalcolours like the iridescentmetallic colours of beetles andbutterflies are due to the pheno-menon of interference. Pigment-ary colours like the green colourof grasshoppers are the result ofconjugated double bonds ofpigments in the cuticle. Thelatter are not persistent and fadeaway, for instance the greenish-yellow colour of a grasshopperturns into brown, after theanimal dies and its body dries.

Fig. 2-3: General structure of the insect cuticle(reproduced from Coulson, R.N. and Witter, J.A., 1984)

2.1.2 Head

The head (cranium or caput), is the anteriorcapsule-like structure that bears the brain,mouthparts and sense organs like antennae andeyes. The insect head consists of six fusedsegments, the labral segment, the antennalsegment, the postantennal segment, themandibular segment, the maxillary segmentand the labial segment. The shape of the headvaries considerably in relation to how theinsect feeds. Insects with chewing mouthpartsnormally have large heavy heads that aredirected downward or forward, whereas insectswith piercing-sucking mouthparts have smallheads that are quite variable in appearance andposition. The generalised view of the head isshown in figs. 2-4 and 2-6.

Fig. 2-4: General view of agrasshopper head (reproduced

from CSIRO, 1991)


2.1.2.1 AntennaeThe antennae or feelers are a pair of mobile,segmented appendages located on the anteriorportion of the head between the compoundeyes. The antennae are part of the antennalsegment (segment 2) and are homologous tothe true legs of the thorax. All insects exceptProtora possess one pair of antennae. Theprimary function of the antennae is sensory.Various types of small hairs (sensilla) locatedon the antennae act as mechano- (tactile),chemo-, hydra-, sound, and temperature recep-tors. Antennae play an important role in themating process of many insects. Feelers aresometimes greatly enlarged in males toincrease the surface area and consequently theefficiency of the sense of smell. Antennae arecommonly used as a taxonomic characteristicin identifying insects because of the distinctivevariations in size and shape shown in fig. 2-5.

2.1.2.2 MouthpartsA basic understanding of the different types ofmouthparts is important because feeding tracesand type of damage often tell to which orderthe insect that has caused the particular damagebelongs. It is also important to recognise

mouthpart types since they vary considerablyand are always used in classifying insects.Mouthparts are generally divided into twomajor types, mandibulate or chewing andpiercing-sucking. All types of mouthpartshave evolved from chewing mouthparts asadaptations to different sources of food. Thebiting mandibles of a cockroach for instanceare homologous to particular parts of a bug’sproboscis, even though the latter is used forpiercing its plant or animal host. Themandibles of these two different mouthpartshave evolved from the same ancestral structurebut have different functions.Mouthparts are composed of several compo-nents, as shown in figs. 2-6 to 2-17. The fivebasic parts are

• the labrum, a movable flap attached to thefront part of the head, covering the mouthlike the upper lip

• the mandibles, a paired appendage of headsegment 4, typically hard and sclerotizedwith various sets of teeth (endites) orbrushes used like jaws

• the maxillae, a paired appendage of headsegment 5, consisting of several parts andused for tasting and uptake of food

Fig. 2-5: Types of antennae (A) filiform (thread-like), (B) moniliform (bead-like), (C) clavate orcapitate (clubbed), (D) serrate (saw-like), (E) pectinate (comb-like), (F) flabellate (fan-shaped), (G)geniculate (elbowed), (H) plumose (with whorls of setae), (I) aristate, (J†) setaceous (tapering), (K†)lamellate (leaf-like) (reproduced from Gullan, P.J. and Cranston, P.S., 1994; Ross, H.H. et al., 1982†)

A B C

D E F

GH I

J K


Fig. 2-6: Components of the mouthparts of an earwig (Dermaptera). Frontal view of head at topand dissected mouthparts at bottom (reproduced from Gullan, P.J. and Cranston, P.S., 1994)

• the hypopharynx, an unpaired, tongue-likeorgan

• the labium, a paired appendage of segment6, forming the lower lip. The labium is alsoreferred to as a fused pair of secondmaxillae, consisting of various parts. Thefunction is similar to the first pair ofmaxillae.

Chewing MouthpartsThis type, shown in figs. 2-4, 2-6 and 2-7generally occurs in primitive insect orders likesilverfish (Thysanura), crickets, locusts andgrasshoppers (Orthoptera) and cockroaches

(Blattodea). Chewing mouthparts are alsofound in larval instars of higher developedorders like beetles (Coleoptera), moths andbutterflies (Lepidoptera), true flies (Diptera),ants, wasps and bees (Hymenoptera) and inadults of Coleoptera, Hymenoptera and otherorders. The mandibles might be reduced as inweevils (see fig. 5-38), so that the mouthpartscan be confused with a piercing-suckingproboscis. Chewing mouthparts are easilyrecognised by a large head that bears themuscles operating the heavy sclerotizedmandibles. Insects with chewing mouthpartsbite and chew their food.


Fig. 2-7: Chewing mouthparts of a Mecopteralarva (reproduced from Ross, H.H. et al., 1982)

Filtering typeThe filtering type of mouthparts, as shown infig. 2-8, can be found in some aquatic insectsand is derived from mandibulate mouthparts.

Fig. 2-8: Filtering mouthparts of the black fly(A) Crozetia sp., (B) Simulium sp. (Simuliidae)(reproduced from Ross, H.H. et al., 1982)

Cutting-sponging typeThis type, shown in fig. 2-9, can be found inhorse flies and some other Diptera. The sharpmandibles are for cutting the integument of amammal host, causing blood to flow from thewound. The blood is then collected by asponge-like development of the labium andsucked into the mouth.

Fig. 2-9: Cutting sponging mouthparts of ahorse fly (Tabanidae) (reprod. from Seifert, G., 1995)

Chewing-lapping typeThe chewing-lapping type of mouthparts, asshown in fig. 2-10 can be found in bees andwasps (Hymenoptera). Mandibles and labrumare of the chewing type for grasping prey,maxillae and labium are channelled to probedeep into the nectaries of blossoms.

Fig. 2-10: Chewing-lapping mouthparts of thehoney bee Apis mellifera (Apidae) (reproducedfrom Gullan, P.J. and Cranston, P.S., 1994)

A

B


Piercing-sucking mouthpartsAs shown in figs. 2-11 to 2-14 this type usuallyappears as a rod-like beak, called a proboscisand is adapted for piercing plant or animaltissues and sucking sap or body juices. Insectswith piercing-sucking mouthparts are found inorders such as Hemiptera (aphids, scaleinsects, cicadas, true bugs, etc.), Siphonaptera(fleas), some Diptera like mosquitoes andsome Phthiraptera (lice). Labrum, mandiblesand maxillae are slender and long and fittogether to form a hollow needle. The labiumforms a sheath to hold the needle rigidly.Mosquitoes inject pain killers and anti-coagulants prior to sucking.

Fig. 2-11: Piercing-sucking mouthparts of atrue bug (reproduced from Seifert, G., 1995)

Fig. 2-12: Piercing-sucking mouthparts of amosquito (reprod. f. Gullan, P.J. & Cranston, P.S., 1994)

Fig. 2-13: Piercing-sucking mouthparts of afirst larval instar of the plant hopper Macro-steles spp. (reproduced from Ross, H.H. et al., 1982)

Fig. 2-14 (left): A phytophagous Hemipterafeeding with piercing-sucking mouthparts(reproduced from Gullan, P.J. and Cranston, P.S., 1994)


Sponging mouthpartsSponging mouthparts as shown in figs. 2-15and 2-16 are fitted for using either liquid orfood soluble in saliva and can be found inmany non-biting flies. This type is similar tothe cutting-sponging type but the parts forchewing are non-functional. The apex of theproboscis called the labella, extrudes salivaand draws up the dissolved or pre-digestedfood into the mouth.

Fig. 2-15: Sponging mouthparts of the house flyMusca domestica (Muscidae) (reproduced withpermission from Seifert, G., 1995)

Fig. 2-16: Sponging mouthparts of the vinegarfly Drosophila melanogaster (Drosophilidae)(reproduced from Ross, H.H. et al., 1982)

Siphoning-tube typeMouthparts of the siphoning-tube type, asshown in fig. 2-17, are found in adult mothsand butterflies (Lepidoptera). These insectssuck nectar and other liquid foods by means ofa long proboscis. The proboscis is composedonly of united parts of the maxillae, forming atube that opens into the oesophagus. Theproboscis is coiled when not in use andstretched out, when the animal is drinking.

Fig. 2-17: Siphoning-tube type mouthparts of abutterfly (Lepidoptera) (reprod. from Seifert, G., 1995)

2.1.3 Thorax

The thorax is the middle body region of aninsect, usually composed of three segments, theprothorax, mesothorax and metathorax asshown in fig. 2-1. The modified last thoracicsegment of apocritan Hymenoptera is shown infig. 5-59. The main function of the thorax islocomotion. Each segment bears one pair oflegs. Winged insects (Pterygota) have one pairof wings attached to the second segment(mesothorax) and a second pair attached to thethird segment (metathorax). All segments havean internal skeleton for the attachment ofmuscles. In grasshoppers (Orthoptera) andbeetles (Coleoptera) the prothorax has a shield-like structure called the pronotum. The platesthat make up the thorax, shown in fig. 2-18, areoften of importance for the identification of aninsect.


Fig. 2-18: Cross section of a thoracic segment(reproduced from Ross, H.H. et al., 1982)

2.1.3.1 LegsThe walking leg is the generalised leg fromwhich all other types have evolved. It consistsof coxa, trochanter, femur, tibia, tarsus andseveral pretarsi, as shown in figs. 2-19, 2-20and 2-31. Insect legs are adapted for walking(gressorial), running (cursorial), jumping(saltatorial), clasping, grasping (prehensile orraptorial), holding, swimming (natatorial),and digging (fossorial) as can be seen from fig.2-20. Even though all these legs look differentand are used for different purposes, they arehomologous, because they have evolved fromthe same ancestral origin and are made up ofthe same parts. Apart from thoracic or truelegs, some caterpillars possess false abdominal

legs that are not homologous but analogousstructures. Leg characteristics are often used ininsect identification owing to the greatvariations in leg size, shape, number of tarsalsegments (tarsal formula, tarsomeres), thenumber, shape and location of spines.

Fig. 2-20: Types of insect legs: (A) walking orrunning leg, (B) jumping hindleg of a grass-hopper, (C) grasping foreleg of a prayingmantid, (D) clasping foreleg of a bug, (E)digging foreleg of a mole cricket, (F) holdingleg of an aquatic beetle; (c) coxa, (t) trochanter,(f) femur, (tb) tibia, (ts) tarsus (reproduced fromRoss, H.H. et al., 1982)

Fig. 2-19: Parts of the generalised walking leg with enlarged ventral surface of pretarsus and lasttarsomere shown on the left (reproduced from Gullan, P.J. and Cranston, P.S., 1994)

A B

C D

E F


2.1.3.2 WingsThe wings of insects are unique structures notfound in other organisms. The wing of a bird ora bat is a modified foreleg, whereas the insectwing is an outgrowth of the body wall. Wingshave no muscles attached inside them. Wings,giving the power of flight, are one of the mostimportant reasons for the evolutionary successof insects.The winged insect (Pterygota = wing) usuallyhas two pairs of wings attached to the meso-and metathorax. Wings are thin, flap-likeextensions of the body wall with an upper andlower membrane and a set of strengtheningveins and cross-veins, as shown in fig. 2-21,which are important diagnostic features.Wings usually can be folded, except in Palaeo-ptera (= old winged) orders like damsel anddragon flies (Odonata). Usually wings aretransparent but in butterflies and moths (Lepi-doptera = scale wings) they are covered withscales. The first pair of wings in beetles (Coleo-ptera = cover wings) is reduced to hardenedwing covers (elytra), to protect the second pairof wings during rest. The leathery, hardenedforewings of earwigs (Dermaptera = skin-likewings) and grasshoppers (Orthoptera = straightwings) are called tegmen. In flies (Diptera =two wings only) the second pair of wings isreduced to a pair of slender knobbed balancingorgans called halteres. Insect orders of theApterygota (= no wings) are primarily wing-less, whereas in some pterygote orders thewings are reduced secondarily, like in fleas

(Siphonaptera) and lice (Phthiraptera). Thefore- and hindwings of most moths are hookedtogether by a frenulum or wing-couplingmechanism, as shown in fig. 5-49. Thus theflight of moths is stabilised and the flight per-formance enhanced compared to butterflies.

2.1.4 Abdomen

The abdomen is the posterior body region of aninsect. The generalised insect abdomen iscomposed of eleven or fewer rather uniformsegments with the last segment forming theappendages. Many insects have eight or fewersegments due to fusion. The abdominal seg-ments are composed of tergites (terga or dorsalplates) sternites (sterna or ventral plates), anda connecting lateral membrane, as shown infig. 2-22. One pair of spiracles can be foundlaterally on the first eight segments. Thespiracles are openings of the tracheal system,the ‘respiratory’ system of insects. The anus ishoused in segment 10. Adult insects lackabdominal legs but these false legs can befound for instance in some caterpillars. Manyinsects however have a number of appendagesat the posterior end of the abdomen homo-logous to the true legs of the thorax. Append-ages of a non-reproductive nature like thetactile organs on the terminal segment arecalled cerci. Reproductive appendages, formthe ovipositor in females for laying eggs andcopulatory organs in males.

Fig. 2-21: Venation of an insect wing H: humeral vein , Sc: subcosta, R: radius, S: sector, M:media, Cu: cubitus, P: plical vein, E: empusal vein, 1A, 2A, etc.: anal veins (reproduced from Ross,H.H. et al., 1982)


Fig. 2-22: Abdomen of an insect (reproduced from Ross, H.H., et al, 1982)

2.2 Internal Anatomy and LifeProcesses

Compared to vertebrates, insects are tiny littlecreatures. Despite their small size, they arevery complex organisms, composed of millionsof cells. Insects like vertebrates posses a heart,a brain, intestines, muscles, sense organs andother highly specialised organs and tissues.Their senses are in many regards much moreoutstanding, subtle and sensitive than thesenses of most other animals. Insects developfrom an egg cell via several larval instars,appearing to be completely different life formsfrom the adults at the end of the development.One advantage of being small in size is theinsects’ rapid development, which takes insome cases only a few days for the egg to turninto an adult. The price for it however, is agenerally short lifespan.

2.2.1 Digestion and Excretion

Living organisms continuously require energyto maintain vital functions. The energy isgenerated by a process called respiration inwhich nutrients are oxidised in cells usingmolecular oxygen. The nutrients are suppliedby food, but most of the substrates have to bebroken up during digestion. The insect’s

digestive or intestinal tract is divided into threespecialised major parts, the foregut, midgutand hindgut as shown in fig. 2-23.The food is gathered and prepared by themouthparts. Prior to ingestion, the food under-goes the enzymatic effects of saliva either inthe mouth or externally. The pharynx andesophagus carry the food to the muscular croplocated in the foregut. There the food issubject to further mechanical break-down inthis so-called ‘gastric mill’. The midgut is theorgan corresponding to a vertebrate’s stomach.Acid and enzymes of the digestive juices areproduced and released by the gastric caeca todegrade proteins, fats and carbohydrates intotheir respective smaller compounds, ie. aminoacids, fatty acids and sugars. The Mid- andhindgut are responsible for the up-take of thenutrients. A further task of the latter part is toabsorb water and thicken the contents of theguts for final excretion of faeces.

The Malpighian tubules drain between themid- and hindgut. Their primary function is, inanalogy to the vertebrate’s kidneys, theelimination of ‘waste’ from the insect blood(hemolymph). These ‘wastes’ are particularmetabolic end products such as ureic acid,excess water and minerals. The fat body coatsthe guts and is closely associated with muscles.It fulfils the functions of the vertebrate’s liveras the central metabolic organ and energy store.


Fig. 2-23: Subdivisions and outgrowths of an insect’s alimentary channel (reprod. f. Ross, H.H. et al., 1982)

2.2.2 Circulatory System

Vertebrates have a closed circulatory system ofblood vessels with arteries; veins and capil-laries. In insects however, ‘blood’ or hemo-lymph is simply flowing through the bodycavities driven by a primitive tube-like heartlocated in the dorsal part of the abdomen asshown in fig. 2-24. This dorsal vessel is theonly blood vessel in the insect body. Haemo-lymph from the abdominal body cavity, rich innutrients, enters the dorsal vessel throughvalve-like openings, the ostia and is thenpumped towards the head. Apart from theheart, independent accessory pulsatile organscan be found at the base of the antennae andlegs to enhance their supply of blood.

Fig. 2-24: The dorsal blood vessel or ‘heart’ ofan insect (reproduced from CSIRO, 1991)

The major function of the hemolymph is thesupply of various organs and tissues withnutrients. Furthermore blood is the carrier ofhormones and other transmitters and has toremove metabolic end products. Anotherfunction is to maintain a certain internalhydraulic pressure, supporting the exoskeletonin its task of keeping the body in shape.The hemolymph consists of a liquid part, theplasma and of cellular compounds calledhemocytes. The latter are responsible for theinsect’s immunity and wound healing. Thehemolymph usually has a greenish-yellow,transparent colour and does not contain anyrespiratory pigments. In vertebrates oxygen istransported by the pigment hemoglobinlocated in red blood cells, but in insects theoxygen is supplied directly to tissues via thetracheal system and not via the blood.

2.2.3 Gas Exchange

Heterotrophic organisms like insects requiremolecular oxygen for a process called respira-tion. During this process nutrients are oxidisedfor energy generation and water and carbondioxide are produced. Therefore oxygen has tobe supplied and carbon dioxide removed con-tinuously.For the purpose of gas exchange insectspossess a system of hollow tubes calledtracheae (fig. 2-25). Distally the tracheae openinto the paired spiracles found laterally onmost thoracic and abdominal segments asshown in figs. 2-22, 2-25 and 2-26. Theproximal ends of the tracheae form narrowbranches or tracheoles that are closely associ-ated with the internal organs and tissues.


Fig. 2-25: The tracheal system of an insect(reproduced from Ross, H.H. et al., 1982)

Oxygen passively moves from outside throughthe tracheae to all parts of the body, where it isrequired for the essential respiratory process.Carbon dioxide in return, also moves passivelyfrom the inside to the outside.The driving force for gas exchange is diffusionand to a certain extent squeezing of gases bymovement of the body. Insects are restrictedfrom growing larger than 20 cm, since there isno active ventilation system like in amphi-bians, reptiles, birds and mammals.Aquatic insect larvae possess gills for gasexchange with the surrounding water. Otherslike adult water beetles carry an air bubbleentrapped between the elytra and the abdomen.

Fig. 2-26: Abdominal spiracles of Theretra sp.(Sphingidae) (photo Schneider, M.F.)

Most animals are highly susceptible to oxygendeficiency (anoxia) and usually die after a fewminutes without an oxygen supply. Insectshowever can sustain anoxic conditions for avery long time without being harmed. Carbondioxide on the contrary can be used as asedative, reversibly putting an insect to sleepfor a while.

2.2.4 Nervous System

Insects have a highly developed nervoussystem whose outstanding abilities to co-ordinate senses, muscles, etc. are demonstratedimpressively for instance by the flight per-formance of dragon flies and damsel flies(Odonata).

The basic unit of the nervous system is thenerve cell. Millions of such nerve cells arelinked together for information exchange likein a tremendously complex computer network.A nerve cell has long extensions (dendrites)that transmit the information as electricalcurrent to other nerve cells or muscle cells, likea power cable. The neurotransmitter acetyl-choline then carries the signal through a tinysynaptic gap between the membranes of thesending and receiving cell. Certain chemicalinsecticides like organophosphates specifi-cally interfere with the process of signaltransduction between nerve cells and as a resultparalyse and kill target insects.

The nervous system consists of a centralnervous system and stomodeal nervoussystem. The central nervous system is com-posed of the brain as major coordinating organlocated in the head plus paired nerve centresper segment called ganglia. Brain and gangliaconsist of masses of nervous tissue joinedtogether by nerve cords. The ganglia are fusedin some insects as shown in fig. 2-27.The stomodeal or stomatogastric nervoussystem, also referred to as the sympatheticnervous system, controls involuntary motionsof parts of the guts and the dorsal blood vessel.This part of the nervous system is located infront of the brain and next to the esophagus.


Fig. 2-27: The central nervous system ofinsects with brain and several ganglia (A) andfused ganglia (B) (reproduced from Gullan, P.J. andCranston, P.S., 1994)

2.2.5 Endo- and Exocrine System

Endocrine or Hormon SystemPhysiological as well as behaviouralresponses of insects are controlled bythe central nervous system in con-junction with the endocrine or hor-monal system. Central nervous stimuliusually cause instant reactions such asthe reflex of an insect escaping upondisturbance. Endocrine control is ofteneffective within the range of minutes orhours thus controlling medium or longterm processes.Endocrine glands or hormone glandsshown in fig. 2-28 produce messengerscalled hormones or neuropeptides,that are released into the haemocoeland carried by haemolymph to therespective target organs. There havebeen almost thirty insect hormones

discovered so far. Some important hormonesare Ecdysone produced in the paired pro-thoracic glands and Juvenile Hormone pro-duced in the paired corpora allata. Bothhormones play an important role in controllingthe moulting process during the developmentof insects as further outlined in chapter 2.2.9.Others are smaller neuropeptides like Adipo-kinetic Hormone which controls lipid andcarbohydrate metabolism eg. during sustainedflight and Diapause Hormone which activatesdormancy in insect eggs.

Exocrine GlandsUnlike endocrine glands, exocrine glandsrelease their products to the outside of aninsect. Main glands of this type are for instancethe salivary glands which are located in thethorax and drain saliva with digestive enzymesinto the mouth. Scent glands like pheromoneglands or stink glands and other defensiveglands have openings dispersed over thecuticle. Scent glands are further discussed inchapter 3.1.3.

Fig. 2-28: Secretion sites of the maininsect hormones (reproduced from Ross,H.H. et al., 1982)


2.2.6 Sense Organs

Insects can perceive light, sound, scent,gravity and temperature in minute quantitiesoften far beyond what can be detected by otheranimals. The subtle chemical sense of somemoths for instance, allows the males to sense afemale from as far away as one kilometre.Ticks and fleas can sense even the slightestchanges of temperature in the environmentcaused by the body temperature of a mammalpassing by at some distance and eventuallyjump onto the host. An attempt to kill a flywith bare hands is almost impossible since itreacts much faster then our hands can strike.

MechanoreceptionMechanoreceptors are stimulated by soundwaves, vibrations and touch. Touch is moni-tored by hair-like trichoid sensilla or setae,shown in fig. 2-3, scattered over the surface ofthe body and appendages like antennae, cerciand tarsi. Hearing is the ability to perceivesound waves that either stimulate trichoidsensilla as shown in fig. 2-3 or membrane-likestructures. The latter type is called thetympanal organ and can be found on thetibiae of Tettigoniidae forelegs (fig. 2-29), onwings of some moths, on the thorax of noctuidmoths and on the abdomen of some otherinsects. The membrane or tympanum swingsas a result of the impact of sound waves likethe membranous skin of a kundu drum whenheld close to a speaker. The movement of themembrane is then transformed into a nervoussignal. Often the tympanal organs are closelyassociated with other structures like thetracheal system as shown in fig. 2-29 in orderto enhance sound reception. Some insects likenoctuid moths are able to detect ultra soundproduced by bats (see chapter 4.4).

VisionMost adult insects and larvae have a pair ofcompound eyes and up to three ocelli orsimple eyes, as shown in fig. 2-30. Thecompound eyes are complex and variable.Generally they are large and located at the topand to the sides of the head. Each compound

Fig. 2-29: Tympanal organ on the tibia of agrasshopper’s foreleg associated with trachealsystem (reproduced from CSIRO, 1991)

eye is composed of individual sensory unitscalled ommatidia. The number of ommatidiavaries from one in some ants to 30,000 or morein some flies, beetles, and dragon flies. Eachommatidium contains a lens and sense cells

Fig. 2-30: Compound eyes and ocelli of ahymenopteran insect (top) and longitudinalsection (A) and cross-section (B) of anommatidium (bottom) (reprod. from CSIRO, 1991)


(pigment cells). The process of photoper-ception happens in the pigment cell, thatconverts light into nerve signals. An individualommatidium can perceive only a small portionof the environment. However the combinedimages of all ommatidia form a mosaic view ofthe insect’s environment. Most adult insectsand larvae have simple eyes, called ocelli,located on the dorsal portion of the head. Thenumber of ocelli can vary from zero to three,depending on the taxon. Their function is notfully understood. They are not important asimage formers but are light sensitive and act as‘stimulatory organs’ in reaction to majorchanges in illumination.The colour spectrum seen by insects oftenexceeds the range of light visible for humans.Bees for instance are able to detect ultravioletlight (UV), invisible for humans. Apart fromthat, bees are also able to sense polarised light,as further outlined in chapter 3.2.2.

ChemoreceptionChemoreceptors respond to chemical aspectsof the environment, taste and olfaction arechemosenses. Chemoreceptors for taste areabundant on mouthparts and other body partslike tarsi. The receptors for smell are locatedon the surface of the cuticle and antennae.These olfactory sensilla are highly sensitive tospecific scents. Male moths can perceivepheromones released by females in minuteconcentrations, amounts so small that theycannot be detected by the most sophisticatedanalytical devices of chemists. The outstandingolfactory abilities of insects are further outlinedin chapter 3.1.3.

ThermoreceptionThe sense for temperature is housed inantennae and other appendages of the head.Qualities like warm and cold are mostimportant for poikilothermic organisms likeinsects that cannot maintain a constant bodytemperature. Therefore, insects try to find anambient environment, for instance in theshade, if it is hot, in order to keep the body ata suitable temperature. During early morninghours butterflies can be observed, spreadingand directing their wings towards the sun so

that the radiant solar energy influences theirbody temperature. Another possibility ofthermoregulation is to generate warmth bymeans of the flight muscles. Due to the highmetabolic rate of some moths so much heat isproduced that it can raise their bodytemperature up to 10 °C higher than the sur-rounding environment.The temperature optimum of insects liesbetween 21 °C and 35 °C. Usually insectscan’t survive temperatures above 42 °C.Tropical species can be also very susceptibleto low temperatures, and might die, when thetemperature drops below 10 °C. Insects oftemperate areas are inactive and hibernateduring the coldest months of the year. Theycan survive temperatures of a deep freezer anddie if it becomes colder than -30 °C.

2.2.7 Locomotion

There is hardly any locomotive action thatcannot be performed by insects: they can fly,swim, dive, jump, walk, run, creep and dig.Movement requires the action of muscles ofwhich an insect houses a considerable numberin its body and limbs. For instance inside agrasshopper’s jumping leg, as shown in fig. 2-31, a complex system of muscles can be found.

Fig. 2-31: Jumping hindleg of a grasshoppershowing arrangement of muscles (reproducedfrom CSIRO, 1991)


Fig. 2-32: Direct (A, B) and indirect (C, D)flight muscles of insects (reproduced from Ross,H.H. et al., 1982)

The massive flight muscles occupy most of thespace in the wing-bearing segments of thethorax. There are two different systems, thedirect and indirect flight muscles as shown infig. 2-32. The direct flight muscles as found forexample in dragonflies, are directly attached tothe wings. A contraction of the inner dorso-ventral muscles moves the muscles upward,whereas a contraction of the outer pair ofmuscles forces the wings downward. In thecase of indirect flight muscles, which are notdirectly attached to the wings, a contraction ofthe dorso-ventral muscles results in a depres-sion of the tergum, lifting the wings up,whereas a contraction of the longitudinalmuscles ‘pops’ the tergum out resulting in adownward movement of the wings. This typecan be found for instance in houseflies.A further distinction can be made betweensynchronous and asynchronous flight mus-cles. Synchronous muscles require one nervousimpulse per contraction whereas asynchronousmuscles contract several times per nervousimpulse. A nerve cell needs to recover forabout 100 msec or / of a second (refractoryperiod) before it can ‘fire’ again. This factmakes asynchronous muscles much faster.Therefore, insects with a rapid wing beatfrequency like houseflies possess asynchronousflight muscles.

2.2.8 Reproduction

Insects have developed quite a number ofsexual and asexual mechanisms for theproduction of offspring.

The major case is sexual reproduction, thefusion of a haploid egg cell with a haploidsperm cell, resulting in a diploid fertilisedegg cell. The advantage of sexual repro-duction is reshuffling of genes with the out-come of producing genetically diverse anddifferent offspring.

The paired glands producing the reproductiveproducts - either sperm or eggs - are calledgonads and are shown in fig. 2-33. Sperm orsemen is produced in the male testes, whereasegg cells, also called oocytes, are produced inthe female ovaries.

Copulation is often introduced with court-ship behaviour like the ’dancing’ of butter-flies or particular calls of crickets. Duringcopulation or insemination the male animaltransfers sperm into the vagina of a female.Various specialised reproductive appendagesof the male and female abdomen are involvedin the process of copulation. The sperm mightbe temporarily stored in the spermatheca orreceptaculum seminis of the female, shownin fig. 2-33. Termite queens can store spermand keep it alive for a considerable time, upto many years and are consequently inde-pendent of further contact with males.

During ovulation, the mature egg cells leavethe ovaries and pass through the oviducts.Then the egg cells are fertilised by a smallamount of sperm released from the sparma-theca. Thus, copulation and fertilisation areindependent of each other and do not neces-sarily have to take place at the same time.

After the fertilisation of the eggs, females ofmost insects lay the eggs (oviposition). Thesuccessive process of ovulation, fertilisationand oviposition is called oviparity. Ovi-position is carried out in various ways.Females of some species possess ovipositors,enabling eggs to be laid into soil or injectedinto fruits or other hosts. Eggs are either laidsingly or in clusters. The latter might be

A

B

C

D


Fig. 2-33: Male (A) and female (B) reproductive system (reproduced from CSIRO, 1991)

enclosed in spittle masses produced by spittlebugs or in foam-like pods made from protein,called ootheca as is done by praying mantids,locusts and cockroaches. Eggs can be gluedonto a substrate such as leaves and bark, butare always laid on or at least in closeproximity of the larval source of food. Femalebutterflies and moths lay their eggs on therespective food plants of the caterpillars. Afemale Birdwing for instance carefully checksa potential Pararistolochia or Aristolochiaplant, before she lays the eggs. She makessure, that there are enough young and softleaves available for the developing caterpillar.The female also looks for the presence ofother eggs in order to avoid competitionbetween the caterpillars. Finally she lays oneto two eggs on the underside of a leaf, so thatthe eggs are protected from sun and rain andhidden from egg parasites.

Such care-taking actions of females endingright after oviposition are called brood care.Parental care refers to the actions of parentsafter the laying of the eggs, for protection ofthe brood and provision of food. Parental careis common for example in social insects,spiders and cockroaches and as shown in fig.2-34 in Harlequin bugs.

Apart from oviparity, there are several lesscommon but very interesting strategies of

viviparity. Eggs of ovoviviparous speciesare incubated in the reproductive tract of themother for a certain period of time. In thecase of viviparity the incubation time isextended and eventually ‘birth’ is given tolarvae. Some strange and rare cases of adeno-trophic viviparity occur in some flies. Thepoorly developed larvae hatch in the ‘uterus’and orally feed from ‘milk’ glands of thefemale’s reproductive system. The fullydeveloped larvae are then deposited andpupate instantly. Embryos of haemocoelousviviparous Strepsiptera develop in thehaemolymph of the female. The larvae leavethe mother through a brood canal. A reallyalienating case of haemocoelous viviparityare the larvae of particular gall midges(Diptera) that develop in the mother andsubsequently consume her.

The second basic reproductive strategy apartfrom sexual reproduction is parthenogenesis.It is a form of asexual reproduction that can befound for instance in some cockroach, wasp,bee and ant species. These females can produceoffspring without being fertilised by males.The involved egg cells are then not subject tomeiosis as they remain diploid and thereforedo not require any fertilisation. The price forbeing independent of males is, that onlygenetically identical individuals (clones) or at

A B


least individuals without great genetic variationare produced. Parthenogenesis might be obli-gatory or facultative and result in theproduction of female or male eggs only.Females of partial parthenogenetic speciesproduce mainly parthenogenetic generations offemales only. Occasionally generations of bothmales and females are produced sexually.Other asexual strategies are hermaphroditism,polyembryony and paedogenesis.

Fig. 2-34: Female Tectocoris diophthalmus(Hemiptera: Scutelleridae) guarding her eggs(reproduced with permission from Monteith, G., 1991)

The sex of most insects is genetically deter-mined by the number of sex or X- chromo-somes (heterochromosomes). Usually femalespossess two X-chromosomes (XX) whereasmales have one only (X0), but this allocationvaries between taxonomic groups. A lesscommon sex determination system found inbees amongst others, is called haplodiploidy.Female bees develop from fertilised eggs andare thus diploid. The male drones are haploid,have only one set of chromosomes and developfrom unfertilised eggs. Haplodiploidy allowsthe bee’s queen to control the sex of heroffspring by fertilising eggs to producefemales. In humans, by the way, two hetero-chromosomes, X and Y can be found. Thecombination of XX results in females, whereasXY results in male humans.

The large number of eggs as well as theshort generation time is the major reasonfor the reproductive potential of insects.

2.2.9 Development

Ontogeny is the development of an eggthrough several stages into an adult or imago.During development tissues differentiate andan insect gains size. The fully sclerotizedcuticle of an insect is inflexible and rigid, sothat it cannot grow any more (see chapter2.1.1). Therefore the insect has to undergoseveral moults during its development, ie. toproduce a new expandable cuticle and to castoff the hardened cuticle of the previous stage.Prior to each moult, a new cuticle is growingbelow the old one. Then the latter separatesfrom the newly formed cuticle, during aprocess called apolysis. Finally during ecdysisthe old cuticle dorsally opens up to release thenext stage and the old cuticle (exuviae) is castoff. Initially the cuticle of the newly emergedinsect is soft and flexible. Thus the ecdysedinsect is able to pump air into its trachea inorder to gain body size, like inflating a balloon.According to Dyar’s law, the increase in sizefrom instar to instar is often of the factor 1.4.After each moult the cuticle hardens or sclero-tizes, a tanning process which takes from thirtyminutes up to several hours. During this timethe insect has to hide since it is quitevulnerable, eg. to predation and desiccation.The development of an insect takes from aboutone week to several years and depends on thesize of the insect and the surroundingtemperature. Generally the larger the insect isthe longer it takes for its development.

prothoracic gland corpus allatum

PTTH

ECDYSONE JUVENILE HORMONE

larval-larval larval-pupal pupal-adultmoult moultmoult

brain

Fig. 2-35: Simplified diagram of the endocrinecontrol of moulting and metamorphosis in endo-pterygote insects; for more information see text


Moulting is a complex process that is con-trolled by three major hormones as shown infig. 2-35. Ecdysone is produced in the pro-thoracic glands and released upon stimulus byProthoracicotropic Hormone (PTTH). Eachmoult is mediated by Eclosion Hormone andEcdysone. The presence of Juvenile Hormonesuppresses the final moult in larval instars, itsabsence makes a larval instar turn into a pupaor an adult.Three different major patterns for thedevelopment from the larva to the adult insectcan be found. The primitive ametaboly iswithout marked change in form. Insects withmetamorphosis show a major change in formbetween immature and mature, winged stages.Metamorphosis can be further divided intopartial metamorphosis (hemimetaboly) andcomplete metamorphosis (holometaboly). Box2-1 summarises differences between hemi- andholometabolous life cycles or life histories.

Ametaboly without metamorphosis can befound in wingless orders, the Apterygota suchas silverfish and bristletails. These insectsundergo more than ten moults and continue tomoult after sexual maturity. There are nomarked changes in body form between imma-ture and mature insects.

Fig. 2-36: Hemimetabolous life cycle of thecockroach Methana marginalis (Blattidae)with gradual metamorphosis (after CSIRO, 1991)

Fig. 2-37: Hemimetabolous life cycle of thehemipteran bug Amorbus alternatus (Coreidae)with gradual metamorphosis (after CSIRO, 1991)

Hemimetaboly with partial metamorphosis isa characteristic of dragonflies and damselfliesand exopterygote insects like locusts, bugs andcicadas. These insects usually have four to fivelarval instars before they directly change intoadults. The larval instars of terrestrial forms arecalled nymphs, those of aquatic insects areknown as naiads. After each moult wing padsand genitalia increase in size. The immaturestages resemble the adults, except that they aresmaller and there are no functional wings.Partial metamorphosis can be further dividedinto incomplete (dragonflies, damselflies) andgradual metamorphosis (grasshoppers, bugs,cockroaches, etc.). See also figs. 2-36, 2-37and 3-9.

Holometaboly with complete metamorphosisis a feature of endopterygote insects whoselarvae differ completely from adults as shownin figs. 2-38 to 2-41 and 6-28 ff. A caterpillaris very different from an adult butterfly or


Fig. 2-38: Holometabolous life cycle of thebeetle Phyllophaga sp. (Scarabaeidae) withcomplete metamorphosis (after Ross, H.H., 1982)

moth, a grub does not resemble an adult beetleat all and a maggot differs considerably froman adult fly. There is no progressive change inform towards the adult. Usually four or five,sometimes up to 20 larval instars occur, beforethe last larval instar turns into the pupal stage

Fig. 2-39: Holometabolous life cycle of theparasitic wasp Apanteles melanoscelus (Bra-conidae) with complete metamorphosis (repro-duced from Ross, H.H. et al., 1982)

Fig. 2-40: Holometabolous life cycle of the flyDidea fasciata (Syrphidae) with completemetamorphosis (reprod. from Ross, H.H. et al., 1982)

or pupa from which an adult emerges. Thepupa of a butterfly is also called chrysalis. Thepupal stage is a resting stage without feedingand without locomotion but with extensiverestructuring of tissues. The pupal stage canlast from several days to years.

Fig. 2-41: Holometabolous life cycle of thebutterfly Papilio woodfordi (Papilionidae) withcomplete metamorphosis (reproduced from CSIRO,1991; photo Schneider, M.F.)


Hemimetabolous Development Holometabolous Developmentl EÜL1ÜL2ÜL3ÜL4ÜL5ÜA l EÜL1ÜL2ÜL3ÜL4ÜL5ÜPÜAl no pupal stage l pupal stage presentl wings develop externally l wings develop internally

(exopterygote insects) (endopterygote orders)l larvae are generally similar in l larvae look very different from

appearance to adults adults

Box 2-1: Differences between hemimetabolous and holometabolousdevelopment; E = egg; L1 - L5 = larval instar 1 to 5; P = pupa; A = adult

In some holometabolous insect orders a changein the type of mouthparts during metamorpho-sis can be observed. This is an adaptation to achange of the type of food of the particularinstar. Some examples are given in box 2-2.Such drastic alterations require a completemetamorphosis and never occur in hemimeta-bolous insect orders which typically showconstancy in this regard.

Since larvae and pupae of holometabolousinsects greatly differ from their respectiveadults, the identification of larvae is often notan easy task. A classification of larvae assuggested in fig. 2-42 considers more func-tional rather than taxonomic features.

Polypod larvae possess cylindrical bodies withshort thoracic legs and false abdominal legs.Most of these larvae are phytophagous and

cannot walk long distances. Commonlypolypod larvae can be found in Lepidoptera,symphytan Hymenoptera and Mecoptera.Oligopod larvae lack false legs and often haveprognathous mouthparts. Some are fast pre-dators, others are slowly moving phytophagesor detrivores living in soil. Oligopod larvae canbe found in most holometabolous orders, butnot in Mecoptera, Lepidoptera, Strepsiptera,Diptera and Siphonaptera. The worm- ormaggot-like apod larvae lack true legs and livein substrates like soil, mud, dung, rottingplants, carrion or as parasitoids in bodies ofother organisms. This type can be found inSiphonaptera, aculeate Hymenoptera, Coleo-ptera and Diptera.

A pupa usually forms in the puparium, thehardened cuticle of the final instar larva. The

Fig. 2-42: Larval types: polypod larvae (A) Lepidoptera: Sphingidae, (B) Lepidoptera: Geometridae,(C) Hymenoptera: Diprionidae: oligopod larvae: (D) Neuroptera: Osmylidae, (E) Coleoptera:Carabidae, (F) Coleoptera: Scarabaeidae; apod larvae: (G) Coleoptera: Scolytidae, (H) Diptera:Calliphoridae, (I) Hymenoptera: Vespidae (reproduced from Gullan, P.J. and Cranston, P.S., 1994)

A

B

C

D

E

F

G

H

I


Fig. 2-43: Pupal Types: exarate decticous: (A) Megaloptera: Sialidae, (B) Mecoptera: Bittacidae;exarate adecticous: (C) Coleoptera: Dermestidae, (D) Hymenoptera: Vespidae, (E), (F) Diptera:Calliphoridae; obtect adecticous: (G) Lepidoptera: Cossidae, (H) Lepidoptera: Saturniidae, (I) Lepi-doptera: Papilionidae, (J) Coleoptera: Coccinellidae (reproduced from Gullan, P.J. and Cranston, P.S., 1994)

pupa might be surrounded by a cocoon or aprotective cell. The fully developed adult thatis still enclosed in the puparium is referred toas the pharate adult. Most pupae are exarate,ie. their appendages like antennae, legs, etc. arefree and not fused with the body. Obtect pupaehave the appendages closely attached to thebody. Exarate pupae can be decticous witharticulated mandibles for biting through andescaping from the cocoon during eclosion.Pupae with immovable or non-articulatedmandibles are called adecticous.

These conditions are important diagnostictools. Their occurrence within particular insectorders is shown in fig. 2-43.

Polymorphism describes the occurrence ofmorphological variations between individualsof a population and might be of a genetic orenvironmental nature. Polymorphism can alsoinclude physiological, ecological as well as

behavioural differences. Examples for poly-morphism like the cast system of socialinsects or the phase polymorphism as itoccurs for instance in plague locusts arediscussed in chapter 3.2. Another obviousexample is sexual dimorphism. It is referredto as distinct sets of phenotypic secondarysexual characteristics for females and males ofa species. In other words: a male looksdifferent from a female. Males and femalesmight differ eg. in body size, coloration ofbody or wings, patterns of songs, presence andshape of wings and appendages like antlers,feelers, etc. Some examples are:

• male stag beetles (Lucanidae) have longextended mandibles that the females lack• only male rhinoceros beetles (Scarabaeidae)possess two long horns as shown in fig. 2-44• the antennae of male longicorn beetles(Cerambycidae) are much longer than thefemales’ antennae

A

B

C

D

E

F

G

H

I

J


• the ventral side of the male crusader bugMictis profana (Coreidae) is red and has twohumps, the underside of the female isbrownish and lacks the two humps• the females of some grasshoppers, stickinsects, praying mantids, moths and butterflieslike Birdwings are larger than their malecounterparts as can be seen in figs. 2-44 andplate 7 D and E• the songs of male crickets and cicadas aredifferent from the songs of their females• the wing coloration greatly differs betweensome female and male Lepidoptera like theBirdwings shown on plate 9. Other examplesare shown on plates 7 D, E, 8 J, K and 8 M, N• the males of some flies use their antlers oreye-stalks to fight for females, as shown infigs. 5-45 and 5-46 J, K, L• the male stalk-eyed fly Achias spp. (Platy-stomatidae) uses its stalked eyes for territorialfights. The eyes of a female are far lessmodified as shown in fig. 5-46 J, K• the wings of some female cockroach, aphidand praying mantid species are reduced oreven absent as shown in fig. 2-44• some male moths (Lepidoptera) haveenlarged pectinate feelers. This enables themto detect tiny quantities of pheromones re-leased by the females and to eventually meet

them. In the case of Lymantria ninayi thelarger female has filiform antennae, as shownon plate 7 D and E.• the wingless female stylops (Strepsiptera)spends all her life as an entomoparasite ininsects. The male is much larger and has fullydeveloped wings as shown in fig. 5-39 B and C• kings and queens, the male and femalereproductives of social insects like termites,ants, bees and wasps look very different. Themales are always much smaller. Fig. 2-44shows the difference between a termite queenand king

The development of an insect markedlydepends on a set of environmental factors likehumidity, temperature and photoperiod or daylength. Each of these abiotic factors on itsown or in conjunction with other factors candirectly or indirectly influence insect develop-ment. Temperature for instance can. have aneffect either on the availability of foodplants oron the temperature-dependent biochemicalprocesses during the complex life cycle ofpoikilothermic insects. Biotic and abioticeffects are outlined in detail in chapter 4.

Development from the egg to an adult can beinterrupted for a certain period, known asdormancy, when environmental conditions are

ORDER LARVA ADULTDiptera:mosquitoes aquatic larvae feed on algae adult female mosquitoes feed on blood

and microorganisms of vertebrates and invertebrateschewing mouthparts piercing-sucking mouthparts

horseflies larvae feed on various solid adults cut open cuticle of preyfood and suck body juiceschewing mouthparts cutting-sponging mouthparts

Hymenoptera:wasps larvae feed on plant material adults feed on liquid food (nectar)

or animals and solid foodchewing mouthparts chewing-lapping mouthparts

Lepidoptera:butterflies/moths larvae feed on vegetation adults drink nectar or water

chewing mouthparts mouthparts of siphoning-tube type

Box 2-2: Change in mouthparts during the holometabolous development of selected insect groups


unsuitable. This is common in temperate areasduring winter. It would not make sense forinsects to develop because it might be too coldand foodplants be absent. Dormancy can alsooccur during drought, as could be observedduring the prolonged dry-spell in PNG in 1997,when most of the insect diversity vanished.Once the rain started at the end of 1997, naturewoke up again providing the necessary foodand within a few weeks many insect speciesarose from dormancy, too.

Quiescence is the simplest form of dormancy.It is a direct response to adverse environmentalconditions, halting or slowing down the de-velopment which resumes immediately, oncethe conditions become more favourable.In contrast, diapause is arrested development,which persists for some time, even if suitableconditions are prevailing. Diapause is oftenassociated with physiological changes and theresumption of development requires internalstimuli. Obligatory diapause occurs during afixed time of the year in univoltine insects andis usually genetically controlled. Insects withone generation per year need to extend their

short life cycle in order to avoid a secondgeneration that would conflict with unsuitableconditions. An optional facultative diapausecan be found in bivoltine or multivoltineinsects with two or more generations per year.Only those generations that have to sustainadverse conditions are subject to diapause.Various abiotic and biotic factors as well asinternal physiological stimuli like DiapauseHormone trigger diapause. Diapause can lastfrom days to months or up to seventeen yearsas in the case of the northern Americanperiodical cicada Magicicada septemdecim.Any stage of the life cycle can undergodiapause, but egg and pupal diapause are thecommonest. These stages are resting stageswith little activity thus increasing the chancesof survival.Diapause allows insects to adapt their lifecycles to the seasonal rhythm of theenvironment. Thus, active stages, that are notable to escape from unsuitable conditions bymeans of migration, are present only duringfavourable conditions. Diapause also enablesthe synchronisation of adult emergence forreproduction or migration.

A B C

D

Fig. 2-44: Sexual Dimorphism: (A†) ♀ and ♂ Archimantislatistylus (Mantodea: Mantidae), (B††) termite king (♂) andqueen (♀) (Isoptera), (C) ♂ and ♀ rhinoceros beetleXylotrupes gideon (Coleoptera: Scarabaeidae), (D) ♀ and ♂Scapanes australis (Coleoptera: Scarabaeidae) (reproduced

from CSIRO, 1991†; Hadlington, P., 1992††, photos Schneider, M.F.)


The time required to complete one full lifecycle varies from several days for a few minuteinsects to several weeks for the majority ofinsects in the tropics up to several months forlarger insects or insects of temperate areas.However, diapause or quiescence might mark-edly extend the life cycle of a particular species.The information about the life history of aparticular pest species as well as the durationof its stages is most valuable and even arequirement for the prediction of outbreaks andeffective control strategies. This is furtheroutlined in chapter 4.7.

Further reading:

Abercrombie, M. et al. (19928): Dictionary of Biology;Penguin Books; London; UK

Chapman, R.F. (1982³): The Insects: Structure andFunction; Hodder and Stoughton; London; UK

Commonwealth Scientific and Industrial ResearchOrganisation (CSIRO) (19912): The Insects ofAustralia - A Textbook for Students and ResearchWorkers; Volume 1 & 2; Melbourne UniversityPress; Carlton; Australia

Commonwealth Scientific and Industrial ResearchOrganisation (CSIRO) (19962): Insects - Little Crea

tures in a Big World; CD-ROM; CSIRO Publishing;Collingwood, Australia

Coulson, R.N. and Witter, J.A. (1984): Forest Entomo-logy, Ecology and Management; Wiley; New York;USA

Gullan, P.J. and Cranston, P.S. (1994): Insects - AnOutline of Entomology; Chapman and Hall;London; UK

Hadley, N.F. (1986): The Arthropod Cuticle; ScientificAmerican 255: 98-106

Kerkut, G.A. and Gilbert, L.I. (eds.) (1985): Compre-hensive Insect Physiology, Biochemistry andPharmacology; Pergamon Press; New York; USA

Lapedes, D.N. (ed.) (19782): Dictionary of Scientific andTechnical Terms; McGraw-Hill; New York; USA

Naumann, I.D. (1994): Systematic and Applied Ento-mology - An Introduction; Melbourne UniversityPress; Melbourne; Australia

Pearson, I., (1978): English in Biological Science;Oxford University Press; Glasgow; UK

Pyenson, L.L. (19802): Fundamentals of Entomologyand Plant Pathology; AVI Publ.; New York; USA

Roberts, M.B.V. (19854): Biology - A Functional Ap-proach; ELBS; Walton-o.-T.; UK

Ross, H.H. et al. (19824): A Textbook of Entomology;Wiley; New York; USA

de la Torre-Bueno, J.R. (1989²): The Torre-BuenoGlossary of Entomology; New York EntomologicalSociety in Cooperation with the American Museumof Natural History; New York; USA

Fig. 2-45: “Poem Case”, by Dodd, F.P. The words are spelled out in tiny pyralidmoths while the signature is in metallic green beetles (reproduced from Monteith, G., 1991)

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