biology of animals topic 11.3 arthropods iii: insects … · page 1 arthropods iii: insects! zoo...

Arthropods III: Insects!ZOO 2040 !Biology of Animals !Topic 11.3!

Subphylum Hexapoda!  A controversial taxon, often combined with the myriapods

(centipedes, millipedes, pauropods and symphylans) in the (equally controversial)subphylum Uniramia!

  Primarily terrestrial, with a few found in freshwater and coastal habitats!

  The insects have reduced their body tagmata to a head, thorax and abdomen!• Abdominal appendages are greatly reduced or absent!• The common ancestor of insects probably resembled the myriapod

body form!  Only one pair of antennae; appendages always uniramous!• Uniramian gills in aquatic larvae not homologous with crustacean

gills!  Use tracheae to distribute respiratory gases, similar to

onychophorans and annelids!  Excretion usually involves Malpighian tubules!

Class Insecta! Most diverse and abundant of all animals (at least"

1 million spp. in 28 orders). Have three pairs of legs"and often 2 pair of wings(!) on the thoracic region"of the body. Range from < 1 mm to 20 cm in length.!

 Found in nearly all habitats except the sea!• Common in freshwater, brackish water and salt marshes!• Very abundant in soils and forest canopies!• Can be found in deserts and wastelands (a 5 mm "

wingless fly is Antarctica’s largest land animal)! Most feed on plant tissues or juices, but utilize all"

possible diets !• Many caterpillars are specialized to eat only certain species of plants!• Certain ants and termites cultivate fungus gardens for food!• Many beetles and other insect larvae eat dead animals and are saprophagous!• Some insects are predaceous on other insects or other animals!• Many species are parasitic as adults and/or larvae (most animals and plants have insects as

parasites externally or internally)!• Parasitoids live inside a host until they eventually kill the host; they are important in pest

control!• Many of these have major medical and economic effects on humans, and are critical to

plant and animal ecology!• Many parasitic insects, in turn, have parasites (hyperparasitism)!

Major Insect Orders!Coleoptera (beetles, scarabs,"weevils, ‘fireflies’)–front pair of "wings is a protective sheath"(elytra) >360,000 spp. (20% of all"described living species!)!Lepidoptera (butterflies and moths)–"large, scale-covered wings"160,000 spp.!Diptera (flies, gnats, mosquitoes,"and midges)–rear pair of wings"reduced (halteres) 150,000 spp.!Hymenoptera (bees, wasps and ants) 130,000 spp.!Hemiptera (true bugs)–wings cross over each other 50,000 spp.!Homoptera (cicadas, aphids, plant lice, and leaf hoppers) 35,000 spp.!Orthoptera (crickets, grasshoppers, locusts, katydids) 20,000 spp.!

Adaptive Traits of Insects! Flight and small size makes insects widely distributed! Well-protected eggs withstand rigorous conditions and are readily

dispersed! A wide variety of structural, feeding, and behavioral adaptations gains

them access to every possible ecological niche!• Most structural modifications are in wings, legs, antennae, mouthparts and

alimentary canal!• Specialization for eating only one part of a host plant allows many insect species

to coexist on a single individual plant! The hard protective exoskeleton is well-adapted to life in desert regions!• Holds in water which, along with excretory system that saves water, allows for

desert existence!• Made of complex plates (sclerites), connected by hinge joints!• Muscles attaching sclerites allow precise movement!• Rigidity is due to scleroproteins and not mineral matter; this lightness allows

flight! The insect is more homogenous in tagmatization than the variable

crustaceans! The cuticle of a somite is composed of a dorsal notum, a ventral

sternum and a pair of lateral pleura!

Insect Body Regions!Head!  Usually there is a pair of "

large compound eyes!  One pair of antennae vary "

greatly in form; they can"feel, taste and hear!

  Mouthparts consist of a"labrum, a pair each of of "mandibles and maxillae,"a labium and a tongue-like hypopharynx!

Thorax!  Prothorax, mesothorax, and metathorax; each with 1 pair of legs!Abdomen!  Abdomen has from 9 to 11 segments; the last is reduced to a pair of cerci!  Larval and nymphal forms may have abdominal appendages that are

lacking in adults!  The external genitalia are usually at the end of the abdomen!

Locomotion!Walking!  Insects walk using the first and last leg on one side and the middle leg on

the opposite side in a tripod arrangement (provides stability)!• Walking legs end in terminal pads and claws!• Hind legs of grasshoppers and crickets are enlarged for jumping. (Fig. 20-6)!• Mole crickets have front legs adapted for burrowing in the ground!• Forelegs of the praying mantis allow it to grasp prey. (Fig. 20-7)!• Honeybees have leg adaptations for collecting pollen. (Fig. 20-8)!• A water strider has non-wetting footpads that do not break the surface water tension

(Fig. 20–11)!

Flight!  Insect wings are not homologous with vertebrate wings!• Formed as outgrowths of cuticle from the mesothoracic and metathoracic segments!• Consist of a double membrane. Veins strengthen the wing; the vein pattern is used to

identify insect taxa! Most flying insects have two pairs of wings; although only one may be

functional for flying!• In Diptera, halteres are reduced wings that balance the fly during flight!• Beetle have elytra that cover and protect the flying wings!• Butterflies wings are covered with scales; caddisfly wings are covered with hairs!• Worker ants and termites are wingless; lice and fleas have also lost their wings!


Insect Flight! Direct flight muscles attach to a wing directly!  Indirect flight muscles alter the shape of the"

thorax to cause wing movement!• The wing is hinged on a pleural process that forms a"

fulcrum; all insects cause the upstroke by contracting "indirect muscles that pull the tergum down toward"the sternum!

• Dragonflies and cockroaches contract direct muscles"to pull the wing downward!

• Bees, wasps and flies arch the tergum to cause the downstroke indirectly!• Beetles and grasshoppers use a combination of direct and indirect muscles!

 Flight muscle contraction has two kinds of control:!• Synchronous muscle control uses a single volley of nerve impulses to stimulate a

wing!• Asynchronous muscles stretch the antagonistic muscle and cause it to contract in a

recoil fashion (potential energy can be stored in resilient tissues); they only need occasional nervous stimulation!

 Wing beats vary from 4/s in butterflies to>1000/s in flies!• Direct flight muscles also alter the angle of wings to twist the"

leading edge to provide lift. Figure-8 movement drives insect forward!• Fast flight requires long, narrow wings and a strong tilt,"

(e.g., dragonflies)!

Digestion and Nutrition!  Insects have various types of "

mouthparts, depending on diet!• Sucking mouthparts form a tube"

to pierce tissues of animals or "plants!

• Flies have sponging "mouthparts; soft lobes"at the tip absorb food!

• Biting mouthparts can seize and"crush food!

 The foregut consists of the mouth"with salivary glands, esophagus, crop and gizzard!• Some digestion, but no absorption, occurs in the crop as salivary enzymes mix with

food.The gizzard grinds food before it enters the midgut (main site of digestion and absorption). Ceca may increase digestive/absorptive area. The hindgut is primarily a site for water absorption!

Circulation! A tubular heart in the pericardial cavity moves hemolymph forward

through the dorsal aorta!• The heartbeat is a peristaltic wave!• Accessory pulsatile organs help move the hemolymph into wings and legs!• Hemolymph has plasma and amebocytes but does not function in oxygen transport!

Gas Exchange! Terrestrial animals are faced with the dilemma of exchanging gases while

reducing water loss to an acceptable level! The tracheal system is a network of thin-walled tubes that branch

throughout the body!• Composed of a single layer of cells lined with cuticle that is shed at each molt!• Spiral thickenings of cuticle, called taenidia, prevent the tracheae from collapsing!

 Spiracles open to the tracheal"trunks; there are two on the"thorax and 7-8 on the abdomen!

 A valve on the spiracle often"cuts down on water loss; and"prevents dust entry!

 The tracheae branch out into fluid-"filled tubules called tracheoles that"reach individual body cells!

 This system provides direct gas"transport to the cells without use"of oxygen-carrying pigments!

Excretion and Water Balance!  Both insects and spiders utilize Malpighian tubules in conjunction with

rectal glands!• Malpighian tubules vary in number but join between the midgut and hindgut!• The blind ends of the tubules float freely in the hemocoel bathed in hemolymph!

  Potassium is actively secreted into the tubules; other solutes follow the gradient!

  The main waste product is uric acid; it flows across at the upper end that is mildly alkaline!

  In the lower end of "the tubule, K+"combines with CO2"and is reabsorbed!

  Rectal glands then"reabsorb Cl–, Na+,"and H2O; the wastes"pass on out of the body!

Nervous System!  Nervous system resembles that of larger crustaceans,"

with fusion of ganglia!  Some have a giant fiber system!  A stomadeal system corresponds to autonomic"

system of vertebrates!  Neurosecretory cells in the brain control molting "

and metamorphosis!Sense Organs!  Many insects have sophisticated sensory "

perception abilities!  Most sense organs are microscopic and located in the body wall!  Different organs respond to mechanical, auditory, chemical, visual and

other stimuli!Mechanoreception: touch, pressure, vibration, etc. are picked up by sensillae

(may be single hair-like setae or a complex organs) that are distributed widely over the antennae, legs and body!

  Setae (hair sensilla) or tympanal organs in the legs of Orthoptera, Homoptera, and Lepidoptera can detect vibrations of the substrate!

Chemoreception: sensory cell processes located in sensory pits on mouthparts, antennae, and legs!

  Some insects can detect odors for several km!  Feeding, mating, habitat selection and host-parasite relationships are mediated

through chemoreception!Visual Reception: two types of eyes (simple and compound)!  Studies indicate that the ocelli"

monitor light intensity but do "not form images!

  Compound eyes may contain"thousands of ommatidia; their "structure is similar to that of "crustaceans!

  Insects can see simultaneously "in almost all directions; the"image is fuzzy and myopic!

  Flying insects have a higher flicker-fusion rate; they distinguish 200-300 flashes per second!

  A honeybee can distinguish ultraviolet light we cannot see, but cannot detect shades of red!

Other Senses: insects are very sensitive to temperature, especially by cells in antennae and legs Also detect humidity, proprioception, gravity and other physical properties!


Reproduction!  Sexes are separate in insects and fertilization is usually internal!  Parthenogenesis is common in Homoptera and Hymenoptera!  Sexual Attraction!

• Female moths secrete a powerful pheromone to attract males from a great distance!• Fireflies use flashes of light to detect mates!• Some insects use sounds, color signals and other courtship behaviors!

  Many insects deposit sperm in the vagina during copulation!  In some orders, spermatophores are transferred or deposited on substrate!  During evolution from aquatic to terrestrial life, spermatophores were

used first!  The female may only mate once and store the sperm to fertilize eggs

throughout her life!  Females may lay a few eggs and provide care of young, or lay huge

numbers!  Butterflies and moths must lay eggs on the host plant if the caterpillars are

to survive!  Wasps may have to locate a specific species that is the only host to their

young!

Metamorphosis!  Various forms of metamorphosis produce degrees of change among

different insect groups!• Most insects change form after hatching from an egg!• Each stage between molts is called an instar!• Insects develop wings during the last stage where they are useful in reproduction!

Holometabolous Metamorphosis!Stages are egg-larva-pupa-adult!  About 88% of insects undergo this"

complete metamorphosis!  This separates the physiology of "

larval growth, pupal differentiation"and adult reproduction!

  Larvae and adults often live in"completely different environments"and therefore do not compete!• After several larval instars, a larval"

lepidopteran becomes a pupa inside"a cocoon or chrysalis!

• Pupae often overwinter in this stage; final molt occurs and adult emerges in spring!

Metamorphosis!Hemimetabolous Metamorphosis!  Some insects undergo a gradual"

metamorphosis!

  Grasshoppers, cicadas, mantids, true bugs, mayflies and dragonflies exhibit this pattern!

  Young are called nymphs!  Bud-like growths in early instars show where the adult wings will

eventually develop!  Stages are egg-nymph-adult!Direct Development!  Silverfish and springtails have young similar to adults except in size and

sexual maturation!  Stages are egg-juveniles-adult!  These are primitive wingless insects!

Physiology of Metamorphosis! Hormones regulate insect metamorphosis!• The brain and nerve cord ganglia produce brain hormone or ecdysiotropin!• Neurosecretory cells send axons to the corpora cardiaca that stores and

ultimately releases prothoracicotropic hormone (PTTH)!• PTTH circulates in the hemolymph to the prothoracic gland in the head or

prothorax!• The prothoracic gland produces molting hormone or ecdysone in response!• Ecdysone starts the molting process!

 The corpora allata produces juvenile hormone!• Molting continues as long as juvenile hormone (neotenine) is sufficiently present!• In later instars, the corpora allata release less and less juvenile hormone!• When juvenile hormone reaches a low level, the larva molts to become a pupa!

 Cessation of juvenile hormone production in the pupa leads to an adult at the last molt!• In hemimetabolous insects, cessation of juvenile hormone occurs in the last

nymphal instar!• In adults, the corpora allata becomes active again in normal egg production!• The prothoracic glands degenerate in adult insects and adults do not molt!

Diapause and Dormancy!  Diapause is a period of dormancy in the annual life cycle

that is independent of conditions!  Winter dormancy is called hibernation; summer

dormancy is called estivation!  Any stage (eggs, larvae, pupae or adults) may remain

dormant to survive adverse conditions!  This allows them to synchronize with the environment

(e.g., warm or rainy seasons when food is available!  Diapause is genetically determined but it may be

triggered by environmental cues such as day length!  Diapause always occurs at the end of an active growth

stage; the insect is then ready for another molt: many larvae thus do not develop beyond this point until spring even in mild temperatures!

Malaysian mantis ��Hymenopus coronatus

Defensive Adaptations!  Some insects are aggressive, with venomous stings, bites,"

or fluids!• The bombardier beetle can spray an attacking enemy with"

irritating chemicals that are scalding hot!  Protective coloration, warning coloration and mimicry are"

also defensive adaptations!• Many insects are amazingly well-camouflaged!• Many insects store poisonous substance from their "

food plant; they often advertise their poisonous or "repulsive odors and tastes (aposomatism)!• Edible insects often coevolve to ‘copy’ the color "

patterns colors of distasteful ones (Batesian mimicry)!• Development of a common signal in forms such as"

bees and wasps (Mullerian mimicry) is also common!

leaf butterfly Kallima sp.

Batesian

Mullerian

camouflage


Defensive Adaptations (cont.)! Some species minimize predation by "

having a long life cycle that is a prime"number of years!

 Periodical cicada brood populations are very "susceptible to predation as they emerge"from the ground and molt into adults!

 They minimize the opportunity for predators to specialize on them by having 7, 13, or 17-year "life cycles!

 Major emergences of "broods in adjacent"regions are"generally separated"by four-year intervals!

 Larvae are not in"diapause. They feed"underground all during "their long wait!

Other Adaptations!Sound Production and Reception!  Sounds are used as warning devices, advertisement of

territory, and courtship songs!  Crickets chirp for courtship and aggression!  The male cicada vibrates paired membranes on its

abdomen to attract females!Tactile Communication!  Tactile communication involves tapping, stroking,

grasping and antennae touching!  Some beetles, flies and springtails use bioluminescence!  Some female fireflies mimic another species’ flash

pattern and attract males of other species and then eat them!

Insect Behavior!  Due to very sensitive perception, many insects respond to many

environmental stimuli!  Responses are governed by both the physiological state of the

animal and its nerve pathways!  Many insect behaviors are complex sequences of responses!  Most insect behaviors are innate but some involve simple

learning!Pheromones!  These chemicals are secreted by one individual to affect the

behavior of another individual!  Pheromones attract the opposite sex, trigger aggregation, fend

off aggression and mark trails!  Bees, wasps and ants can recognize nestmates and signal an

alarm if strangers enter the nest!  Pheromones can be used to trap insects to monitor populations!

Social Behavior! Some social communities are temporary and uncoordinated; other

social groups are highly organized and depend on chemical and tactile communication!

 Caste differentiation is common in the most organized social groups!Honeybees! Honeybees have a few male drones, a fertile female queen and many

sterile female workers! Males come from unfertilized eggs; fertilized eggs produce females that

are diploid! The development of a fertile queen occurs because she alone is fed

royal jelly throughout development (workers and drones only get it for the first three days)!

 A queen secretes “queen substance” to prevent workers from maturing or feeding larvae any royal jelly!

 A honeybee hive of 60,000-70,000 individuals continues indefinitely! Honeybees have a communication system; scouts can inform workers of

the location of flowers with a “dance”!

Termites! A fertile king and queen fly away to start a new colony; they mate and

lose their wings! Sterile members are wingless and become workers and soldiers! Soldiers have large heads and defend the colony! Reproductive individuals secrete inhibiting pheromones that produce

sterile workers! Nymphs feed from each other in trophallaxis, thus spreading the

pheromone about! Worker castes also produce worker and soldier substances; drops in

these pheromones cause more of these castes to be produced!Ants! Ants differ from termites; ants are darker, hard-bodied and have a

thread-like waist! In ant colonies, the male ant dies after mating! Ants have wingless soldiers and workers, and often have variations of

these castes! Ants have also evolved slavery, fungus farming, sewing nests together,

tool use, and herding ‘ant cows’ (aphids)!

Insects and Human Welfare! Insects produce honey, beeswax, silk, shellac and other products!• Bees pollinate more than $100 billion worth of food crops in the U.S. annually!• Pollinating insects and flowering plants are tightly co-evolved!

 Insects play many vital environmental roles!• Predaceous and parasitoid insects are vital in controlling many pest species!• Dead animals are rapidly consumed by fly maggots!• Insects are critical components of most food chains, especially for fishes and

birds! Harmful insects eat and destroy crop plants and fruits!• Nearly every cultivated crop has several insect pests: this requires substantial

money and effort for insect control!• Bark beetles, the gypsy moth, and others are serious forest pests!• Insects also destroy food, clothing and property!

 Medically important insects include vectors for disease agents!• Warble and bot flies attack humans and domestic livestock!• Malaria is carried by Anopheles mosquitoes!• Lice carry typhus and fleas carry plague, diseases that have produced pandemics

that changed human history in the Middle Ages!• Other insects (especially flies) carry a variety of viral, bacterial, protozoan

diseases, and even animal parasites!


Control of Insects! Pesticides (insecticides)!• Broad-spectrum insecticides damage beneficial insect populations along with

the targeted pest!• Some chemical pesticides persist in the environment and accumulate as the

move up the food!• Some strains of insects have evolved a resistance to common insecticides!

 Biological control is the use of natural agents, including diseases, to suppress an insect population!• Bacillus thuringiensis (Bt) is a bacterium that controls lepidopteran pests; the

gene coding for the toxin produced by the bacteria has been genetically engineered into other bacteria and crop plants!• Some viruses and fungi may be economical pesticides!• Some natural predators or parasites of insect pests can be raised and released to

control the pest!• Release of sterile males can eradicate the few insect species that only mate once!

 Pheromones can monitor pests and hormones may have a role in disrupting their life cycle!

 Integrated pest management is the combined use of all of the possible and practical techniques listed above!

Phylogeny and Adaptive Radiation!  First terrestrial arthropods were Silurian scorpions and

millipedes!• The first insects were wingless and appeared in the Devonian period!• Several orders of winged insects evolved in the Carboniferous

period!  Some researchers believe Myriapoda is paraphyletic; with

Diplopoda and Pauropoda as sister groups to insects!  Molecular data and eye enervation suggest insects and

myriapods are remotely related and insects arose within the Crustacea!

  Some sequence analyses suggest that myriapods are more closely related to chelicerates than to insects!• The ancestral insect had a head and trunk of similar somites!• Insects have specialized the first three post-cephalic somites as

thorax and lost the remaining ones!

 Subclass Apterygota,"the wingless insects,"is apparently "paraphyletic!

 Wings unite the"remaining "ectognathous orders!• Wings probably "

evolved for other "purposes until they "were large enough"to provide lift!• Winged insects evolved in"

two lineages based on their "ability to flex the wing!

 Metamorphosis also distinguished insects by the Permian period!• Hemimetabolous metamorphosis group the Orthoptera, Thysanoptera,

Hemiptera,!• Other orders have holometabolous metamorphosis and are the most

specialized!

biology of animals topic 11.3 arthropods iii: insects … · page 1 arthropods iii: insects! zoo...

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