Lab 5 - Arthropods


Introduction to Arthropods

This is not, as the Victorians called it, the Age of Mammals. Theplanettoday is almost completely dominated by a single phylum of animal life.On land, in the sea, even in the air itself, they are the true mastersof the Earth. They are the arthropods. Arthropods areeucoelomateprotostomes, dominating the protostome branch of the animal tree, justas vertebrates dominate the deuterostome branch. Arthropods share acommonancestor with polychaete worms, and may even be a direct descendant ofpolychaetes. But unlike other eucoelomate invertebrates, the arthropodcoelom is greatly reduced in the adult animal.

You are watching: The arthropod skeleton is composed of ______.

There are over 800,000 named species in the Phylum Arthropoda,named from the Greek arthros (= jointed) and poda (=foot),including the familiar arachnids, crustaceans, and insects, togetherwitha host of less familiar critters, like centipedes, millipedes and seaspiders.All arthropods have jointed appendages. This evolutionaryinnovationis probably the key to the stunning success of this diverse group.Thereare about 1018 (10 billion billion) arthropods alive at any one time.Thereare over three times as many species of arthropods as there are of allother animals on Earth, and there may be millions more that we haven"teven discovered. Arthropods do everything with legs or modified legs.Theywalk, they swim, they creep and crawl, they use legs to sense with (theantennae), to bite and sting with, and even to chew with. That"s onereasonarthropods look so alien when we see them up close. They chew sideways,and it"s all done with legs.

Their bodies are protected by an tough cuticle made of proteins andchitin,a polysaccharide with added nitrogen groups. A cuticle is atoughouter layer of non living organic material. The cuticle of arthropodsactsas an exoskeleton. Most are very small, though a few lobstersreachup to a meter, and one giant crab grows to 3.5 meters long.

Fossil insects were also very large. Ancient dragonflies hadwingspansof a foot or more. But living insects are uniformly small. Perhapssmallerinsects were better at hiding or escaping from their many predators.Terrestrialarthropods remain small primarily because of the limitation imposed bytheir exoskeleton. A large insect would need such a thick exoskeletontowithstand its strong muscles that the weight of the cuticle would betoogreat for the animal to carry around. For a small animal, having yourskeletonon the outside is as logical as having it on the inside. But it poses afundamental problem for arthropods. They must shed their exoskeleton,ormolt,in order to grow. The exoskeleton splits open. the animal emerges andswellsto a larger size until the newer, larger exoskeleton is hardened. Whilethe animal molts, it is especially vulnerable - just ask a plate ofsoft-shelledcrabs!

Arthropods have segmented bodies, like the annelid worms. Thesesegmentshave become specialized, however, with one pair of jointed appendagesaddedto each segment. Among living arthropods, the millipedes most closelysuggestwhat the ancestral arthropod might have looked like. Arthropod segmentshave also fused together into functional units called tagma.Thisprocess of segment fusion, or tagmosis, usually results in anarthropodbody that consists of three major sections, a head, thorax, andabdomen.Sometimes the head and thorax are fused together into a cephalothorax.Each of these body sections still bear the appendages that went withit,though these appendages are often highly modified. Arthropods are veryhighly cephalized, often with intricate mouthparts and elaboratesensoryorgans, including statocysts, antennae, simple eyes andcompound eyes. Sensitive hairs on the surface of the body candetecttouch, water currents, or chemicals. Their nervous systems are highlydeveloped,with chains of ganglia serving various parts of the body, and threefusedpairs of cerebral ganglia forming a brain.

Aquatic arthropods respire with gills. Terrestrial formsrelyon diffusion through tiny tubes called trachea. Trachea arecuticle-linedair ducts that branch throughout the body, and open in tiny holes calledspiracles, located along the abdomen. Insects can open and closethesespiracles, to conserve water that would otherwise be lost toevaporationfrom the open tubes. Their reliance on diffusion for respiration is oneof the reasons that insects are small.

Arthropods excrete by means of malphigian tubules,projectionsof the digestive tract that help conserve water. Terrestrial formsexcretenitrogen as uric acid, as do birds. Their waste is nearly dry, asuperb adaptation to life on land. Arthropods have an open circulatorysystem, and separate sexes. Fertilization is usually internal, anotheradaptation for terrestrial life. Males and females often showpronouncedsexualdimorphism.


Phylum Arthropoda:

Subphylum Chelicerata

Class Merostomata - horseshoe crabs,

Class Arachnida - spiders,scorpions,ticks, mites

Subphylum Crustacea - crustaceans

Subphylum Uniramia

Class Chilopoda - centipedes

Class Diplopoda - millipedes

Class Insecta - insects

Order Hymenoptera- ants, bees, wasps

Order Coleoptera -beetles

Order Lepidoptera- butterflies, moths

Order Diptera -flies,mosquitoes

Order Orthoptera -grasshoppers, crickets, roaches

Order Odonata -dragonflies

Order Isoptera -termites

Terms jointed appendages cuticle chitin exoskeleton molt tagma tagmosis head thorax abdomen cephalothorax statocyst antennae simple eyes compound eyes gills trachea spiracles malphigian tubules uric acid sexual dimorphism chelicerae fangs pedipalps spinnerets webs Rocky Mt. Spotted Fever Lyme Disease biramous appendages uniramous appendages nauplius larvae detritus detritivores pheromones maxillae simple metamorphosis complete metamorphosisCharacteristics of Subphyla and Classes

Subphylum Chelicerata:

In chelicerates, the first pair of appendages are called chelicerae,and are modified to manipulate food. They are often modified as fangsorpincers. Chelicerates lack antennae.

Class Merostomata - horseshoe crabs (Limulus)

Horseshoe crabs have larvae that are very similar to trilobites, andthey may be descendants of this long vanished group. Horseshoe crabsarenocturnal, feeding on annelids and molluscs. They swim on their backs,or walk upright on five pairs of walking legs. They live in the deepocean,migrating inshore in large numbers in the spring to mate on the beachesduring moonlight and high tide - much like undergraduates on SpringBreak.

Class Arachnida - (57,000 sp.), spiders, scorpions, ticks,mites,and daddy longlegs

This very successful group of arthropods have four pair of walkinglegs(8 legs). The first pair of appendages are the chelicerae, andthesecond pair are pedipalps, appendages modified for sensoryfunctionsor for manipulating prey. They are mostly carnivorous (many mites areherbivores).Most secrete powerful digestive enzymes which are injected into thepreyto liquify it. Once dissolved in its own epidermis, the prey is sippedlike a root beer float.

Order Scorpiones (2,000 sp.) - Scorpions have pedipalpsmodifiedas pincers, along with a venomous sting in their tail. Scorpions datebackto the Silurian, about 425 mya, and may be the first terrestrialarthropods.

Order Araneae (32,000 sp.) - Spiders have special modifiedposteriorappendages called spinnerets, which they use to spin their webs.Not all spiders spin webs. Wolf spiders are the tigers of the leaflitter,and the common jumping spider leaps several times its body length tocatchits prey. Spiders use pedipalps as copulatory organs. Spiders breathebybook lungs

Order Acari - (30,000 sp.) - Ticks and mites are the largestand most diverse group of arachnids. Most are very tiny, less than 1 mmlong. The thorax and head are fused into a single unit (cephalothorax).Ticksarebloodsucking parasites, and can carry diseases like Rocky MountainSpottedFever and Lyme Disease.

Order Opiliones (5,000 sp.) - Daddy Longlegs is a familiararachnid.It has an oval body with extremely long legs, which they frequentlylosein various accidents and brushes with predators. They are predators,herbivores,and scavengers. Look at them closely next time you see one. They carrytheir eyes atop a little tower on their back (weird!).

Subphylum Crustacea - (38,000 sp.), crabs,shrimp,lobsters, crayfish, isopods, barnacles, brine shrimp

Crustaceans are mostly marine, and dominate the ocean to the samedegreethat insects dominate the land and air. Despite their aquaticdiversity,there are very few terrestrial crustaceans, just as there are very fewtruly aquatic insects. Crustaceans have biramous appendages.Eachleg has an additional process, like a little miniature leg branchingofffrom the main leg. Many groups of crustaceans have lost this extraappendageduring subsequent evolution. The Order Decapoda have five pairofwalking legs, and include the familiar crabs, lobsters, and crayfish.Thefirst pair of appendages are usually modified as antennae.Crustaceanshave two pair of antennae. Another set of anterior appendages aremodifiedas mandibles, which function in grasping, biting, and chewingfood.Male crayfish also use one pair of legs as a copulatory organ. Allcrustaceansshare a common type of larva called a nauplius larva.

Order Isopoda, Isopods have many common names, such as Pillbugs,Roly-Polys, Woodlice, Bibble Bugs, Cheesybugs, Cud-worms,Coffin-cutters,Monkey Peas, Penny Pigs, Sink-lice, Slaters, Sowbugs, Tiggyhogs, and(inNew Orleans) Doodlebugs. They are one of the few successful terrestrialcrustaceans. They feed on decaying vegetation in the leaf litter.

Subphylum Uniramia - centipedes, millipedes,insects

Uniramians have a single pair of antennae, and uniramousappendages.They probably evolved from oligochaete worms.

Class Chilopoda - (2,500 sp.) Centipedes dwell in damp placesunder old logs and stones. They are carnivorous, eating mostly insects.They are highly segmented, and have one pair of legs per segment.Despitethe name, the number of legs comes out to considerably less than onehundred(centi = 100). The first trunk segment bears poison fangs.Centipedesare very dangerous, and their bite is extremely painful.

Class Diplopoda - (10,000 sp.) Millipedes share the samehabitatas centipedes, but they are mostly herbivorous, feeding on decayingvegetationin the leaf litter. Animals that feed on detritus are called detritivores.They have two pair of legs per segment, (less than a thousand <=milli>,but lots more than a centipede). Each segment of the millipede isactuallytwo segments fused together (hence the double set of legs). They cansecretea defensive fluid that smells bad, and a few species actually secretetinyamounts of cyanide gas to protect themselves!

Class Insecta - (750,000 sp.) If we knew all the differentinsectson Earth, there could be as many as 30 million species. Insects evolvedabout 200 mya, with cockroaches and dragonflies among the first toappear.Insects have a head, thorax, and abdomen, with three pair of legs (6legs)on the thorax. (Crustaceans have legs on the abdomen as well as on thethorax). Most insects have one or two pairs of wings. They are the onlyinvertebrates that fly. Most have compound eyes, and cancommunicateby sound and scent, using powerful chemical hormones calledpheromones.

Insects have extremely elaborate mouthparts, consisting of pairs ofappendages fused into a lower lip (labium), and an upper lip (labrum),with other appendages called maxillae aiding in chewing. Thesemouthpartsare highly modified in various groups for chewing, sucking, andpiercing.Insects undergo metamorphosis as they develop, changing fromoneform to another as they mature. Some (about 10%) show simplemetamorphosis,in which there is no resting stage. The juvenile stages look like tinyversions of the adults. Most (90%) show complete metamorphosis,in which one stage is an inactive pupa, like the cocoon of themothor the chrysalis of the butterfly. Their larvae are often radicallydifferentfrom the mature adult (like the butterfly and the caterpillar). Theynotonly look different, they live in different places and eat differentfood.

To Do and View

Observe the preserved arthropods on display. How do thevariousgroups use their legs to walk, swim, feed or mate?

Watch the way the millipede moves. Look at the legs. See howthe waves of muscle contraction pass down through the segments? Thepolychaeteworm Nereis moves in exactly the same way. Handle themillipedesvery gently. They are someone"s pets. They also make great pets fordormrooms - they need little care, don"t take up much room, and don"t makenoise or messes, unlike your roommate.

Disturb the centipedes to get them moving around. Can you seethe poison fangs? Notice how flat the body is, and contrast the numberof legs with those of the millipede. Why does each container hold onlya single centipede? Don"t open the jars unless you have a thing forextremepain.

Play around with the roly-polys. Oh, go ahead, it"s cool.Theywon"t bite. Watch the way they roll up into a ball when disturbed. Notall isopods can do this, but rolling up into an armored ball is a greatdefensive tactic. Compare our teeny tiny terrestrial version with theenormous(preserved) marine isopods.

Look at the live brine shrimp, hermit crabs and fiddler crabs.Treat them gently (more pets). Watch the way they use their legs,includingthe modified legs that form their mouthparts. You may see the malefiddlercrabs raise their large claw and wave it about to claim a territoryinsidethe tank, in the hopes of attracting a mate (Can you blame them?).

Observe the live crayfish. What does the crayfish do when itfeels threatened? How does it use its swimmerets when it is stationary?

Observe the diversity in insect mouthparts etc. Don"t worryaboutbeing able to identify the individual slides. Try to get a feel for theway modified legs are employed in these animals for a wide variety ofsucking,sponging, piercing and biting.

Observe the insects on display. You should be familiar (forlaband lecture) with the common orders of insects listed in this guide.

Tips for Dissection

Crayfish are relatively easy to dissect. Many of you have had amplepractice dissecting them at Jazz Fest. Your first task is to determinewhether you have a male or female crayfish. Turn the animal on itsback,and examine the area of the thorax where the legs join the body. Femalecrayfish have a circular opening, like a tiny doughnut, which is theirseminalreceptacle. Male crayfish have a hardened pair of swimmerets (legson the abdomen) that extends back towards the head, and fits neatlyintothe groove between the walking legs. These modified legs are stiff,likehard plastic. They are curved like half a soda straw, and when they arejoined together, they make a tiny tube through which the sperm travelduringcopulation. Crayfish literally copulate with their legs.

Observe their external anatomy. Identify the following structures: rostrum,antennae, eyes, thorax, carapace, chelae (claws), cheliped,walkinglegs, abdomen, swimmerets, telson, and uropod. Examine the variousappendages and modified appendages closely. Note that some are biramous(ex. uropods, most swimmerets), while some are uniramous (ex.cheliped).The uniramous appendages result from the evolutionary loss of thesecondbranch. Note that each pair of antennae are biramous appendages.Examinethe telson and uropod. How does the crayfish use these biramousappendagesto escape predators? Using a probe, try to find the mouth and anus.Notethe thick triangular mandibles, a primary trait of crustaceans.

Place the crayfish in the pan with its dorsal side up. Carefully cutthe carapace just to one side of the midline with your scissors, anddownalong both sides. Peel it back to expose the gills. Notice howthegills interface with the legs, and observe the second underlying row ofgills. Cut away the gills where they join the body. Try to find thetinyheart(good luck!). Just under the heart are the gonads (ovaries ortestes).Look for the esophagus and stomach (you can alwaysinserta probe through the mouth to see where it emerges). Carefullyremovethe internal organs, and look for the tiny brain near the baseofthe antennae.

Crayfish Anatomy seminal receptacle rostrum antennae eyes thorax carapace chelae cheliped walking legs abdomen swimmerets telson uropod mandibles gills heart gonads esophagus mouth stomach brainEconomic, Ecological, and Evolutionary Importance

The many ways that arthropods help us and hurt us are almost toonumerousto mention.

They provide seafood, and pollinate fruit crops.

The also cause billions of dollars a year in crop damage.

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They cause or carry a host of diseases, such as malaria and theplague.

Ecologically, they are critically important herbivores. Arthropodsarethe primary converters of plant tissue to animal tissue on the planet!

Consider This

How do segmentation and tagmosis account for the success ofarthropods?

Why aren"t bugs the size of Buicks?

Trilobites were among the most successful arthropods on Earth, oncenumbering over 10,000 species. Why are they all gone?

How does the smooth flow of muscle contractions in the movingmillipederelate to the evolution of segmentation in annelids and arthropods? (Hint:Why is a segmented body plan useful for a burrowing animal?)

Links to Explore

There are hundreds of arthropods waiting quietly in obscure cornersof the web, ready to pounce on the unwary undergraduate and grab yourattention.Where to start? The UCMP server is great for all groups exceptcrustaceans: incredible wealth of material on spiders, scorpions, ticks and mitesawaits you at the center of the arachnid web. Includes systematics,arachniddatabases, stuff for kids, art, literature and movies: crawl throughout the murky reaches of the cybersea. You"llfind a good starting point at the home page of the Crustacean Society,which includes lots of links to these crunchy critters: are almost as many insect home pages as there are species ofbutterflies.A good place to start is Gordon"s Entomological home page, whichincludesa wealth of links to all major orders, and lots of cool stuff aboutbugs: the drop on jumping spiders, including PG-13 Quicktime movies oftheircurious courtship rituals at: an faq on scorpions, including beaucoup links, at: power of lice compels you, so learn about ticks and mites andthingsthat bite from the Lyme Disease Network"s home page at: http://www.lymenet.orgRead the Cockroach Control Manual at: of butterflies and moths have been caught in the web: A goodpictorialguide to common species, with tips on their host plants can be found at: about the moths of North America at: books, videos, caterpillars etc. at: of many pages dedicated to raising Painted Lady butterflies:

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