Arthropods. Insects were a winning combination Insects cause hugh economic losses (but a lot of benefits, as well) each year They are the dominant group.

Arthropods

Insects were a winning combination

• Insects cause hugh economic losses (but a lot of benefits, as well) each year

• They are the dominant group of animals on the earth today

• The diversity of insects is far greater than all other taxa combined

• There are an estimated 200 million insects alive for every human today

Characteristics of Arthropoda• Arthropods includes spiders, scorpions, ticks, mites,

crustaceans, millipedes, centipedes, insects, and some smaller groups as well

• There is a rich fossil record extending back to the mid-Cambrian period

Characteristics

• Arthropods have a well-developed organ system and a chintinized cuticular exoskeleton

• Segments have coalesced into tagmata• They range in size from .1 mm to 13 feet!• Arthropods compete with us for food and

spread disease; they also produce silk, honey, and beeswax

Ecological Relationships• They are found in all environments and

virtually all altitudes and latitudes• Species are adapted to land and to fresh,

brackish, and marine water• Most species fly to their favored habitats• All modes of feeding occur among arthropods

though the majority are herbivorous• Nothing else matches their divesity

Characteristics of the arthropoda• An exoskeleton with a cuticle that is highly

protective but is jointed, providing mobility• Both layers of the cuticle contain chitin bound

with protein• Chitin is a tough resistant polysaccharide

insoluble in water

Characteristics of the arthropoda

• Terrestrial arthropods use an efficient tracheal system that delivers oxygen directly to cells

• Aquatic arthropods respire by various forms of efficient gills

• Arthropods have highly developed sensory organs– Sensory organs vary from a compound mosaic eye

to other senses of touch, smell, hearing, balancing and chemical reception

Segmentation and Appendages

• Each segment usually has a pair of jointed appendages

• Segments and appendages are modified for various adaptive functions

• Appendages may function in sensing, food handling, walking or swimming

Characteristics of the arthropoda

• As the cuticle is thin between segments, it allows movement at the joints

• Muscles attach to the cuticle• The cuticle also folds inward to line the foregut,

hindgut, and the trachea• Ecdysis, or molting, is the process of shedding its

outer exoskeleton• Arthropods usually molt 4 to 7 times; the weight

of the exoskeleton limits their body size

Taxonomy of the arthropods• There are four subphyla:– Trilobita– Chelicerata– Crustacea– Uniramia

Subphylum Trilobita• Trilobita arose before the Cambrian, flourished, and then

became extinct 250 million years ago• They have a trilobed body shape due to a pair of longitudinal

grooves• They were bottom dwellers and probably were scavengers• They ranged from an inch to 20 inches or more and could roll

up like pill bugs

Subphylum Chelicerata

• Include the horseshoe crabs, spiders, ticks, mites, scorpions, and sea spiders

• Chelicerates have six pairs of appendages including chelicerae, pedipalps and four pair of legs

• They lack mandibles and antennae• Most suck liquid food from prey

Class Merostomata• Includes the modern horeshoe crab; Limulus

polyphemus• This species is nearly unchanged from its

Cambrian ancestors• Five species in three genera survive

The Horseshoe Crab

• Most live in shallow water• They have an unsegmented carapace that covers

the body in front of a broad abdomen• A telson or spinelike tail• Book gills are exposed on some of the abdominal

appendages• They walk with their walking legs and swim with

abdominal plates• They feed at night on worms and small molluscs

Class Pycnogonidathe sea spiders

• They vary from a few millimeters to centimeters; all have small, thin bodies

• Some males use legs to carry developing eggs• The mouth at the tip of a proboscis, drinks juices from

cnidarians (hydroids) and soft-bodied animals• They have a greatly reduced abdomen attached to an

elongated cephalothorax

Class Arachnida

• Consists of a great diversity among scorpions, mites, ticks, daddy longlegs and others

• Of 80,000 species, most are free living and more common in warm, dry regions

• Arachnids are dived into a cephalothorax and abdomen

Order Araneaethe spiders

• About 40,000 species of spiders are known• The body consists of an unsegmented

cephalothorax and abdomen joined by a slender pedicel

Order Araneaethe spiders

• The anterior appendages are a pair of chelicerae with terminal fangs

• All spiders are predaceous, mostly on insects, which are killed b poison and fangs

• The injected venom liquefies and digests the tissues which are then sucked into the spider’s stomach

• Spiders breath by book lungs and/or trachae

Characteristics of the spiders• Book lungs are found only in the spiders; parallel air

pockets extend into a blood-filled chamber• Air enters the chamber through a slit in the body wall• The tracheae system is less extensive than in insects; it

carries air directly to tissues• Spiracles are openings to the trachea• Most spiders have eight simple eyes, each with a lens,

optic rods and a retina• They detect movement and may for images• Sensory setae detect air currents, web vibrations, and

other stimuli

Web-spinning Habits

• Spinning silk is a critical ability for spiders and some other arachnids

• Two or three pairs of spinnerets contain microscopic tubes that run to silk glands

• A liquid protein secretion hardens as it is extruded from the spinnerets

• Silk threads are very strong and will stretch considerably before breaking

• Spiders are often camouflaged or cryptic

Web-spinning Habits• Jumping spiders (Salticidae) have excellent vision and stalk their prey• Silk is used for orb webs, lining burrows, forming egg sacs, and wrapping

prey• Wolf spiders, jumping spiders, and fisher spiders chase and catch their

prey

Spider Diversity

Reproduction in spiders• Males court females before mating• The male spins a small web, deposits a drop of sperm

on it, and then stores the package in his pedipalp• Mating involves inserting the pedipalps into the female

genital openings and depositing the spermatophore• Sperm are stored in a seminal receptacle for weeks or

months until the eggs are ready• Eggs may develop in a cocoon in the web or may be

carried by the female• The young hatch in a few weeks and may molt before

leaving the cocoon

Venomous spiders

• Most are feared for no reason at all• Spiders help to control the populations of insects• American tarantulas rarely bite and the bite is not

dangerous• The black widow (Lactrodectus mactans) however, can

be fatal– The venom is neurotoxic

• The brown recluse spider, Loxosceles recluse, has hemolytic venom that destroys tissue around the bite

• Some Australian and South American Spiders are the most dangerous and aggressive

Order Scorpionida: Scorpions

• More common in tropical and subtropical (such as Florida!) zones but do occur in temperate areas

• They are nocturnal and feed largely on insects and spiders

• The short cephalothorax has the appendages and 1 to 6 pair of eyes

• The postabdomen has the long slender tail of five segments that end in a stinging apparatus

• Scorpions bear live young carried on the mothers back

Order Opiliones: Harvestmen

• Harvestmen or Daddy longlegs are common, especially in tropical areas

• Unlike spiders, their abdomen and cephalothorax join broadly without a narrow pedicel

• They can lose one or mor legs of their eight legs without ill effect

• Their chelicerae are pincer-like and they feed more as scavengers than do spiders

Order Acari: Ticks and Mites

• They are medically and economically the most important arachnids

• Their mouthparts are on the tip of the anterior capitulum

• They are both aquatic and terrestrial; some parasitize vertebrates and invertebrates

• About 40,000 species have been described; many more are estimated to exist

Diversity of mites and ticks

• House mites are free-living and cause allergies• Spider mites are one of the many important agricultural pest

mites that suck out plant nutrients• Chiggers are larval Trombicula mites; they feed on dermal tissues

and cause skin irritation• The hair follicle mite Demodex is harmless but related species

cause mange in domestic animals• Ticks are usually larger than mites• Tick species of Ixodes carry Lyme disease• Tick species of Dermacentor transmit Rocky Mountain Spotted

Fever• The cattle tick transmits Texas cattle fever

Diversity of Mites and Ticks

Subphylum Myriapoda

• Includes the centipedes, millipedes, pauropods and symphylans

• Several classes have two tagmata – a head and trunk with paired appendages on the trunk

• Myriapods only have one pair of antennae, mandibles, and maxillae

• Legs are always uniramous• Respiration occurs through the body surface,

trachea, or gills in juveniles

Class Chilopoda: Centipedes• Centipedes are terrestrial carnivorous predators found under

logs, bark and stones eating earthworms, cockroaches and other insects

• Their flattened bodies have up to 177 segments

Class Chilopoda: Centipedes• Each segment, except the one behind the head and the last

two, bears a pair of appendages• Appendages of the first body segment form venom claws• The head has a pair of eyes on either side of the head that

consist of groups of ocelli• A pair of spiracles in each segment allows air to diffuse

through branched air tubes of the trachae• The sexes ar separate; all are oviparous and the young

resemble the adults• One genus of house centipede has 15 pairs of legs and

another has 21 pairs• Most are harmless to humans but a few large, tropical

centipedes are dangerous

Class Diplopoda• Millipedes have many legs, but not a thousand as they

are sometimes called• Their cylindrical bodies have from 25 to 100 segments• Two pairs of legs are present per segment, probably

from the fusion of two segments• Each abdominal segment has two pair of spiracles

opening into air chambers and tracheal air tubes• Most eat decayed plants but a few eat living plant tissue• After copulation, the female lays eggs in a nest and

guards them• Larvae have only one pair of legs to each segment

Millipedes

Subpylum Crustacea• 67,000 species include lobsters, crayfish,

shrimp, crabs, and copepods• Most are aquatic, and free living, many are

sessile, commensal, or parasitic• The main distinguishing characteristic of

crustaceans is that they have two pairs of antennae

• The head also has a pair of mandibles and two pair of maxillae

Typical Body Plan

Subphylum Crustacea

• There is one pair of appendages on each of the additional segments; some segments lack appendages

• All appendages, except perhaps the first antennae, are biramous with two main branches

• Primitive crustaceans may have up to 60 segments; modern crustaceans have 16-20

• The tagmata are usually head, thorax and abdomen• The dorsal covering is the carapace; it may cover

most of the body or just the cephalothorax

Form and Function of Crustacea• Crayfish and lobsters show modifications to their

appendages• Swimmerets retain the primitive biramous condition

and consist of an endopod and exopod which are attached to one or more basal segments collectively called a protopod

Form and Function of Crustacea

• Crayfish appendages have evolved into walking legs, mouthparts, swimmerets, etc from modification of the basic biramous appendage– The evolutionary trend is reduction and

modification of appendages• Three pairs of thoracic appendages are called

maxillipeds; the first pair of walking legs are called chelipeds, and the last pair of appendages are called uropods

Form and Function of Crustacea

• Abdominal swimmerets are used in locomotion, the first pair are named gonopods

• Gonopods in males are modified for copulation; females attach eggs and young to them

• Uropods serve as paddles for swift backward movement

• The telson also protects eggs and young on the swimmerets

External Structure of Crayfish

Important Internal Features

• Major body space is a blood-filled hemocoel• Muscular and nervous sysems show the

metamerism of annelid-like ancestors• Most muscles are antagonistic; flexors draw a limb

toward the body and extensors straighten a limb out• Abdominal flexors of a crayfish allow it to swim

backward• Strong muscles located on each side of the stomach

control the mandables

The molting process

• Molting is necessary for a crustacean to increase in size; the exoskeleton does not grow

• The physiology of molting affects reproduction, behavior and many metabolic processes

• Inorganic salts are withdrawn from the old cuticle during premolt• The underlying epidermis secretes the cuticle• Enzymes released into the area above the new epicuticle dissolve

the old endocuticle• The animal then swallows water or air to expand and burst the old

cuticle• The new soft cuticle stretches and then hardens with the

deposition of inorganic salts during postmolt• Molting occurs often in young animals and may cease in adults• Temperature or day length may trigger molting

Ecdysis or Molting

Molting sequence of a lobster

Feeding Habits• Many crustaceans shift from one type of feeding

to another, depending on food availability• The same fundamental mouthparts are adapted

to a wide array of food availability• Mandibles and maxillae ingest food; maxillipeds

hold and crush food• Suspension feeders generate water currents in

order to eat plankton, detritus and bacteria• Predators consume larvae, worms, crustaceans,

snails and fishes

Feeding Habits

• The shrimp-like Lygiosquilla pierces prey with a specialized digit on a walking leg

• The pistol shrimp Alpheus catches prey with a large chela that snaps shut

• Scavengers eat dead animal and plant matter• Crayfishes have a two-part stomach; a gastric

mill grinds up food in the first compartment

Internal anatomy of crustaceans

• Gills that vary in shape• Excretory and osmoregulatory organs are located

in the head• Decapods have antennal glands called green glands• Waste products consist of ammonia with some

urea and uric acid• Crustaceans and other arthropods have an open

circulatory system; there is no system of veins to separate blood from interstitial fluid

Internal anatomy of crustaceans• Movement of organs and limbs circulate blood in the open

sinuses• Hemocyanin and hemoglobin are respiratory pigments; clotting

also occurs• There is a brain with a double ventral nerve cord• A median eye and compound eyes are present• The median eye consists of three pigment cups, retinal cells,

and possible a lens• Crustacean compound eyes are similar to insect eyes• Attached to moveable stalks, compound eyes detect motion

and analyze polarized lights• Statocysts, tactile setae, and chemosensitive setae are also

present

Reproduction and Life Cycles• Crustaceans have separate sexes with specializations for

copulation• Almost all barnacles are monecious but generally cross-

fertilize• In some ostracods, males are scarce and reproduction is by

parthenogenesis• Most crustaceans brood eggs in brood chambers, in brood

sacs attached to the abdomen, or attached to abdominal appendages

• Crayfish develop directly without a larval form• Most crustaceans have a larva unlike the adult in form, and

under metamorphis• The Nauplius is a common larval form

Life Cycle of Gulf Shrimp

Brief Survey of Crustaceans• Ostracods are enclosed in a bivalve carapace and

resemble tiny clams and are less than 1/16 inch long• Most live in marine or freshwater sediments but

some scavenge or feed on detritus

• Copepods have numerous species• They lack a carapace and retain the simple,

median eye in the adult• They have four pairs of flattened, biramous,

thoracic swimming appendages• Free-living copepods may be the dominant

primary consumer in aquatic communities• Parasitic forms are highly modified and

reduced, often unrecognizable as arthropods

• Brachiurans lack gills and most are parasites of fish

• Found on both marine and freshlwater fish

Cirripedia – the barnacles• Barnacles as adults are sessile and attach

directly or by a stalk to the substrate• The carapace surrounds the body and secretes

a set of calcareous plates• The head is reduced, the abdomen is absent

and the thoracic legs are long with hair-like setae

• The many-jointed cirri that bear the setae are extended from the plates to feed on small particles

The barnacle

• Isopods are dorsoventrally flattened, lack a carapace and have sessile compound eyes

• The abdominal appendages bear gills• Common land forms include the sow bugs and pill bugs• Some isopods are highly modified as parasites of fish or

crustaceans

• Amphipods resemble isopods except they are somewhat compressed laterally

• They lack a carapace and have sessile compound eyes

• Many are marine, others are beach-dwelling, freshwater or parasitic

• Euphausiacids or “Krill” has only 90 species but includes the important ocean plankton called krill

• They form a major component of the diet of baleen whales and of many fishes.

• Some are over 2 inches long

• Decapods have five pairs of walking legs, the first forming pincers or chelae

• They range from a few millimeters to the larges arthropod, a Japanese crab with a 12 foot leg-span

• They are true crabs with a broader cephalothorax and reduce abdomen, compared to crayfish or lobsters

• Fiddler crabs have a reduced abdomen and burrow in the sand

• Hermit crabs are adapted to live in snail shells

Variety of Decapods