43.0 Introduction

1. New Coelomate Innovation: Jointed Appendages
1. Arthropod Segmented Bodies Show Relationship to Annelids
1. Segments visible during early development
2. Fuse together into functional groups in adults

2. Jointed Appendages Provide Functional Flexibility fig 43.1

43.1 The evolution of jointed appendages has made the arthropods very successful

1. Arthropods
1. Yet Another Major Innovation
1. Jointed appendages first accomplished by the arthropods
2. Necessary adaptation with advent of rigid exoskeleton

2. Jointed Appendages
1. Phylum name means "jointed feet"
1. Number of appendages reduced in more advanced members of phylum
2. Appendages modified into antennae, mouthparts and legs

2. Some appendages like wings are not homologous to other appendages
3. Importance of jointed appendages
1. Necessary for walking, grasping objects
2. Arthropods also use antennae for sensing environment, mouthparts for feeding

4. Rigid exoskeleton is a limitation of arthropod body plan
1. Skeleton functions as attachment for muscles
2. Arthropod muscles attach to interior surface of chitin shell
1. Shell protects animal from predators
2. Impedes water loss

3. Chitin is tough, but brittle, cannot support great weight
1. Exoskeleton must be thicker to bear pull of muscles
2. Most arthropods cannot attain great size

3. The Most Successful of All Animals
1. Includes nearly two-thirds of all named species on earth fig 43.2
1. May include 30 million species of insects alone
2. Are abundant in all habitats, but dominate terrestrial regions

2. Most arthropods are relatively small, but a few may be as large as 3.6 meters
3. Well represented in fossil record
1. Trilobites an early important group fig 43.3
1. Extinct for 250 million years
2. First animals with eyes capable of substantial resolution

2. Insects groups exist as fossils and common living forms

4. Phylum Onychophora shares several features with arthropods
1. Commonly called velvet worms
2. Appear to be related to arthropods fig 43.4

5. Economically important, especially insects fig 43.5
1. Compete with humans for food
2. Pollinate crops, control insects and weeds
3. Cause extensive damage to food crops, important herbivores
4. Spread diseases

2. General Characteristics of Arthropods
1. Body Segmentation
1. Some members have large numbers of segments
2. In others the segments are fused into functional units or tagmata
1. Example head or thorax of an insect fig 43.6
2. Process called tagmatization
3. Segmentation may be more obvious during development of larvae
4. Head and thorax may be fused into a cephalothorax

2. Head Appendages Define Key Groups
1. Arthropods divided into three subphyla
1. Chelicerata: Spiders, horseshoe crabs, sea spiders
2. Crustacea: Lobsters, sowbugs, barnacles
3. Uniramia: Insects, centipedes, millipedes

2. Arthropods also grouped by type of mouthparts
1. Chelicerates lack jaws, include Chelicerata, mouthparts are chelicerae
2. Mandibulates have jaws called mandibles, include Crustacea and Uniramia

3. Chelicerates
1. First pair of appendages are the chelicerae fig 43.7a
2. May appear as pincers or fangs
3. Second chelicerate appendages are pincer or feelerlike
4. Remaining appendages are legs
5. Mandibulates and chelicerates evolved independently, neither gave rise to other

4. Mandibulates
1. Most anterior appendages are sensory antennae
2. Next appendages are mandibles fig 43.7b
3. Crustacean appendages are biramous (two-branched) fig 43.8
1. Some have single-branched appendages
2. Resulted from reduction during evolution

4. Uniramia appendages are uniramous (one-branched)

3. External Features
1. Exoskeleton
1. All arthropods covered by hardened chitinous skeleton or cuticle
1. Tough outer covering is secreted by and fused with epidermis
2. Varies in toughness and thickness
3. Crustaceans add calcium carbonate, making it less flexible

2. Functions of exoskeleton
1. Prevents excessive water loss
2. Protects from predators, parasites and injury

3. Molting
1. Growth requires periodic ecdysis, shedding of outer cuticle
2. New exoskeleton grown beneath old one
3. Controlled by hormones
4. Separated by a fluid that dissolves components of old skeleton
5. Old skeleton cracks open and is shed
6. New skeleton is initially quite soft and must be expanded to full size
7. Hardens with exposure to air or water

2. Compound Eye fig 43.9a
1. Composed of many ommatidia: Independent visual units
1. Each covered with a lens
2. Linked to eight retinula cells and central light-sensitive rhabdom

2. Apposition eyes
1. Example: Bee
2. Each ommatidium acts in isolation
3. An image inverted on each ommatidium retina
4. Surrounded by pigment cells
5. Individual images formed in brain

3. Superposition eyes
1. Example: Moth
2. Images from ommatidia are combined on cornea at rear of eye
3. Not associated with screening pigment cells
4. Single right-side-up image is formed

4. Ocelli are simple eyes with single lenses fig 43.9b
1. Sometimes occur together with compound eyes
2. Function in distinguishing light and darkness
3. May also serve as horizon detectors in locusts and dragonflies

4. Internal Features fig 43.6
1. Reduction of Coelom Through the Course of Evolution
1. Consists of cavities housing reproductive organs and some glands
2. Arthropods completely lack cilia
3. Have tubular gut that extends from mouth to anus

2. Circulatory System fig 43.10
1. Open system, blood does not flow through closed vessels
2. Longitudinal heart along dorsal thorax and abdomen
1. With contraction, blood flows into head
2. When heart relaxes blood returns it
3. Series of one-way valves in posterior of heart allows blood to flow inward only
4. Blood from anterior end flows through spaces to posterior end
5. Flow is more rapid with greater activity
6. Blood delivers nutrients, transports wastes

3. Respiratory System
1. Functions in Uniramia to transport oxygen directly to tissues
2. All parts of body must be near air passage limiting body size
3. Possess no single respiratory organ, but a system of branched tracheae fig 43.11
1. Become smaller tracheoles that are in contact with individual cells
2. Air passage controlled through external spiracles
3. Closing spiracles conserves water
4. Air flow assisted by muscular movements in larger organisms

4. Many chelicerates like spiders have book lungs
1. A series of leaflike plates within a chamber
2. Air drawn in and out by muscular contractions
3. May exist along with or in place of trachaea
4. Horseshoe crabs have book gills

5. These respiratory systems unique to arthropods and Onychophora
6. Crustaceans have typical gills

4. Excretory System
1. Several forms of excretory systems
2. Principal components of the land uniramians are Malpighian tubules fig 43.12
1. Slender projections of the digestive tract
2. Located at the junction of the midgut and hindgut
3. Fluid of blood passes through walls of tubules
4. Nitrogenous wastes are precipitated as fluid passes toward hindgut
5. Waste emptied into hindgut and eliminated
6. Most water and salts reabsorbed by hindgut and returned to body
7. Efficient mechanism to conserve water, necessary for adaptation to land

5. Nervous System
1. Predominant double chain of ganglia runs along ventral surface
2. Anterior end possess three fused pairs of dorsal ganglia form the brain
3. Much control of activities regulated by ventral ganglia
1. Many activities continued with brain removed
2. Brain appears to be inhibitor, not stimulator as in vertebrates

43.2 Scorpions, spiders, and mites all have fangs or pincers

1. Introduction to Chelicerates
1. Distinct Evolutionary Line Evolved Chelicerae
1. Most anterior appendages
2. Function as fangs or pincers
3. Different evolutionary origin than mandibles in crustaceans and uniramians

2. Three Classes of Chelicerates
1. Arachnids
2. Horseshoe crabs
3. Sea spiders

2. Class Arachnida: The Arachnids
1. General Biology
1. Largest class of chelicerates
2. Possess a pair of chelicerae, pair of pedipalps, four pair of legs
1. Chelicerae are frontmost appendages, fangs with poison glands
2. Pedipalps are next set of appendages, like legs but one less segment
1. Have specialized functions as copulatory organs, sensory organs
2. Scorpion pedipalps are large pincers
3. Often used for catching and handling prey
4. May also chew with basal portion, as in some spiders
5. Rarely used for locomotion

3. Other general characteristics
1. Most are carnivorous, mites are herbivorous
2. Most ingest only preliquified foods, thus digestion is external
3. Are generally terrestrial, evolved direct transfer of sperm
4. Breathe by trachea, book lungs or both
5. Include eleven orders, four presented here

2. Order Opiliones: The Daddy Longlegs fig 43.13
1. Possess compact, oval bodies with extremely long, slender legs
2. Respire by a single pair of trachea
3. Engage in direct copulation, unusual among arachnids
1. Males possess penis
2. Females use ovipositor to deposit eggs

4. Most are predators, some feed on plant juices or are scavengers

3. Order Scorpiones: The Scorpions fig 43.14
1. Pedipalps are modified into pincers to handle and tear food apart
2. Have venomous stings on terminal segment
3. Distinctive elongated, jointed abdomens
4. Extremely ancient group of terrestrial arthropods

4. Order Araneae: The Spiders
1. Important predators of insects and small animals
2. Hunt prey or catch it in silk webs
1. Silk formed from fluid protein, forced out spinnerets fig 43.15
2. Up to six pairs of modified appendages
3. Variety of adaptive modifications to webs

3. Many forms are active hunters fig 43.16
1. Tarantulas do not spin webs, wolf spiders line burrows with silk
2. Water spider envelopes body in bubble of air

4. Have poison glands leading through chelicerae
1. Some are poisonous to man and large mammals
2. Examples: Black widow and brown recluse fig 43.17

5. Reproduction
1. Males produce sperm web, add drop of sperm, pick up with pedipalps
2. May involve elaborate courtship
3. Male fits pedipalps into special plate on female's abdomen
4. Female may eat male once fertilization is complete
5. Eggs enclosed in silken egg sac
6. Young resemble adults, go through several molts

5. Order Acari: The Mites
1. Most diverse in terms of numbers and species fig 43.18
2. Generally very small in size
3. Cephalothorax and abdomen fused into an ovoid body
4. Respiration occurs through trachea or directly through exoskeleton
5. Development occurs on many complex successive stages
1. Various juvenile stages have become reproductive
2. Called paedomorphosis

6. Many mites live on humans, have irritating bites
1. Some transmit diseases
2. Follicle mites live in facial hair follicles, no adverse symptoms fig 43.19

7. Ticks are blood-feeding ectoparasites, transmit various diseases fig 43.18c
8. Cause extensive plant damage

3. Class Merostomata: The Horseshoe Crabs
1. General Characteristics
1. Example: Limulus, common on North Atlantic coasts fig 43.20
2. Evolution
1. Ancient group, fossils identical to 220 million years old Limulus
2. May be derived from trilobites due to resemblance of larvae

3. Live in deep water, migrate to shallow coastal waters to mate
4. Feed at night on mollusks and annelids
5. Structural adaptations
1. Swim on backs by moving abdominal plates
2. Shell protects most body parts
3. Possess four pairs of walking legs
4. Also covers chelicerae and pedipalps fig 43.21
5. Breathe via five pairs of book gills posterior to legs located under opercula

4. Class Pycnogonida: The Sea Spiders
1. General Characteristics
1. Common, but rarely observed because of small size
2. Adults are generally parasites or predators on other animals fig 43.22
3. Structural adaptations
1. Have sucking proboscis with terminal mouth
2. Body consists mostly of cephalothorax, no well-defined head
3. Possess four to six pairs of legs
4. Males exhibit parental care of young, carry eggs on legs
5. Lack excretory and respiratory systems, exchange by diffusion

4. Not closely related to other two classes

43.3 Crustaceans have branched appendages

1. Crustaceans
1. Arrangement and Nature of Appendages
1. Two pairs of antennae, three pairs of chewing appendages
2. Number of legs varies with the species
3. All appendages are biramous
1. Excluding first pair of antennae
2. Single-branched appendages previously biramous, one branch lost in evolution

4. Evolution of crustaceans
1. All descended from common ancestor as evidenced by nauplius larvae fig 43.23
2. Some groups lack larvae and undergo direct development into adult

2. General Biology
1. Have legs on abdomen and thorax like millipedes and centipedes (unlike insects)
2. Only arthropods with two pair of antennae
3. Mandibles evolved from limbs that developed a chewing function
4. Many have compound eyes and tactile hairs over whole body
5. Larger forms have feathery gills near base of legs
6. Excretion of nitrogen wastes occurs mostly across surface of cuticle
7. Variety of sex strategies and care of young

3. Decapod Crustaceans
1. Decapods ("ten-footed") include lobsters, shrimp and crabs fig 43.24
2. Structural adaptations
1. Exoskeleton reinforced with calcium carbonate
2. Body segments fused into cephalothorax, covered by carapace
3. Crushing pincers common, used to obtain food
4. Abdominal swimmerettes used in reproduction and locomotion
5. Snapping of telson and uropods causes forceful, rapid movements
6. Crabs have larger broader carapace than lobsters fig 43.24b
7. Shrimps have smaller carapace than crabs or lobsters fig 43.24c

4. Terrestrial and Freshwater Crustaceans
1. Terrestrial forms include pillbugs, sowbugs, isopods fig 43.25a
2. Sand and beach fleas, order Amphipoda are semiterrestrial intertidal species
3. Planktonic crustaceans
1. Copepods, order Copepoda fig 43.25b
2. Water fleas, order Cladocera
3. Ostracods, order Ostracoda
4. Fairy shrimp and brine shrimp, order Anostracoda

5. Sessile Crustaceans
1. Include barnacles, order Cirripedia fig 43.26
2. Are sessile as adults, but have free-swimming larvae
3. Head attached to submerged object, food swept into mouth by feathery legs
4. Protected by calcareous plates attached to substrate
5. Are hermaphroditic, unusual for crustaceans, but cross-fertilize

43.4 Insects are the most diverse of all animal groups

1. Subphylum Uniramia
1. General Characteristics
1. Evolved from annelids similar to oligochaetes
2. Respire via trachea
3. Filter waste products through Malpighian tubules

2. Classes Chilopoda and Diplopoda: The Centipedes and Millipedes
1. Common Characteristics
1. Both possess head region followed by numerous segments
1. Segments are nearly identical, possess paired appendages
2. Centipedes have one pair of legs per segment fig 43.27
3. Millipedes have two pairs of legs per segment fig 43.28
1. Millipede segment is a tagma
2. Formed from fusion of two segments, thus two pairs of legs

2. Share similar reproductive strategies
1. Fertilization is internal, direct transfer of sperm
2. Sexes separate, all species lay eggs
3. General appearance of young is similar to adult

3. Centipedes
1. Are carnivorous, eat mainly insects
2. Appendages of first trunk segment modified into poison fangs

4. Millipedes
1. Most are herbivorous
2. Can roll bodies into a flat coil
3. May secrete defensive fluids and cyanide gas

3. Class Insecta: The Insects
1. General Ecology of Insects
1. Largest group of organisms on earth
2. Especially numerous in the tropics
3. Enormous diversity fig 43.29

2. External Features
1. Primarily terrestrial organisms,aquatic forms had terrestrial ancestors
2. Typical body organization
1. Have three body segments: Head, thorax, abdomen
2. Have three pairs of legs, all attached to thorax
3. Have one pair of antennae
4. May have one or two pairs of wings

3. Basic structure of mouthparts similar with modifications by feeding habits
4. Most possess compound eyes, many have ocelli
5. Thorax consists of three fused segments (tagmata)
1. Each has a pair of legs
2. Legs may be absent in some larvae, flies for example fig 43.31

6. Structure of insect wings
1. If two pairs, attach to middle and posterior segments
2. If one pair, attach to middle segment
3. Arise as saclike outgrowths, are solid excluding veins
4. Are not homologous to other appendages
5. Two pairs are the basic construction for winged insects
6. One pair lost in the evolution of groups like flies
7. Most wings folded at rest, except for outstretched dragonfly wings
8. Forewings may be hard and tough, form covering for hindwings
9. Most wings composed of sheets of chitin with strengthening veins
10. May possess detachable scales, like butterflies and moths fig 43.32
11. Some groups like springtails never evolved wings
12. Others, like fleas and lice, are derived from winged ancestors

3. Internal Organization
1. Tubular, somewhat coiled digestive tract
1. Greater coiling associated with sucking mouthparts
2. Dilute digestive enzymes less effective on liquids than solids
3. Anterior and posterior digestive regions lined with cuticle
4. Digestion occurs within stomach or midgut

2. Excretion through Malpighian tubules
3. Trachea extend throughout body
1. May form air sacs with muscles that create a bellows system to move air deeper
2. Spiracles closed via actions of muscles, retards water loss
3. Spiracles are permanently closed in some aquatic, parasitic forms

4. Fat body is food-storage organ or is similar to vertebrate liver
1. More prominent in immature forms
2. May be completely depleted when metamorphosis is finished
3. Non-feeding adults rely on fat body for nutrition through short lives

4. Sense Receptors
1. Possess wide variety of sensors in addition to eyes
1. Sensory hairs located all over bodies fig 43.33
2. Abundant on antennae and legs
3. Sound detected by tympanum, associated with tracheal air sacs
4. Sensory hairs may also detect sound waves

2. Insect communication
1. Produce sounds which are mostly inaudible to humans
2. Produce chemicals called pheromones

4. Insect Life Histories
1. General Characteristics of Development
1. Most insects hatch from eggs outside of the mother's body
1. Zygote develops within egg into young insect
2. Escapes egg by chewing or bursting through it
3. May have specialized projections to aid in escape

2. Rarely eggs develop within mother's body
3. Young insects undergo regular ecdysis, stages called instars
1. Exoskeleton is soft immediately after molting
2. Young is in greater danger from predators

2. Simple Versus Complete Metamorphosis fig 43.34
1. Simple metamorphosis
1. Wings develop during juvenile stages
2. No "resting stage" before last molt
3. Immature stages generally called nymphs
1. Include primitive orders, springtails and silverfish, that never had wings
2. Larvae of mayflies and dragonflies are aquatic, have gills
3. Nymphs and adult grasshoppers look similar, live in same habitat

2. Complete metamorphosis fig 43.35
1. Wings appear only during resting stage just prior to final molt
2. Resting stage called a pupa or chrysalis
1. Pupa does not normally move, except mosquitos
2. Substantial amount of cellular reorganization occurs

3. Juveniles and adults live in distinct habitats
4. Development is indirect, larvae are wormlike
5. Larva do not have compound eyes
6. Larvae may or may not have legs
7. Generally have chewing mouthparts, even if adults have sucking mouthparts
8. Pupa generally are usually inactive and do not feed
9. Include moths, butterflies, beetles, bees, wasps, ants, flies, fleas

3. Molting hormone, ecdysone, controls ecdysis and molting
1. Released from gland in thorax on stimulus from brain hormone
2. Effects of molting hormone determined by juvenile hormone
3. Amount of juvenile hormone decreases as insect passes through successive stages
1. When high produces another larva
2. When lower produces pupa, then adult