This document provides an overview of arthropods, including their defining characteristics, success, and major groups. It discusses their versatile exoskeleton, jointed appendages, open circulatory system, and molting process. Key points covered include: - Arthropods are the most diverse and successful phylum, with over 1 million described species. - Their exoskeleton provides structural support and protection while allowing for growth. - They have segmented bodies, jointed appendages, and metabolize oxygen directly in tissues. - Molting and metamorphosis allow for growth and development with an exoskeleton. - Major groups include chelicerates, crustaceans, hexap

1. INVERTEBRATES (Zool-02506)
3(2-1)
Presented by: Nabeel Tahir
M.Phil. Zoology
Class: ADP 5 Semester
Lecture: 13

2. What is an Arthropod?
• Arthro – joint + podas
– foot
• Crayfish, lobsters,
spiders, mites,
scorpions, and insects.
• + 1 million species
described.
• Estimated to be 30 –
50 million
undescribed species.
• Most successful
Phylum in the Animal
Kingdom.
Arthropoda - Jointed Legs

3. Arthropods - Most Successful Animals
• Number of species
• Diversity
• Distribution
• Longevity

4. Reasons for Arthropod Success
• Versatile exoskeleton
• Segmentation
• Oxygen piped directly
to cells (terrestrial)
• Highly developed
sensory organs
(compound eye)
• Complex behavior
• Metamorphosis

5. • Metamerism modified by tagmatization
• Chitinous exoskeleton
• Paired, jointed appendages
• Ecdysis
• Ventral nervous system
• Coelom reduced to cavity around gonads
• Open circulatory system
• Complete digestive tract
• Metamorphosis often present
General Characteristics of Arthropods

6. Classification and Relationships to other
Animals
• Ecdysozoans
• Cuticle, ecdysis, loss of epidermal cilia
• Monophyletic with five subphyla
• Chelicerata, Crustacea, Hexapoda, Myriapoda, Trilobitomorpha (entirely
extinct)

7. Metamerism and Tagmatization
• Metamerism evident externally
• Segmental body wall
• Segmental appendages
• Metamerism reduced internally
• No septa
• Most organs are not metameric
• Tagmatization obvious
• Specializations for feeding, sensory perception, locomotion, and visceral
functions

8. The Exoskeleton
• Exoskeleton or cuticle
• External jointed skeleton
• Functions
• Structural support
• Protection
• Prevents water loss
• Levers for muscle attachment and movement
• Covers all body surfaces and invaginations
• Secreted by epidermis (hypodermis)

9. The Exoskeleton
• Epicuticle
• Lipoprotein
• Impermeable to water
• Barrier to microorganisms and pesticides
• Procuticle
• Chitin
• polysaccharide
• Outer procuticle hardened by sclerotization or deposition of calcium
carbonate
• Inner procuticle less hardened and flexible
• Articular membranes at joints
• Modifications include sensory receptors
• Sensilla

10. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
From A Life of Invertebrates, Copyright © 1979, W. D. Russell-Hunter.
Modifications of Exoskeleton
Invaginations for
gas exchange
Soft and flexible at joints
for movement

11. Growth with an Exoskeleton
• Growing arthropods must periodically shed the
exoskeleton by molting – ecdysis.
• Triggered by hormones (ecdysone) – which causes the
old procuticle to breakdown and separate from the
epidermis.
• A new epicuticle and procuticle are made underneath
by the epidermis.
• Old exoskeleton splits open as the animal stretches
with air or water intake and wriggles out of old
exoskeleton.

12. How do Arthropods grow?

13. The Hemocoel
•An internal cavity for the open circulatory
system of arthropods; results from no
internal segmentation.
•Internal organs are bathed by body fluids
(hemolymph) for exchanges of nutrients,
wastes and gases.
•Because of an exoskeleton, the coleom is
no longer used as a hydorstatic skeleton.

14. Open Circulatory System Closed Circulatory System
Review of Circulatory Systems
Ostia

15. Metamorphosis
•Changes in body form and physiology as
arthropods grow and develop.
•Immature stages (larvae) often are
radically different than adult forms.
•These differences reduce competition
between adult and young (different
body forms, behaviors, and habitats).
•Many types of metamorphosis
(discussed more with insects).

16. Metamorphosis
• Metamorphosis is a
• radical change in body form and
physiology as an immature stage,
usually called a larva, becomes an
adult.
• Intraspecific competition
(between members of one
species) is reduced because of
metamorphosis.
• Larval forms may be quite
different from adults.

17. Metamorphosis in a Monarch Butterfly

18. Phylum Arthropoda Groups
• Subphylum Trilobitomorpha (trilobites)
• Subphylum Chelicerata
– Class Merostomata (horseshoe crabs)
– Class Arachnida (spiders,scorpions,ticks)
– Class Pycnogonida (sea spiders)
• Subphylum Crustacea
– Class Malacostraca (lobsters, crabs, shrimp)
– Class Branchiopoda (brine shrimp, water fleas)
– Class Maxillopoda (barnacles, copepods)
• Subphylum Hexapoda (insects)
• Subphylum Myriapoda (millipedes and centipedes)

20. Subphylum Trilobitomorpha
• Trilobites, dominant life form that went
extinct 345 million years ago.
• Trilobites had a trilobed shape.
• Three tagmata:
• Head (cephalon) with a mouth, compound
eyes, antennae, and 4 pairs of leglike
appendages.
• Trunk with a variable number of segments
each with a pair of biramous appendages.
• One of the branches of biramous appendage
was fringed and may have been a gill.
• Pygidium – segments fused into a plate.
• All body parts could roll up (like a rollie-pollie).
• Probably benthic scavengers.

21. Subphylum Chelicerata
• Spiders, mites, ticks, horseshoe crabs
• Two tagmata
• Prosoma (Cephalothorax)
 Six appendiges
• Chelicerae
• Often chelate
• Usually feeding appendages
• Pedipalps
• Sensory, feeding, locomotion, reproduction
 Walking legs
• Have Eyes
• Lack mandibles and antennae.
• Opisthosoma (abdomen)
• Digestive, reproductive, excretory, and respiratory organs

23. Class Merostomata
• Class Merostomata includes
the eurypterids and
Xiphosura crabs.
• Eurypterids were giant
water scorpions up to 3 m in
length.
• Cambrian through Permian.
giant water scorpions
(extinct 280 mya).
• Predators, some with large
crushing claws.

24. Class Merostomata -- Subclass
Xiphosura
• Three genera of
horseshoe crabs live
today.
• Limulus polyphemus,
found in North America,
Atlantic Ocean and the
Gulf of Mexico
• Has existed on earth
almost unchanged for
over 200 million years

25. Class Merostomata
• Horseshoe crabs have an unsegmented carapace (hard dorsal shield),
a broad abdomen, and a long telson (tail piece).
• Cephalothorax
• Chelicerae
• Pedipalps
• 4 pairs walking legs
• Abdomen
• 6 pairs of thin appendages
• Book gills found on 5.

26. Class Merostomata
• Horseshoe crabs have simple and
compound eyes.
• Feed at night on worms and small molluscs.
• Come to shore in large numbers to mate at
high tide.
• Trilobite larvae resemble trilobites.

27. Class Arachnida
• Class Arachnida includes spiders, scorpions, mites,
and ticks.
• Some of the first terrestrial animals – exoskeleton
helps retain water; many other adaptations for
land.
50 µm

28. Form and Function
• Carnivores
• Chelicerae to hold prey or as fangs
• Gut
• Foregut
• Cuticular
• Pumping stomach
• Hindgut
• Cuticular
• Water reabsorption
• Midgut
• Noncuticular
• Secretion and absorption

29. Form and Function
• Excretion
• Coxal glands
• Paired sacs bathed in blood of body sinuses
• Homologous to nephridia
• Excretory pores at base of posterior appendages
• Malpighian tubules
• Blind ending diverticula of gut tract
• Empty via digestive tract
• Uric acid
Digestive tract
Midgut
(stomach)
Malpighian
tubules
Rectum
Intestine
Hind
Salt, water, and
nitrogenous
wastes
Feces and urine
Anus
Malpighian
tubule
Rectum
Reabsorption of H2O,
ions, and valuable
organic molecules
HEMOLYMPH

30. Form and Function
• Gas Exchange
• Book lungs
• Paired ventral invaginations of body
wall
• Gas exchange between air and
blood across book lung lamellae
• Tracheae
• Branched, chitin-lined tubes
• Open at spiracles along abdomen
Air
sac
Body
cell
Trachea
Tracheole
Tracheoles Mitochondria
Myofibrils
Body wall
2.5 µm
Air
(a) The respiratory system of an insect consists of branched internal
tubes that deliver air directly to body cells. Rings of chitin reinforce
the largest tubes, called tracheae, keeping them from collapsing.
Enlarged portions of tracheae form air sacs near organs that require
a large supply of oxygen. Air enters the tracheae through openings
called spiracles on the insect’s body surface and passes into smaller
tubes called tracheoles. The tracheoles are closed and contain fluid
(blue-gray). When the animal is active and is using more O2, most of
the fluid is withdrawn into the body. This increases the surface area
of air in contact with cells.

31. Form and Function
• Circulation
• Open with dorsal contractile vessel
• Pumps blood into tissue spaces of hemocoel
• Returns to dorsal vessel via ostia
• Nervous system
• Ventral with fusion of ganglia
Lateral view of body showing
relative position of circulatory
(yellow), digestive (green), and
nervous (blue) systems.

32. Form and Function
• Senses
• Mechanoreceptors
• Modifications of
exoskeleton
• Sensilla respond to
displacement.
• Chemical sense
• Pores in exoskeleton
• Vision
• Eyes detect movement and
changes in light intensity.
Figure 14.10 An arthropod seta (a) and an
eye (ocellus) (b).

33. Form and Function
• Reproduction
• Dioecious
• Indirect sperm transfer
• Male deposits spermatophores, which are transferred to the female.
• Courtship rituals common
• Copulation occurs in spiders via modified pedipalp of male.
• Development
• Direct

34. Order Scorpionida
• Prosoma
• Shieldlike carapace
• Opisthosoma
• Preabdomen
• Postabdomen (“tail” with sting)
• Courtship prior to mating
• 3 Types of Birth in Scorpions
• Oviparous – females lay eggs that develop outside of body.
• Ovoviviparous – young develop in large, yolky eggs held internally in body, then are
born, fully developed.
• Viviparous – mother provides nutrients to embryos; eggs develop in special
chambers close to female digestive tract. After birth, young crawl onto mother’s
back for about 1 month.

35. Figure 14.11 (a) Hardrurus arizonensis (b)
External anatomy.
(a)
(b)

36. Order Araneae
• +34,000 species spiders; largest group of arachnids
• Prosoma
• Chelicerae with poison glands and fangs
• Pedipalps leglike
• Sperm transfer in males
• 6-8 eyes
• Opisthosoma
• Connected to prosoma via pedicel
• Swollen or elongate
• Visceral functions and spinnerets
• Spinnerettes at the posterior end that make silk using silk glands.

37. Figure 14.12 External structure of Argiope.

38. Figure 14.13 Prosoma of a spiderling.

39. Order Araneae
• Silk
• Protein
• Repeating sequence of glycine and alanine
• Beta sheet
• Stored as gel prior to spinning
• Chemical modification when forced through spinnerets
• Webs, line retreats, safety lines, wrapping eggs, dispersal of young
(ballooning)

40. Figure 14.14
Members of the
family Araneidae are
the orb weavers.

41. Order Araneae
• Feeding
• Insects and other arthropods
• Hunt or capture in webs
• Paralyze prey
• May wrap in silk
• Inject enzymes into prey body wall
• Two spiders are venomous to humans.

42. Figure 14.15 (a) Black widow spiders (Lactrodectus mactans) has a neurotoxic
venom. (b) Brown recluse spiders (Loxosceles reclusa) have a histolytic venom.
(a)
(b)

43. Order Araneae
• Reproduction
• Complex behaviors
• Chemical, tactile, and visual signals
• Male’s pedipalps enlarged into embolus
• Male deposits sperm on web and collects with pedipalps.
• Transfers sperm to female during mating
• Female deposits eggs in silk case.
• In webbing, a retreat, or carries with her

44. Order Opiliones
• Harvestmen or daddy longlegs
• Prosoma broadly joins
opisthosoma
• Legs long and slender
• Omnivores
• External and internal digestion
Figure 14.16 Order Opiliones (Leiobunum sp).

45. Order Acarina
• Mites
• Prosoma and opisthosoma fused
and covered by single carapace
• 1mm or less
• Free-living
• Herbivores or scavengers
• Many pest species
• Ectoparasites
• Chigger (Trombicula)
• Follicle mite (Demodex)
Figure 14.17 Dermatophagoides farinae is
common in homes and grain storage areas.

46. Order Acarina
• Ticks
• Ectoparasites in all life stages
• Up to 3cm
• Females lay eggs after engorging with blood.
• Important in disease transmission
• Rocky Mountain spotted fever
• Lyme disease

47. Figure 11.18 Ixodes scapularis transmits the bacteria that causes Lyme
disease.

48. Class Pycnogonida (Subphylum
Cheliceriformes?)
• Sea spiders
• Marine
• Feed on cnidarian polyps
• Feed by sucking prey tissues
through a proboscis.
• Tear at prey with their
chelicerae.
• Dioecious
• Molecular, developmental,
and morphological
characters are being used to
reevaluate taxonomic status.
Figure 14.19 Class Pycnogonida

49. Subphylum Crustacea
• Crayfish, shrimp, lobsters, crabs,
copepods cladocerans and others
• Almost all are aquatic
• Terrestrial isopods and crabs are exceptions.
• Two pairs of antennae
• Biramous appendages
• Protopodite: Basal segment, with two rami
(distal processes that give the appendage a Y
shape) attached.
• Endopodite: The medial ramus
• Exopodite: The lateral ramus

50. Figure 14.20 Crustacean body form. (a) External anatomy. (b) Biramous
appendages.
Crustacean Body Form. (a) External anatomy of a generalized crustacean. Gills are formed as
outgrowths of the body wall and protected under extensions of the exoskeleton called the carapace.
(b) Pair of appendages, showing the generalized biramous structure. A protopodite attaches to the
body wall. An exopodite (a lateral ramus) and an endopodite (a medial ramus) attach at the end of
the protopodite. In modern crustaceans, both the distribution of appendages along the length of the
body and the structure of appendages are modified for specialized functions.

51. Class Malacostraca
• Crabs, lobsters, crayfish, shrimp, krill, amphipods, isopods
• Order Decapoda
• Largest order
• Shrimp, crayfish, lobsters, crabs
• Order Euphausiacea (yah-fah-see-a’see-ay)
• The krill -> zooplankton

52. Order Decapoda
• Crayfish external structure
• Cephalothorax
• Fusion of head and thorax
• Covered dorsally and laterally by
carapace
• Sensory, feeding, locomotion
• Abdomen
• Muscular “tail” in crayfish
• Locomotor and visceral functions in
others
• Paired appendages
• Serially homologous (derived from a
common ancestral pattern)

53. Crayfish external structure
• Cephalothoracic appendages (13)
• The 1st and 2nd pairs: first and second antennae.
• The 3rd pair: Mandibles -> chewing or grinding structures
• The 4th and 5th pairs: Maxillae -> food handling.
• The second maxilla (5th pair) bears a gill and a thin, bladelike structure, called a scaphognathite (gill bailer), for
circulating water over the gills.
• The 6th through the 8th pairs: Maxillipeds -> derived from the thoracic tagma ->
• They are accessory sensory and food-handling appendages. The last two pairs of maxillipeds also bear gills.
• The 9th to 13th : Pereopods (walking legs).
• The first pereopod (9th pair): cheliped -> enlarged and chelate (pincherlike) and used in defense and capturing
food.
• Abdomen appendages (6)
• The 1st to 5th pairs: Pleopods (swimmerets) -> used for swimming.
• In females, developing eggs attach to pleopods, and the embryos are brooded until after hatching.
• In males, the first two pairs of pleopods are modified into gonopods (claspers) used for sperm transfer during
copulation.
• The 6th pair: Telson -> a median extension
• The telson bears the anus and is flanked on either side by flattened, biramous appendages of the last segment,
called uropods. The telson -> flipperlike structure used in swimming and in escape responses.

54. Figure 14.22 External structure of a male crayfish.

55. Figure 14.23 Serial homology of crayfish appendages.

56. Class Malacostraca (Order Decapoda)
• Crayfish internal structure
• Digestive system
• Complete with foregut, midgut, and hindgut
• Respiratory system
• Gills attach at base of cephalothoracic appendages.
• Lie within gill chamber between carapace and lateral body wall
• Second maxilla circulates water.
• Circulation
• Open
• Dorsal heart and major arteries
• Blood enters hemocoel, and gills before returning to pericardial sinus around heart.

57. Figure 14.24
Internal structure
of a crayfish.

58. Class Malacostraca (Order Decapoda)
• Ventral nervous system
• Cephalization and centralization
• Supraesophageal and subesophageal ganglia process
sensory information and control head appendages.
• Segmental ganglia
• Sensory structures
• Antennae
• Compound eyes
• Chemoreceptors
• Proprioceptors
• Tactile setae
• Statocysts: a pitlike invagination of the exoskeleton that
contains setae and a group of cemented sand grains
called a statolith.
• Crayfish movements move the statolith and displace setae.
Statocysts provide information regarding movement,
orientation with respect to the pull of gravity, and vibrations of
the substrate. Because the statocyst is cuticular, it is replaced
with each molt. Sand is incorporated into the statocyst when
the crustacean is buried in sand

59. Class Malacostraca (Order Decapoda)
• Endocrine system
• Ecdysis, sex determination, color change
• X-organs
• Neurosecretory tissues in eyestalks
• Molt-inhibiting hormone
• Target Y-organ
• Y-organs
• Base of maxillae
• Releases ecdysone when molt inhibiting hormone is not present and ecdysis occurs
• Androgenic glands (males)
• Promotes development of testes and male characteristics

60. Class Malacostraca (Order Decapoda)
• Excretion
• Antennal (green) glands in crayfish -> at the bases of the second antennae
• Maxillary glands in others -> at the bases of the second maxillae
• Homologous to coxal glands of arachnids
• Excretory product: Diluted urine -> Ammonia -> excrete through antennal
gland and diffuses across thin parts of the exoskeleton.
• Reproduction
• Dioecious
• Mating after female molts
• Fertilized eggs attach to female’s pleopods
• Others have planktonic larvae

61. Figure 14.25
(a) Nauplius larva of a
barnacle: Many
crustaceans have a
planktonic, free-swimming
larva
(b) Zoea larvae of a crab: The
nauplius develops into a
miniature adult

62. Order Euphausiacea (yah-fah-see-a’see-ay)
• The krill -> important members of the zooplankton in all oceans of
the world.
• Swarming & Bioluminescent: acquired from the bioluminescent
dinoflagellates that they eat.
• Feed on phytoplankton at the base of the food web
• Krill serve as food for many other organisms.
• Antarctic krill are the food source for 6 species of baleen whales,
more than 100 species of fish, 35 species of birds, 7 species of
seals, and 20 species of squid.
• It is estimated that the biomass of Antarctic krill exceeds 500 million
metric tons, and more than one-half of this biomass is eaten annually.
• Worldwide, commercial fishing also harvests approximately
100,000 to 200,000 metric tons of krill that are used for
aquaculture (e.g., salmon farming), aquarium food, and human
consumption.
• In Japan, krill are eaten as okiami, and krill are processed for sale in
the heath-food industry worldwide.
• The 1990s saw a drastic decline in krill populations around
Antarctica and Japan.
• The Convention on the Conservation of Antarctic Marine Living
Resources (CCAMLR) is a consortium of 24 member countries that
has set catch quotas for krill to ensure a long-term sustainable krill
fishery.

63. Order Isopoda
• “Pillbugs”
• Aquatic and terrestrial
• Dorsoventrally flattened
• Scavenge decaying plant and
animal material.
• Some cling and feeding on other
animals.
• Terrestrial isopods live under
rocks and logs and in leaf litter.
Figure 14.26a Order Isopoda.

64. Order Amphipoda
• Laterally compressed -> gives
them a shrimplike appearance
• Move by crawling or swimming on
their sides along the substrate
• Some species are modified for
burrowing, climbing, or jumping
• Scavengers and parasites.
Figure 14.26b Order Amphipoda.

65. Class Branchiopoda
• Fairy shrimp
• Temporary ponds
• Brine shrimp
• Great Salt Lake
• Cladocera
• Freshwater water fleas
• Large carapace
• Parthenogenesis common
• Flattened, leaflike appendages
Figure 14.27 Order Cladocera.

66. Class Maxillopoda
• Subclass Copepoda
• Most abundant crustaceans
• Important in marine and
freshwater food webs
• Cylindrical body and one
compound eye.
• First antennae are modified for
swimming and no other
appendages.
• Most are planktonic and filter
feeding.
Figure 14.1 Subclass Copepoda.

67. Class Maxillopoda
• Subclass Thecostracea, Infraclass Cirripedia
• Barnacles
• Marine
• Monoecious
• Nauplius larva -> cypris larvae: planktonic larval stage, which has a bivalved carapace.
• Cypris larva settles and metamorphoses into sessile adult.
• Gut tract becomes U-shaped and thoracic appendages used for filter feeding.
• Attach to rocks, ships, whales
• Some are parasitic

68. Figure 14.28 Class Maxillopoda, Infraclass
Cirripedia.
(a) Internal structure of an acorn barnacle.
(b) A stalked barnacle (Lepas).

69. Further Phylogenetic Considerations
• Diverse body forms and lifestyles of Arthropoda arose from single
ancestor.
• Crustaceans very successful in aquatic habitats
• Chelicerata
• First terrestrial arthropods
• Account for evolution of many water conserving features of the phylum
• Exoskeletal, excretory, and respiratory adaptations