9Arthropoda

πŸ“š
PHYLUM ARTHROPODA
β–’ Introduction:
Image 1
❖ Meaning: Arthros = jointed; podos = legs
❖ Definition: Animals with jointed appendages
❖ Rank: Largest phylum of Animalia in number of taxa and biomass
❖ Key Success Factor: Chitinous exoskeleton + jointed appendages
❖ Important Point: First animals with voluntary muscles utilizing jointed appendages
β–’ General Characters:
❖ Body Grade: Organ-system level
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❖ Symmetry: Bilateral symmetry
❖ Germ Layers: Triploblastic
❖ Segmentation: Metameric segmentation with tagmatization
❖ Segment Septa: Septa absent; only external segmentation present
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❖ Body Regions: Head, thorax and abdomen; sometimes head + thorax fuse to form cephalothorax
❖ Coelom: Reduced coelom; body cavity mainly haemocoel
❖ Coelom Type: Schizocoelous
❖ Blood: Haemolymph
❖ True Coelom: Restricted to gonads
β–’ Exoskeleton and Moulting:
Image 1
❖ Exoskeleton: Chitinous exoskeleton
❖ Cuticle: Secreted by epidermis
❖ Function: Protection, support, prevention of water loss and muscle attachment
❖ Ecdysis / Moulting: Periodic shedding of cuticle during growth
Image 1
β–’ Digestive System:
❖ Alimentary Canal: Complete
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❖ Parts: Foregut, midgut and hindgut
❖ Mouth Parts:

Table 1: Types of Mouth Parts

Type
Function / Examples
Biting and chewing
Cutting and chewing food; e.g. grasshopper, termites, beetles
Chewing and lapping
Chewing + lapping nectar; e.g. honeybee, wasp
Piercing and sucking
Piercing tissues and sucking fluid; e.g. mosquito, aphid, bug
Sponging
Absorbing liquid food; e.g. housefly
Siphoning
Sucking nectar through proboscis; e.g. butterfly, moth
β–’ Circulatory System:
❖ Type: Open circulatory system
❖ Heart: Dorsal contractile heart
❖ Blood Vessels: Open into sinuses or lacunae
❖ Capillaries: Absent
❖ Blood: Haemolymph
β–’ Respiration:

Table 1: Respiratory Organs in Arthropods

Organ
Found In
Body surface
Some small arthropods
Gills
Aquatic crustaceans
Tracheae
Insects and myriapods
Book lungs
Arachnids
Book gills
Limulus
β–’ Excretion:

Table 1: Excretory Organs

Group
Excretory Organ
Crustacea
Green glands / antennary glands
Terrestrial arthropods
Malpighian tubules
❖ Major Excretory Product: Uric acid
β–’ Nervous System and Sense Organs:
❖ Nervous System: Dorsal nerve ring and double ventral nerve cord
❖ Sense Organs:
  • β€’
    Antennae
  • β€’
    Simple eyes
  • β€’
    Compound eyes
  • β€’
    Statocysts
  • β€’
    Chemoreceptors
  • β€’
    Taste receptors
β–’ Reproduction and Development:
❖ Sex: Usually dioecious
❖ Fertilization: Internal
❖ Development: Direct or indirect
❖ Larval Stage: May be present
❖ Parthenogenesis: Seen in some forms
❖ Special Point: Few forms are ovoviviparous
β–’ Classification:

Table 1: Major Subphyla and Classes

Group
Important Characters / Examples
Onychophora
Velvet worms / walking worms; Peripatus; connecting link between Annelida and Arthropoda
Trilobitomorpha
Fossil arthropods
Chelicerata
First pair chelicerae, second pair pedipalps; body cephalothorax + opisthosoma
Merostomata
Limulus / horseshoe crab / king crab; living fossil
Arachnida
Cephalothorax + abdomen; 4 pairs of legs; antennae and wings absent; book lungs
Mandibulata
Mandibles present; body commonly head, thorax and abdomen
Crustacea
Mostly aquatic; cephalothorax + abdomen; gills; green glands; hemocyanin
Myriapoda
Many-legged arthropods; includes Chilopoda and Diplopoda
Insecta
Head, thorax, abdomen; 3 pairs of legs; one pair antennae; mostly winged
β–’ Onychophora:
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❖ Common Name: Velvet worms / walking worms
❖ Example: Peripatus
❖ Importance: Connecting link between Annelida and Arthropoda
❖ Status: Most primitive arthropod-like group
β–’ Chelicerata:
❖ Body Division: Cephalothorax and opisthosoma
❖ Appendages: First pair chelicerae; second pair pedipalps
❖ Important Classes:
  • β€’
    Merostomata
  • β€’
    Arachnida
β–’ Merostomata:
❖ Example: Limulus
❖ Common Names: Horseshoe crab / king crab
❖ Importance: Living fossil
❖ Respiration: Book gills
β–’ Arachnida:
❖ Body Division: Cephalothorax and abdomen
❖ Legs: Four pairs
❖ Antennae: Absent
❖ Wings: Absent
❖ Respiration: Book lungs
❖ Eyes: Simple eyes
❖ Special Organs: Spinnerets in spiders
❖ Examples:
  • β€’
    Spider
  • β€’
    Scorpion
  • β€’
    Mite
  • β€’
    Tick
β–’ Crustacea:
❖ Habitat: Mostly aquatic; dominant arthropods of sea
❖ Body Division: Cephalothorax and abdomen
❖ Carapace: Hard shield on dorsal surface
❖ Respiration: Gills
❖ Respiratory Pigment: Hemocyanin dissolved in plasma
❖ Excretion: Green glands / antennary glands
❖ Examples:
  • β€’
    Daphnia
  • β€’
    Cancer
  • β€’
    Palaemon
  • β€’
    Astacus
  • β€’
    Shrimp
β–’ Myriapoda:

Table 1: Chilopoda vs Diplopoda

Feature
Chilopoda
Diplopoda
Common Name
Centipedes
Millipedes
Body Division
Head and trunk
Head, thorax and abdomen
Legs per Segment
One pair per trunk segment
Two pairs per abdominal segment
Poison Claws
First pair of legs modified into poison claws
Absent
Antennae
One pair
One pair
Respiration
Tracheae
Tracheae
Example
Scolopendra
Julus
β–’ Insecta:
❖ Body Division: Head, thorax and abdomen
❖ Segments: Head 6, thorax 3, abdomen 11
❖ Legs: Three pairs; all attached to thorax
❖ Antennae: One pair
❖ Wings: Usually two pairs
❖ Respiration: Tracheae
❖ Excretion: Malpighian tubules
❖ Excretory Product: Uric acid
❖ Largest Class: Largest class of animals living on land and in air
❖ Development Types:

Table 1: Development in Insects

Type
Meaning / Example
Ametabolous
Direct development without metamorphosis; e.g. Lepisma
Paurometabolous
Gradual metamorphosis; e.g. grasshopper
Hemimetabolous
Incomplete metamorphosis; e.g. dragonfly, mayfly
Holometabolous
Complete metamorphosis; e.g. housefly, mosquito
β–’ High-Yield Competitive Points:

Table 1: Must Remember

Question Clue
Answer
Largest animal phylum
Arthropoda
Meaning of Arthropoda
Jointed legs
Exoskeleton
Chitinous
Moulting
Ecdysis
Body cavity
Haemocoel
Circulation
Open type
Heart
Dorsal contractile heart
Main terrestrial excretory organ
Malpighian tubules
Crustacean excretory organ
Green glands
Respiration in insects
Tracheae
Respiration in arachnids
Book lungs
Respiration in Limulus
Book gills
Connecting link
Peripatus
Living fossil arthropod
Limulus
Respiratory pigment of crustaceans
Hemocyanin
Largest class of animals
Insecta
Q1.
The name Arthropoda means
Q2.
Arthropoda is the largest phylum of Animalia mainly in terms of
Q3.
The exoskeleton of arthropods is mainly made up of
Q4.
Periodic shedding of cuticle in arthropods is called
Q5.
The body cavity of arthropods is mainly
Q6.
True coelom in arthropods is mainly restricted to
Q7.
Circulatory system in arthropods is generally
Q8.
The heart of arthropods is generally
Q9.
The complete alimentary canal of arthropods is divided into
Q10.
Malpighian tubules are mainly excretory organs of
Q11.
Green glands are excretory organs of
Q12.
Respiration in insects generally occurs through
Q13.
Book lungs are commonly found in
Q14.
Book gills are found in
Q15.
Peripatus is important because it is a connecting link between
Q16.
The most primitive arthropod-like group is
Q17.
Limulus is commonly known as
Q18.
Which arthropod is called a living fossil?
Q19.
In Chelicerata, the first pair of appendages is
Q20.
Arachnids possess how many pairs of walking legs?
Q21.
Spinnerets in spiders are associated with
Q22.
Crustaceans usually respire by
Q23.
The respiratory pigment commonly present in crustacean plasma is
Q24.
Carapace in crustaceans is
Q25.
Chilopoda includes
Q26.
Diplopoda includes
Q27.
In Chilopoda, the first pair of legs is modified into
Q28.
Diplopods usually have
Q29.
Insects are also called hexapods because they have
Q30.
The three tagmata of insects are
Q31.
In insects, legs are attached to
Q32.
Ametabolous development means
Q33.
Holometabolous development means