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Introduction to Arthropoda

The arthropods are the most successful phylum of animals, both in diversity of

distribution and in numbers of species and individuals.

They have adapted successfully to life in water, on land and in the air.

About 80% of all known animal species belong to the Arthropoda - about 800,000

species have been described, and recent estimates put the total number of species

in the phylum at about 6 million.

Evolution :

• Probably evolved from a Peripatus - like ancestor, which in turn evolved from a

segmented worm

Metamerism

• Metamerism- body is segmented. Exoskeleton and metamerism causes molting

Exoskeleto

• Exoskeleton- body covered with a hard external skeleton

• Why an exoskeleton?

• Why not bones? Exoskeleton good for small things, protects body from damage

(rainfall, falling, etc.).

• Bones better for large things

Bilateral Symmetry :

• Bilateral Symmetry- body can be divided into two identical halves

Jointed Appendages :

• Jointed Appendages- each segment may have one pair of appendages, such as:

• legs , wings , mouthparts

Open Circulatory System :

• Open Circulatory System- blood washes over organs and is not entirely closed by

blood vessels. Our system is a closed one

: Ventral Nerve Cord

• one nerve cord, similar to our spinal column

•

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Open Circulatory System Closed Circulatory System

Classification :

1-Subphylum Trilobitomorpha

2-Subphylum Cheliceriformes

Class Chelicerata

Subclass Merostomata (horseshoe crabs)

3- Subclass Arachnida (spiders, scorpions, mites, ticks)

Class Pycnogonida (sea spiders)

4-Subphylum Myriapoda

Class Chilapoda (centipedes)

5-Subphylum Hexapoda

Class Insecta

6-Subphylum Crustacea :

• a heterogeneous taxon with many subtaxa

• two pairs antennae

• marine, freshwater, and terrestrial

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• Primarily aquatic

• Free-floating larval stage

• 26,000+ known species

Class Branchipoda (brine shrimp)

Class Maxillopoda / Subclass Copepoda

Class Malacostraca

Order Decapoda (crabs, lobsters, shrimp)

Class Malacostraca

Order Decapoda

Order Isopoda (isopods)

Phylum Arthropoda1-Subphylum Trilobitomorpha

2-Subphylum Cheliceriformes

A- Class Chelicerata

Subclass Merostomata (horseshoe crabs)

B- Subclass Arachnida (spiders, scorpions,

mites, ticks)

Class Pycnogonida (sea spiders)

3-Subphylum Myriapoda

Class Chilapoda (centipedes)

4-Subphylum Hexapoda

Class Insecta

5-Subphylum Crustacea

A- Class Branchipoda (brine shrimp)

B- Class Maxillopoda

Subclass Copepoda (copepod)

C-Class Maxillopoda

Subclass Copepoda (copepod)

D-Class Malacostraca

Order Decapoda

(crabs, lobsters, shrimp)

E- Class Malacostraca

Order Decapoda

Order Isopoda (isopods)

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Arthropods support and movement

Cuticle :

Secreted by epidermi

Waxes, lipoproteins, proteins.

Sclerotized - tanning process that hardens

Each segment bound by four plates - dorsal tergite, ventral sternite, and two lateral

pleurites.

Muscle bands attach to apodemes.

Appendages segmented with extrinsic or intrinsic muscles.

How do Arthropods support themselves and move?

Cuticle

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Side view of body showing relative position of circulatory (yellow), digestive (green), and

nervous (blue) systems.

NERVOUS SYSTEM

NERVOUS SYSTEM

General

• nervous system is adapted for the needs of a segmented animal as is that of annelids

• annelid and arthropod nervous systems are similar

• the similarities may be convergent

2. Arthropod Ground Plan

• ladder like nervous system

• dorsal brain in the head

• a pair of circumenteric connectives that encircle the gut

• a paired, ventral, longitudinal nerve cord

• paired segmental ganglia

• transverse commissures between ganglia

• longitudinal connectives

• segmental sensory and motor nerves

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SENSE ORGANS

1. General

• receptors for light, vibrations, chemicals, equilibrium, gravity

• cuticle modified to form a vast array of receptors (sensilla)

2. Sensilla

• specialized for various environmental events

• consist of:

<>modified cuticle, usually a seta

<> a modified cilium

<> sensory neurons

<> support cells

3. Mechanoreceptors

a. Trichobothria

• extremely sensitive mechanoreceptors detect weak, low velocity air currents

• a long, slender, solid seta

• especially common and important in arachnids

• a blinded spider can capture flies using its trichobothria

4. Chemoreceptors

• detect chemicals

• modified hollow setae with substrate-specific chemosensory neurons

• chemoreceptive sensilla resemble mechanoreceptive setae

5. Equilibrium and Gravity

a. Statocysts

• several malacostracan crustaceans have statocysts

• analogous to the vestibule of the vertebrate inner ear

• detects gravitational field and acceleration

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6. Tympanal Organs

• some arthropods can hear, i.e. detect vibrations in air

• tympanal organs are present in several insect groups

• cicadas, crickets, grasshoppers, moths

• a thin sheet of cuticle (eardrum) over an opening

7. Hygroreceptors

• humidity receptors

• very important to terrestrial arthropods

8. Photoreceptors

a. General

• widespread, often well-developed and sophisticated

• two types:

<>"median" pigment-cup ocelli

<>"lateral" compound eyes

<>both often present in the same individual

REPRODUCTION

1. General

• most arthropods are gonochoric

• fertilization may be external or internal in aquatic arthropods

• always internal in terrestrial species

2. Sperm Transfer

• sperm transfer usually by mating or copulation

• females usually store the sperm in a seminal receptacle

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3. Spermatophores

a. General

• many arthropods transfer sperm in packets

• an adaptation for sperm transfer on land

• male abandons spermatophore for female to find or may place it in the female

gonopore

• most insects use copulation

4. Copulation

• present in many aquatic arthropods (crustaceans), also in derived terrestrial forms

• often with courtship and behavior patterns

• anatomical equipment for internal fertilization by copulation

• male inserts an intromittent organ into the reproductive tract of the female

• deposits sperm or spermatophores into her seminal receptacle

Arthropod Development

Zoea MegalopsNauplius

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Arthropods maintain homeostasis

Circulation and respiration :

Open hemocoel as a result of hard exoskeleton and lack of internal segmentation.

Muscular heart required since body movements can’t move blood.

Hemolymph has amebocytes, pigments, and some have clotting factors.

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Osmoregulation and waste excretion :

Closed structure required since open nephrostome wouldn’t with such a large

hemocoel.

Material uptake depends on filtration pressure and active uptake.

Reuptake of salts and nutrients

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How do Arthropods maintain homeostasis?

Antennal and maxillary glands in crustaceans.

Produce ammonia with some urea and uric acid.

How do Arthropods maintain homeostasis?

Malpighian tubules in arachnids and insects. Blind tubes extend into

hemocoel and empty into gut. Produce uric acid.

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Reproduce and develop

Usually some brooding.

Development often mixed with early brooding then larval stages.

Eggs centrolecithal but amount of yolk varies (so does cleavage).

Symbiosis

scarlet

cleaner

shrimp