Phylum Arthropoda: General characteristics and Classification

Gereral Characteristics of Phylum Arthropoda

Arthros; Jointed, podos; Foot)
Kingdom: Animalia
Habitat: mostly terrestrial, also aquatic
Insects are the most successful life form on the planet: they make up more than half of all living things on Earth
Body has Three-part: head, thorax, abdomen.
Body is metamerically segmented
Three pairs of jointed legs (6 legs).
Compound eyes which contain several thousand lenses leading to a larger field of vision.
They possess two antenna.
Symmetry: bilateral
Germ layer: triploblastic
Grade of organization: organ system grade
Coelom: hoemocoel
Chitinous (hard) exoskeleton, no bones or a skeleton
Respiratory system: by general body surface, by gills, tracheae or book lungs
Circulatory system: open type with dorsal heart.
Excretion: malpighian tubules or green gland
Nervous system: dorsal brain with ventral nerve cord
Sexes are separate. Sexually dimorphism is present
Fertilization: internal.
Development: direct or indirect with larval stages.

Arthropoda is classified into five classes on the basis of body divisions, body appendages, habitat, organs of respiration and modes of excretion.

(Crusta: shell)

Habitat: They are mostly aquatic, few are terrestrial and very few are parasitic.
Cephalothorex: Head is often fused with thoracic segments to form cephalothorax.
Thorax and abdomen have a pair of biramous appendages in each segment.
Respiration: through the gills or general body surface.
Excretory organs are modified coelomoducts which may either maxillary glands or antennary glands.
Examples: Cancer (crab), Palaemon (Prawn), Daphnia (water flea) etc.

(Myrios: ten thousand; podos: foot)

Habitat: Mostly terrestrial.
Many appendages: Body is long with numerous segments each having one or two pairs of legs.
Head is distinct with antennae, a pair of eyes and two to three pairs of jaws.
Excretion: by malpighian tubules.
Respiration: by trachea.
Examples: Julus (Millipede), Scolopendra (Centipede), Spirobolus etc

(Insectus: divided)

Habitat: Mostly terrestrial and rarely aquatic
Body divided into three regions: head, thorax and abdomen.
Thorax has three segments, each bearing a pair of leg and a pair of wings found on second and third segments.
Abdomen has 7-11 segments without appendages.
Respiration: by tracheae, gills etc.
Excretion: usually by malpighian tubules.
Examples; Pieris (Butterfly), Periplaneta (Cockroach), Tabernus (Housefly), Mosquiotes, Ants, etc.

(Arachne: spider)

Habitat: mostly terrestrial and rarely aquatic
Body is usually divided into cephalothorax and abdomen.
There are four pairs of legs attached to the cephalothorax.
Respiration: by tracheae or book lungs or gills.
Excretion: by malpighian tubules or coxal gland or both.
Examples: Aramea (Spider), Palamnaeus (Scorpion), Limulus (King Crab), etc

Class 5 Onychophora

(Onychos: claw; phoros: bearing)