ARTHROPODS

Arthropoda

General

Etymology - from the greek name from its distinctive jointed appendages, Arthron "joint" and Pous "Foot" or "Leg"
Over 1,000,000,134,000 species identified, found world-wide
Aquatic species includes Lobsters, Crabs, Hermit Crabs, Crayfish, Shrimps and Barnacles
Non aquatic species include Insects, Spiders, Scorpions
Thought to have originated from Annelids

Defining Characteristics

Protostomes, develops mouth before anus
Most possess a straight gut with an anus, body cavity has a true coelom
Bilaterally symmetry (mostly)
Two main body types in the aquatic realm: shrimp like and crab like
Body is divided into 2 or 3 sections
Has distinctive jointed appendages, may be modified in a number of ways to form antennae, mouthparts and reproductive organ
Do have a nervous system including a brain and ganglia
Do possess a respiratory system in the form of trachea and spiracles (mostly)
Do possess a open or lacunar circulatory system with a simple heart, on or more arteries, and no veins (mostly)

Body

Body possess an external skeleton (mostly), Stiff cuticle, usually a hard outer. Body covering made of chitin and proteins, form an exoskeleton which may or may not be stiffened further with calcium carbonate, like jointed armour.
Segmented bodies with various patterns of segmented fusion (tagmosis) to form integrated units (heads and abdomen)
Body possesses 3 – 400+ pairs of jointed legs

Reproduction / Life Cycle

Sexual Reproduction: Most common, with separate sexes and internal fertilization, although aquatic species can use external fertilization.
Asexual Reproduction: Some can reproduce via parthenogenesis, where unfertilized eggs develop (e.g., honey bee drones).
Sperm Transfer: Can involve direct transfer or indirect methods like spermatophores (sperm packets).

Life Cycles & Development Types

Complete Metamorphosis (Holometaboly): Four distinct stages with radical changes.

Stages: Egg → Larva (e.g., caterpillar, maggot) → Pupa (inactive transformation stage) → Adult.

Examples: Butterflies, beetles, flies.

Incomplete Metamorphosis (Hemimetaboly): Three stages with gradual changes.

Stages: Egg → Nymph (looks like small adult, lacks wings/sex organs) → Adult.

Examples: Grasshoppers, true bugs, cockroaches.

Direct Development: No larval stage; young resemble miniature adults.

Examples: Spiders, some crustaceans.

Key Processes

Molting: Shedding the exoskeleton to grow, a crucial part of development.
Metamorphosis: The transformation from larva to adult, allowing different stages to exploit different resources.