Arthropods are the most successful group of animals on Earth, making up roughly 80-85% of all known animal species. They include insects, arachnids, crustaceans, and myriapods (centipedes and millipedes). For UK invertebrate keepers, understanding the broader arthropod context helps clarify where isopods, springtails, cockroaches, and millipedes fit within this enormously diverse phylum.
What Defines an Arthropod?
Arthropods are invertebrates within the phylum Arthropoda, sharing several key features:
- Hard exoskeleton - made of chitin, providing protection and structural support. In some species reinforced with calcium and other minerals
- Segmented body - with each segment potentially specialised for different functions (movement, feeding, reproduction, sensing)
- Jointed appendages - the name "arthropoda" literally means "jointed feet"
- Bilateral symmetry - left and right sides mirror each other
- Open circulatory system - blood doesn't flow through closed vessels
- Periodic moulting - they shed their exoskeleton to grow
These features combined enable arthropods to occupy nearly every habitat on Earth, from deep ocean to high mountain.
The Four Major Subphyla
Phylum Arthropoda is divided into four major subphyla:
Chelicerata
Spiders, scorpions, mites, ticks, horseshoe crabs. Distinguished by chelicerae (specialised feeding appendages) instead of the antennae that other arthropods have. Mostly terrestrial, with a few aquatic exceptions. PostPods doesn't primarily stock Chelicerata.
Crustacea
The subphylum that includes PostPods's main product range. Crustacea contains both:
- Marine crustaceans - crabs, lobsters, shrimp, krill (gill-breathing aquatic species)
- Terrestrial crustaceans - isopods (woodlice/sowbugs/pill bugs), the only crustaceans to have fully adapted to land
Worth being clear about this: Crustacea isn't just marine. Terrestrial isopods (the suborder Oniscidea) are crustaceans that evolved from marine ancestors during the Carboniferous period (about 300 million years ago) and breathe through pleopodal lungs rather than gills. Browse our isopods collection for the full range. For more on the marine-to-land transition see our are isopods insects article.
Hexapoda
The largest subphylum, containing insects (class Insecta) plus several smaller groups including:
- Collembola (Springtails) - six-legged arthropods, NOT true insects but historically grouped with them. PostPods stocks springtails as bioactive cleanup crew. Browse our springtails collection
- Insecta (true insects) - beetles, flies, bees, butterflies, moths, ants, dragonflies. The largest animal group with millions of species
- Blattodea (cockroaches) - part of Insecta. PostPods stocks species suitable for the invertebrate hobby. Browse our cockroaches collection
Myriapoda
Multi-legged terrestrial arthropods, traditionally split into:
- Diplopoda (millipedes) - slow-moving herbivorous detritivores with two pairs of legs per segment. PostPods stocks both UK-friendly and tropical millipedes. Browse our millipedes collection
- Chilopoda (centipedes) - faster predatory species with one pair of legs per segment and venomous fangs
- Pauropoda and Symphyla - smaller, less well-known groups of soil-dwelling myriapods
Evolutionary History
Arthropods are ancient. Fossils show:
- 500+ million years ago - first arthropods appear in the Cambrian period
- Trilobites - early marine arthropods, the iconic Cambrian fossil group, extinct since the Permian
- ~400 million years ago - first terrestrial arthropods (primitive myriapods and arachnids)
- ~300 million years ago - terrestrial isopods evolve from marine ancestors during the Carboniferous
- ~400 million years ago - early winged insects appear
Each transition required substantial anatomical and physiological adaptations - particularly for the marine-to-terrestrial transitions that produced today's isopods, millipedes, and insects.
Arthropod Anatomy in Detail
The Exoskeleton
The defining arthropod feature. Made of chitin (a tough polysaccharide) with various reinforcements:
- Calcium carbonate - strengthens shells in crustaceans (including isopods)
- Sclerotised proteins - provide flexibility at joints
- Waxy outer layer - prevents water loss in insects and some terrestrial arthropods. Worth noting: woodlice LACK this waxy layer, which is why they need humid environments
Body Segmentation
Most arthropods have body regions specialised for different functions:
- Insects - head, thorax (with three pairs of legs and usually wings), abdomen
- Spiders - cephalothorax (fused head and thorax with four pairs of legs), abdomen
- Crustaceans - head, thorax (often fused as cephalothorax), abdomen with appendages
- Isopods - head, seven-segmented pereon (each with leg pair), six-segmented pleon (with pleopodal lungs)
- Myriapods - head + elongated trunk with many leg-bearing segments
Sensory Organs
Highly developed across the phylum:
- Compound eyes - many insects and crustaceans (though terrestrial isopods have simpler ocelli)
- Antennae - chemical detection and touch
- Sensilla - sensory hairs embedded in the exoskeleton
- Humidity detection - particularly important in terrestrial species like isopods
Respiration
Different arthropod groups have different respiratory systems:
- Gills - marine and some freshwater crustaceans
- Tracheae - branching tubes in insects allowing direct gas exchange
- Book lungs - flat respiratory surfaces in spiders
- Pleopodal lungs - terrestrial isopod adaptation evolved from gills
- Cuticular respiration - through the body surface, supplementary in many species
Life Cycles
Arthropods exhibit diverse developmental patterns:
Complete Metamorphosis
Egg → Larva → Pupa → Adult. The larva is very different from the adult (think caterpillar → butterfly, maggot → fly). The pupa stage involves complete internal reorganisation. Common in beetles, flies, butterflies, moths, wasps, bees.
Incomplete (Hemimetabolous) Metamorphosis
Egg → Nymph → Adult. The nymph resembles a smaller wingless version of the adult and develops gradually through successive moults. Common in cockroaches, grasshoppers, true bugs.
Direct Development
The pattern in terrestrial isopods, myriapods, and arachnids. Juveniles (mancae in isopods, hatchlings in others) emerge looking like miniature adults and grow through successive moults without dramatic transformation. The simplest pattern but works well for invertebrates without flight stages.
Reproductive Variations
- Egg-laying - most arthropods
- Live birth - scorpions, some insects, woodlice (which carry developing young in a marsupium)
- Parthenogenesis - all-female reproduction in some species including Trichorhina tomentosa (Dwarf Whites)
Habitats and Adaptations
Arthropods have colonised every habitat on Earth:
- Deep oceans - giant isopods (Bathynomus), abyssal crustaceans
- Polar regions - Antarctic springtails, Arctic spiders
- Deserts - scorpions, desert beetles, some woodlice
- Tropical forests - the highest arthropod biodiversity
- Caves - including specialised Cubaris isopod species adapted to dark cave environments
- Soil ecosystems - ants, springtails, mites, isopods, millipedes
- Urban environments - thriving alongside humans
Successful adaptations include:
- Diapause - dormancy during unfavourable conditions
- Cryptobiosis - extreme dehydration tolerance (tardigrades among others)
- Flight - unique to insects among invertebrates, allowing dispersal and predator avoidance
- Conglobation - rolling into a ball (Armadillidium and some others)
- Burrowing - many terrestrial arthropods
- Chemical defences - millipedes, bombardier beetles, and others
Arthropods in the UK Invertebrate Hobby
For UK keepers, the most commonly kept arthropods are:
From Crustacea
- Terrestrial isopods (Armadillidium, Porcellio, Porcellionides, Cubaris, Trichorhina, etc.) - PostPods's primary product range
From Hexapoda
- Springtails (various species) - bioactive cleanup crew
- Cockroaches (various pet species) - exotic invertebrate pets
- Tarantulas, mantises, stick insects - outside PostPods's primary range but popular in broader UK invert hobby
From Myriapoda
- Millipedes - slow-moving herbivorous pets, several species suitable for UK hobby
From Chelicerata
- Tarantulas, scorpions - outside PostPods's primary range but popular elsewhere in UK hobby
The Honest Summary
Arthropods are the most successful animal phylum, with around 80-85% of all described animal species. Their success comes from:
- Modular body plan allowing diverse specialisation
- Tough but flexible exoskeleton
- Highly developed sensory systems
- Diverse respiratory strategies for different habitats
- Multiple reproductive approaches
- Capacity for extreme adaptation
For UK invertebrate keepers, the most commercially important arthropod groups are terrestrial isopods (Crustacea), springtails (Hexapoda), millipedes (Myriapoda), and various cockroach species (Hexapoda - Blattodea). Each represents a different evolutionary lineage with different husbandry requirements.
Browse our isopods, springtails, cockroaches, or millipedes collections for current UK stock.
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