Arthropoda: A Complete Guide for Students
Arthropoda is the largest phylum in the animal kingdom, comprising organisms found in nearly every habitat on Earth. These creatures may be aquatic, terrestrial, or even parasitic. They are known for their segmented bodies, jointed appendages, and hard exoskeletons made of chitin. In this guide, we will explore the classification of arthropods, the types, and the characteristics of arthropods. We will also discuss the classes and highlight some fascinating facts that make them one of the most diverse groups of animals on our planet.
Arthropoda Characteristics
Here are some key Arthropoda characteristics that define members of this phylum:
Body Organisation
They show the organ-system level of the organisation.
Their body is triploblastic (deriving from three germ layers), bilaterally symmetrical, and segmented.
The segmentation often groups into distinct body regions such as the head, thorax, and abdomen.
Exoskeleton
A hard exoskeleton made of chitin covers the body.
This exoskeleton provides protection and helps prevent desiccation, especially in terrestrial arthropods.
Jointed Appendages
Arthropods derive their name from “arthro”, meaning join,t and “po,d”, meaning foot (jointed feet/legs).
Their jointed appendages help in various activities like locomotion, feeding, and sensory reception.
Circulatory System
They have an open circulatory system in which blood (haemolymph) circulates through a body cavity called the haemocoel.
Coelom
The true coelom is greatly reduced and is mainly restricted around the reproductive organs.
The main body cavity is the haemocoel.
Respiration
Terrestrial forms respire via tracheae (tube-like structures) or book lungs.
Aquatic forms typically use gills. Some small arthropods also use diffusion through their body surface.
Excretion
Terrestrial arthropods generally excrete via Malpighian tubules.
Aquatic arthropods have specialised glands like green glands or coxal glands.
Sense Organs
They often have well-developed sense organs, including simple eyes (ocelli), compound eyes, antennae, auditory organs, and statocysts for balance.
Reproduction
Most arthropods are unisexual, with external or internal fertilisation.
Their development can be direct or indirect, involving various larval stages.
Classification of Phylum Arthropoda
The classification of arthropods is based on distinct morphological features such as body segmentation, appendages, and mouthparts. Let us explore the major groups (subphyla) along with some of their classes:
1. Subphylum Crustacea
Habitat: Mainly aquatic, though some are terrestrial or parasitic.
Body Structure: The head and thorax are often fused into a cephalothorax, typically covered by a carapace.
Respiration: Gills or general body surface.
Appendages: Two pairs of antennae and five pairs of walking legs (appendages).
Excretion: Green (antennal) glands.
Examples: Crab, Lobster, Daphnia, Palaemon (prawn).
Classes Under Crustacea
Branchiopoda
Remipedia
Cephalocarida
Maxillopoda
Ostracoda
Malacostraca
2. Subphylum Myriapoda
Habitat: Mostly terrestrial.
Body Structure: Head and a long trunk composed of many segments.
Appendages: Each segment (or most of them) bears paired legs.
Respiration: Tracheal system.
Excretion: Malpighian tubules.
Examples: Julus (millipede) and Scolopendra (centipede).
Classes Under Myriapoda
Chilopoda (Centipedes)
Diplopoda (Millipedes)
Pauropoda
Symphyla
3. Subphylum Hexapoda
Habitat: Primarily terrestrial, though some are aquatic or parasitic.
Body Structure: Divided into head, thorax, and abdomen. Thorax has three segments (prothorax, mesothorax, metathorax), each typically bearing a pair of legs.
Appendages: Three pairs of legs and usually two pairs of wings (insects).
Respiration: Tracheal system (some have gills in the larval stage).
Excretion: Malpighian tubules.
Examples: Cockroaches, Tabanus (horsefly), mosquitoes, and ants.
Classes Under Hexapoda
Insecta
Entognatha
4. Subphylum Chelicerata
Habitat: Mostly terrestrial (e.g., spiders, scorpions), some marine (e.g., horseshoe crabs).
Body Structure: Divided into cephalothorax and abdomen. No antennae.
Appendages: The first pair is modified into chelicerae (feeding organs), and the second pair into pedipalps, followed by four pairs of walking legs.
Respiration: Book lungs or tracheae. Aquatic chelicerates (like horseshoe crabs) may have book gills.
Excretion: Malpighian tubules.
Examples: Spiders, scorpions, and Limulus (horseshoe crab).
Classes Under Chelicerata
Arachnida (Spiders, scorpions, ticks, mites)
Merostomata (Horseshoe crabs)
Pycnogonida (Sea spiders)
5. Subphylum Onychophora
Habitat: Small, terrestrial, found in moist habitats.
Body Structure: Segmental arrangement of organs with paired stubby legs.
Respiration: Tracheal system.
Excretion: Nephridia (in each segment).
Examples: Peripatus
6. Subphylum Trilobitomorpha (Extinct)
Fossil Arthropods: Dominant during the Palaeozoic era but now extinct.
Body Structure: Divided into three lobes (one median lobe and two lateral lobes).
Appendages: Biramous (two-branched).
Examples: All extinct forms collectively grouped as Trilobita.
The 7 Classes of Arthropods (With Examples)
While arthropods can be organised into various subphyla and classes, students often refer to the following broad classes to understand the types of arthropods:
Arachnida (e.g., Spiders, Scorpions)
Crustacea (e.g., Crabs, Lobsters)
Insecta (e.g., Cockroaches, Butterflies)
Chilopoda (Centipedes)
Diplopoda (Millipedes)
Merostomata (Horseshoe Crabs)
Pycnogonida (Sea Spiders)
These 7 classes of arthropods are often highlighted for their distinct morphological traits and ecological significance. However, note that the total classification of phylum Arthropoda can also include other subgroups (like Entognatha and Onychophora), depending on different taxonomic schemes.
Additional Unique Insights
Adaptive Success: Arthropods owe their widespread success to adaptive traits such as a protective exoskeleton, efficient locomotion, and varied feeding strategies.
Economic Importance: Some arthropods (like honeybees) are beneficial pollinators, while others (like locusts and mosquitoes) can be agricultural pests or disease vectors.
Diverse Feeding Habits: Arthropods include herbivores, carnivores, omnivores, detritivores, and parasites, showcasing their enormous ecological range.
Fun Mnemonic for Remembering the Major Subphyla
A helpful way to recall the main subphyla of Arthropoda is the phrase:
“Cheese Makes Hungry Ostriches Think”
Che = Chelicerata
M = Myriapoda
H = Hexapoda
O = Onychophora
T = Trilobitomorpha
Quick Quiz (With Answers)
Test your knowledge of the characteristics of arthropods and their classification by attempting this short quiz:
Which subphylum of Arthropoda typically has two pairs of antennae?
A. Myriapoda
B. Hexapoda
C. Crustacea
D. Chelicerata
Answer: C. Crustacea
Which organ is used for excretion in most terrestrial arthropods?
A. Nephridia
B. Green Glands
C. Tracheae
D. Malpighian Tubules
Answer: D. Malpighian Tubules
Name the extinct subphylum of Arthropoda.
A. Chelicerata
B. Trilobitomorpha
C. Myriapoda
D. Onychophora
Answer: B. Trilobitomorpha
Which class of arthropods generally possesses book lungs or tracheae but no antennae?
A. Arachnida
B. Insecta
C. Diplopoda
D. Crustacea
Answer: A. Arachnida
The exoskeleton of arthropods is primarily composed of:
A. Calcium carbonate
B. Chitin
C. Cellulose
D. Silicon dioxide
Answer: B. Chitin
Related Topics
FAQs on Arthropoda: Classification, Characteristics and Detailed Guide
1. What are the three key defining characteristics of the phylum Arthropoda?
The three key characteristics that define members of the phylum Arthropoda are a segmented body (often grouped into head, thorax, and abdomen), the presence of jointed appendages for movement and feeding, and a hard external skeleton called an exoskeleton made primarily of chitin.
2. How is the phylum Arthropoda classified into its major subphyla?
The classification of Arthropoda is based on distinct morphological features like body division, appendages, and mouthparts. The primary subphyla, as per the NCERT syllabus for the 2025-26 session, are:
- Chelicerata: Body divided into cephalothorax and abdomen; no antennae (e.g., spiders, scorpions).
- Crustacea: Mostly aquatic; two pairs of antennae and gills for respiration (e.g., crabs, prawns).
- Hexapoda: Body divided into head, thorax, and abdomen; three pairs of legs (e.g., insects).
- Myriapoda: Head and a long trunk with many leg-bearing segments (e.g., centipedes, millipedes).
- Trilobitomorpha: An extinct group of marine arthropods.
3. What is the difference between an insect and an arachnid?
Although both are arthropods, insects and arachnids belong to different classes. Insects (Class Insecta) are defined by having three body parts (head, thorax, abdomen), six legs, and one pair of antennae. In contrast, arachnids (Class Arachnida), such as spiders, have two body parts (cephalothorax and abdomen), eight legs, and lack antennae and wings.
4. How do arthropods grow despite having a rigid exoskeleton?
Arthropods grow by periodically shedding their rigid exoskeleton through a process known as moulting or ecdysis. After shedding the old, smaller shell, the arthropod undergoes a rapid growth spurt before its new, larger exoskeleton hardens. This process is controlled by hormones.
5. What is the main difference between the respiratory systems of terrestrial and aquatic arthropods?
The primary difference relates to their habitat. Most terrestrial arthropods, like insects, respire through a network of fine air tubes called a tracheal system, while others like scorpions use book lungs. In contrast, most aquatic arthropods, such as crabs and lobsters, use gills to extract dissolved oxygen from water.
6. What is an open circulatory system in arthropods and how does it function?
An open circulatory system means that the blood, called haemolymph, is not always confined within blood vessels. A simple heart pumps the haemolymph into the main body cavity, known as the haemocoel, where it directly bathes the organs and tissues to facilitate nutrient and waste exchange before returning to the heart.
7. Why is the phylum Arthropoda considered the most successful in the animal kingdom?
Arthropoda's remarkable success is attributed to several key evolutionary adaptations. Their versatile chitinous exoskeleton provides protection from predators and prevents water loss, their jointed appendages have been modified for various functions like walking, swimming, and feeding, and their efficient sensory and respiratory systems have allowed them to colonise and thrive in nearly every ecosystem on Earth.
8. What is the economic importance of arthropods to humans?
Arthropods have significant economic importance, with both beneficial and harmful impacts:
- Beneficial: Honeybees are crucial for pollination of crops, silkworms produce silk, and many crustaceans like prawns and crabs are major food sources.
- Harmful: Many insects like locusts are destructive agricultural pests, while others like mosquitoes and ticks act as vectors for serious diseases such as malaria and Lyme disease.
9. What is the key difference between centipedes (Chilopoda) and millipedes (Diplopoda)?
The main differences lie in their body structure and diet. Centipedes (Class Chilopoda) have one pair of legs per body segment, are fast-moving carnivores, and possess poison claws. In contrast, millipedes (Class Diplopoda) have two pairs of legs per segment, are slower-moving, and are primarily detritivores, feeding on decaying plant matter.
