Arthropoda: Classification, Characteristics and Detailed Guide

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:

1. 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.

2. Exoskeleton

• A hard exoskeleton made of chitin covers the body.

• This exoskeleton provides protection and helps prevent desiccation, especially in terrestrial arthropods.

3. 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.

4. Circulatory System

• They have an open circulatory system in which blood (haemolymph) circulates through a body cavity called the haemocoel.

5. Coelom

• The true coelom is greatly reduced and is mainly restricted around the reproductive organs.

• The main body cavity is the haemocoel.

6. 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.

7. Excretion

• Terrestrial arthropods generally excrete via Malpighian tubules.

• Aquatic arthropods have specialised glands like green glands or coxal glands.

8. Sense Organs

• They often have well-developed sense organs, including simple eyes (ocelli), compound eyes, antennae, auditory organs, and statocysts for balance.

9. 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

1. Branchiopoda

2. Remipedia

3. Cephalocarida

4. Maxillopoda

5. Ostracoda

6. 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

1. Chilopoda (Centipedes)

2. Diplopoda (Millipedes)

3. Pauropoda

4. 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

1. Insecta

2. 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

1. Arachnida (Spiders, scorpions, ticks, mites)

2. Merostomata (Horseshoe crabs)

3. 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:

1. Arachnida (e.g., Spiders, Scorpions)

2. Crustacea (e.g., Crabs, Lobsters)

3. Insecta (e.g., Cockroaches, Butterflies)

4. Chilopoda (Centipedes)

5. Diplopoda (Millipedes)

6. Merostomata (Horseshoe Crabs)

7. 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:

1. Which subphylum of Arthropoda typically has two pairs of antennae?
   A. Myriapoda
   B. Hexapoda
   C. Crustacea
   D. Chelicerata
   Answer: C. Crustacea

2. Which organ is used for excretion in most terrestrial arthropods?
   A. Nephridia
   B. Green Glands
   C. Tracheae
   D. Malpighian Tubules
   Answer: D. Malpighian Tubules

3. Name the extinct subphylum of Arthropoda.
   A. Chelicerata
   B. Trilobitomorpha
   C. Myriapoda
   D. Onychophora
   Answer: B. Trilobitomorpha

4. Which class of arthropods generally possesses book lungs or tracheae but no antennae?
   A. Arachnida
   B. Insecta
   C. Diplopoda
   D. Crustacea
   Answer: A. Arachnida

5. The exoskeleton of arthropods is primarily composed of:
   A. Calcium carbonate
   B. Chitin
   C. Cellulose
   D. Silicon dioxide
   Answer: B. Chitin

Related Topics

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• Animal Kingdom

• Cell Division

• Meiosis Definition

• Angiosperms