Phylum Arthropoda: Definition, Characteristics, Classification and Examples of Arthropod

Q: What are some examples of arthropods in each major class?

Insecta: Honeybee (Apis mellifera), butterfly, beetle Arachnida: Spider (Araneae), scorpion , tick Crustacea: Crab (Brachyura), shrimp, lobster Myriapoda: Chilopoda – centipede, Diplopoda – millipede

Arthropoda

Edited By Irshad Anwar | Updated on Jul 02, 2025 05:09 PM IST

Arthropods are invertebrates in the phylum Arthropoda. Phylum Arthropoda is a group of invertebrate animals that includes insects, spiders, crustaceans, and centipedes. These animals have jointed legs, an exoskeleton, and a segmented body. Arthropods are found in nearly every environment on Earth, from land to water. This concept is part of the Class 12 chapter on Animal Kingdom in Biology. Understanding the definition of phylum Arthropoda helps students learn about the diversity of these creatures and how they play important roles in ecosystems.

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Phylum Arthropoda
General Characteristics of Arthropoda
Classification of Arthropoda
Phylum Arthropoda Examples

Arthropoda

Phylum Arthropoda

The Arthropoda phylum is the largest and most diverse group of animals in the Animal Kingdom, which includes insects, spiders, crustaceans, and several others. These animals have segmented bodies, jointed legs, and a hard outer shell. They are important in all ecosystems worldwide because they inhabit both deep ocean environments and land. Arthropods and their distribution are important to understand Earth's biodiversity, which is why this phylum is very prominent in the study of Zoology.

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General Characteristics of Arthropoda

The general characters of Arthropoda that contribute to their evolutionary success and ecological diversity:

Exoskeleton Composition and Molting (Ecdysis)

Both annelids and arthropods have segmented bodies and yet they differ in that the arthropods have an armour-like covering called the exoskeleton, which is composed of a material known as chitin. With their development, they also go through a process of moulting (ecdysis) where they peel their skin and create a new, bigger one.

Segmented Body Plan

Arhopods have limbs and joints and their body is generally segmented, falling under the head, the thorax and the abdomen. This makes possible the specialization of body regions and functions.

Jointed Appendages

The Members of the phylum Arthropoda have bilateral symmetry and paired jointed appendages (legs, antennae) through which movement and manipulation of the environment are made possible hence easy manipulation of the surrounding environment hence the ability to inhabit a variety of places.

Bilateral Symmetry

Arthropods are bi-laterally symmetrical that is their bodies are symmetrical in such a way that they can reflect each other. This is related to the smooth and well-coordinated locomotion of certain parts of the body’s structure.

Open Circulatory System

They have an open circulatory system, in which blood or rather the hemolymph flows without any restrictions within body compartments and directly washes the organs. This system nourishes them metabolically and helps in the distribution of nutrition and oxygen.

Nervous System and Sensory Organs

The main organ, nervous system consists of a brain and a ventral nerve cord that runs along the dorsum of the body. They wield well-developed appendages called sensory appendages that help them perceive stimuli such as light through the compound eye, touch through antennae, chemical signals through chemoreceptors, and many other forms that improve the arthopoda reaction and communication with the surroundings.

Diagram of Arthropoda

Below is the arthropoda diagram with its body parts

Classification of Arthropoda

The subphylum and classes of arthropods are discussed below:

Subphylum Trilobitomorpha

Extinct

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Subphylum Chelicerata

Class Arachnida (spiders, scorpions)
Class Merostomata (horseshoe crabs)
Class Pycnogonida (sea spiders)

Subphylum Myriapoda

Class Chilopoda (centipedes)
Class Diplopoda (millipedes)

Subphylum Crustacea

Class Malacostraca (crabs, lobsters)
Class Branchiopoda (brine shrimp)
Class Maxillopoda (barnacles)

Subphylum Hexapoda

Class Insecta (insects)
Class Entognatha (springtails)

Phylum Arthropoda Examples

The examples of phylum arthropods are given below:

Insecta

Body structure: Insects have a three-part body. It includes a head with sensory organs such as eyes and antennae, a thorax with three sets of legs most of the time wings, and the abdomen which is involved in digestion and reproduction.

Life cycle (metamorphosis): Insects pass through metamorphosis- either holometabolous (egg, larva, pupa, adult) while others are hemimetabolous (egg, nymph, adult) so they can occupy different ecological niches in their life cycle.

Ecological importance: Without insects, the ecosystems would suffer through pollination inability, decomposition and food resources. It is known that they affect ecosystems and agriculture as pollinators and pests.

Example: Honeybee (Apis mellifera), Honeybees are important as they contribute to crop pollination and maintenance of other species.

Honeybee Apis Mellifera

Arachnida

Body structure: Arachnids have a body divided into two main parts, the Cephalothorax which is the fused head and the thorax and the abdomen. While the cephalothorax is responsible for the housing of the eyes, mouthparts, and legs, the abdomen is also assigned functions of the digestive and reproductive organs.

Specialised appendages: Some arachnida such as spiders have special limbs, chelicerae are used in feeding whereas most have fangs and pedipalps that are sensory and feeding organs as well as being reproductive.

Venom and silk production: Most arachnids, for example, spiders, secrete toxins to paralyze their food and threads for building webs to catch the prey or to build nests for the protection of eggs among others.

Example: Spider (Araneae), The spiders are popular since they use silk to twist webs that they use in catching their meal. These are pest management instruments that help to regulate the pest populations and are resident in different ecosystems globally.

Spider

Crustacea

Body structure: Crustacea has a head and thorax fused, which is called the cephalothorax and the remainder of the body is the abdomen. These often consist of more than one pair of legs, mouthparts and/or antennae which help in feeding and/or perceiving the environment.

Aquatic adaptations: In a general, crustaceans are mostly water-dwelling animals and hence they mostly exhibit some traits found in their respiratory system like gills for respiratory purpose and their locomotive and feeding appendages like limbs.

Economic importance: Crustaceans are of vast economic importance, as they serve as food, especially crabs, lobsters, and shrimp, and as a source of fishery and aquaculture in the global market.

Example: Crab (Brachyura), Crabs are marine and freshwater animals that have broad flat heads and thorax that together are known as the cephalothorax, strong claw-like pincers which are used for defences as well as for manpower. It is relevant to both the structure of ecosystems and food chains and it is considered as a source of food for other living beings including human beings.

Crab

Myriapoda

Distinctive features: Class Chilopoda, which includes the centipedes, is characterized by elongated bodies and a pair of legs in every one of the segments of the body. These creatures are carnivorous, and they kill their prey using special claws which are venomous. The class Diplopoda is known as millipedes. They have two pairs of legs on each body segment and are tearing vegetarians, scrap feeders, and decomposers of plant litter.

Habitat and behaviour: Myriapods are usually associated with habitats with high humidity, including soil, litter layer, and logs. While centipedes are very swift when hunting for their prey, millipedes are very slow-moving and to a large extent depend on their shells and chemical secretion to protect them from their predators.

Example: The centipede is associated with speed and predatory nature because of the way it catches its prey. They use five-millimetre-long venomous forcipules for holding and paralyzing their prey, insects and other small animals. These participates in the regulation of some pests in their natural environment.

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Phylum Cnidaria	Phylum Porifera
Phylum Mollusca	Phylum Chordata

Frequently Asked Questions (FAQs)

1. What kinds of animals belong to the phylum Arthropoda?

Arthropods are a group of creatures that include spiders, ticks, centipedes, mites, crabs, lobsters, scorpions, shrimp, and more.

2. How did arthropod evolution occur?

Arthropods and annelids are thought to have descended from the same ancestral root. It is thought that the three arthropod lineages diverged from a single common ancestor.

3. How did arthropods chance to live on land?

The earliest arthropods developed adaptations like breathing tracheae. They also created an exoskeleton, which protects animals' skin from drying out and offers support in the event of buoyant water.

4. When did the earliest arthropods begin to change?

The earliest arthropods are thought to have emerged 545 million years ago, according to fossil records. Initially, only scorpions, centipedes, and millipedes were able to adapt to dry land.

5. Why is the phylum Arthropoda the largest?

Arthropoda makes up over 80% of all animal species. The largest phylum in the animal kingdom is made up of these species, which have evolved specific coping mechanisms over time to endure the varying environmental conditions.

6. How are arthropods classified into different subphyla and classes?

Arthropods are classified into several subphyla based on their body structure and appendages:

Subphylum Chelicerata: Consists of Class Arachnida – the list of animals belonging to this class is spiders and scorpions.
Subphylum Crustacea: Such as; crabs, lobsters, shrimp
Subphylum Myriapoda: Some of it includes Chilopoda which are Centipedes and Diplopoda which are millipedes.
Subphylum Hexapoda: The class include Insecta (insects).

7. What are some examples of arthropods in each major class?

Insecta: Honeybee (Apis mellifera), butterfly, beetle
Arachnida: Spider (Araneae), scorpion , tick
Crustacea: Crab (Brachyura), shrimp, lobster
Myriapoda: Chilopoda – centipede, Diplopoda – millipede

8. What is the difference between an insect and an arachnid?

Insects have three body segments (head, thorax, abdomen), six legs, and usually wings. Arachnids have two body segments (cephalothorax and abdomen), eight legs, and never have wings or antennae.

9. What is the difference between a millipede and a centipede?

While both are myriapods, millipedes have two pairs of legs per body segment and are generally herbivorous, while centipedes have one pair of legs per segment and are predatory. Centipedes are also typically faster and have venomous claws.

10. What is the difference between an open and closed circulatory system in arthropods?

In an open circulatory system, common in most arthropods, hemolymph flows freely in the body cavity. In a closed system, found in some larger crustaceans, blood is contained within vessels, allowing for more efficient circulation.

11. How do social insects like bees or ants organize their colonies?

Social insects use division of labor, with different castes performing specific roles (e.g., workers, soldiers, queens). They communicate using pheromones and sometimes physical interactions to coordinate activities and make collective decisions.

12. How do aquatic arthropods breathe underwater?

Aquatic arthropods may use gills to extract oxygen from water, or in some cases, carry an air bubble with them underwater. Some have adaptations to allow them to surface periodically to replenish their oxygen supply.

13. How do arthropods sense their environment?

Arthropods use various sensory organs including compound eyes, simple eyes (ocelli), antennae for touch and smell, and sensory hairs (setae) that can detect vibrations, air currents, and chemical signals.

14. What is the function of an arthropod's hemolymph?

Hemolymph, the arthropod equivalent of blood, transports nutrients, hormones, and immune cells. It also plays a role in hydraulic movement of appendages and can aid in thermoregulation.

15. How do arthropods' compound eyes work?

Compound eyes consist of many individual lenses called ommatidia. Each ommatidium detects light from a specific direction, creating a mosaic image. This allows for excellent motion detection and a wide field of view.

16. How do arthropods achieve movement without an internal skeleton?

Arthropods use their exoskeleton as an attachment point for muscles. The jointed nature of their appendages, combined with hydraulic pressure from hemolymph, allows for precise and efficient movement.

17. How do arthropods defend themselves against predators?

Arthropods use various defense mechanisms including camouflage, chemical defenses (like venoms or irritants), physical defenses (spines or hard shells), mimicry, and behavioral strategies like playing dead or swarming.

18. What defines an arthropod?

Arthropods are invertebrate animals characterized by a segmented body, jointed appendages, and an exoskeleton made of chitin. This group includes insects, arachnids, crustaceans, and myriapods.

19. How does molting relate to arthropod growth?

Molting is crucial for arthropod growth because their rigid exoskeleton doesn't grow with them. They periodically shed their old exoskeleton and secrete a new, larger one to accommodate their increased size.

20. Why are arthropods considered successful in terms of evolution?

Arthropods are evolutionarily successful due to their adaptable body plan, diverse habitats, efficient respiratory and circulatory systems, and ability to exploit various food sources. They represent over 80% of all known animal species.

21. What is the function of an arthropod's exoskeleton?

The exoskeleton provides protection, support for muscles, prevents water loss, and acts as a barrier against pathogens. It also allows for precise movement through its rigid yet jointed structure.

22. How do arthropods breathe?

Arthropods have various respiratory systems depending on their habitat. Terrestrial arthropods often use tracheal systems (networks of tubes), while aquatic arthropods may use gills. Some small arthropods can absorb oxygen directly through their exoskeleton.

23. How do arthropods cope with extreme temperatures?

Arthropods use various strategies including behavioral (seeking shelter), physiological (producing antifreeze proteins or heat shock proteins), and morphological adaptations (such as light-colored exoskeletons in hot climates).

24. What is the role of arthropods in ecosystems?

Arthropods play crucial roles as pollinators, decomposers, prey for other animals, and predators controlling pest populations. They are key components in food webs and contribute significantly to biodiversity.

25. What is the purpose of an arthropod's molt?

Molting allows for growth, as the rigid exoskeleton doesn't grow with the animal. It also allows for regeneration of lost limbs, repair of damaged exoskeleton, and can facilitate metamorphosis in some species.

26. How do arthropods contribute to human life?

Arthropods contribute through pollination (crucial for agriculture), as food sources (e.g., crustaceans), in pest control (e.g., predatory insects), in decomposition and nutrient cycling, and as model organisms in scientific research.

27. What is the function of an arthropod's antennae?

Antennae are sensory organs used for touch, smell, and sometimes hearing. They help arthropods navigate their environment, find food, detect predators, and communicate with other individuals of their species.

28. What is metamorphosis in arthropods?

Metamorphosis is a process of dramatic change in body structure during development. In arthropods, it can be complete (egg, larva, pupa, adult) as in butterflies, or incomplete (egg, nymph, adult) as in grasshoppers.

29. How do arthropods reproduce?

Most arthropods reproduce sexually, with separate male and female individuals. Some can reproduce asexually through parthenogenesis. Many species have complex mating rituals and may provide parental care to their offspring.

30. What is the role of pheromones in arthropod behavior?

Pheromones are chemical signals used by arthropods for communication. They can mark trails, attract mates, signal danger, or coordinate group behavior, playing a crucial role in social interactions and survival.

31. How do arthropods deal with osmoregulation in different environments?

Terrestrial arthropods have a waxy cuticle to prevent water loss. Aquatic arthropods may have specialized glands to regulate salt balance. Some can adjust the concentration of their hemolymph to match their environment.

32. What is the significance of segmentation in arthropods?

Segmentation allows for specialization of body parts for different functions, such as feeding, locomotion, or reproduction. It also provides flexibility and the potential for diverse body plans through evolution.

33. How do parasitic arthropods adapt to their lifestyle?

Parasitic arthropods often have specialized mouthparts for feeding on host tissues or fluids. They may have reduced sensory or locomotory organs, and their life cycles are often synchronized with their hosts.

34. What is the significance of chitin in arthropods?

Chitin is the main component of the arthropod exoskeleton. It provides strength and flexibility, protects against dehydration and physical damage, and can be modified for various functions like sensory reception or camouflage.

35. How do arthropods maintain water balance in terrestrial environments?

Terrestrial arthropods have a waxy cuticle to prevent water loss, efficient excretory systems, and behavioral adaptations like seeking humid microhabitats. Some can absorb water vapor from the air or obtain water from their food.

36. What is the role of hormones in arthropod development?

Hormones regulate molting, metamorphosis, and reproductive cycles in arthropods. For example, ecdysone triggers molting, while juvenile hormone influences whether a molt will result in another larval stage or progression to adult form.

37. How do arthropods digest their food?

Arthropods have a complete digestive system. Many use enzymes to break down food externally before ingestion. Inside the gut, further enzymatic digestion occurs, with nutrients absorbed through the gut lining into the hemolymph.

38. What is the function of book lungs in some arthropods?

Book lungs, found in some arachnids, are respiratory organs consisting of thin folds of tissue resembling book pages. They increase surface area for gas exchange, allowing efficient respiration in terrestrial environments.

39. How do arthropods perceive color?

Many arthropods can perceive color through specialized photoreceptors in their compound eyes. Some, like bees, can see ultraviolet light, which helps them identify patterns on flowers invisible to human eyes.

40. What is the purpose of silk production in arthropods?

Silk, produced by some arthropods like spiders and moth caterpillars, serves various functions including web building for prey capture, creating protective cocoons, as a lifeline for quick escapes, and in some cases, for courtship displays.

41. How do arthropods cope with the challenges of living in marine environments?

Marine arthropods have adaptations like waterproof exoskeletons, gills for respiration, and specialized appendages for swimming. Some have mechanisms to regulate salt balance, and many have larval stages adapted for planktonic life.

42. What is the significance of mimicry in arthropods?

Mimicry in arthropods is a defensive strategy where a species evolves to resemble another species or object. This can deter predators (if mimicking a dangerous species) or help in camouflage (if mimicking an inanimate object like a leaf or twig).

43. How do arthropods contribute to soil health?

Soil-dwelling arthropods like mites, springtails, and some insect larvae contribute to soil health by breaking down organic matter, aerating the soil, and participating in nutrient cycling. They also serve as food for other soil organisms.

44. What is the function of the arthropod nervous system?

The arthropod nervous system, consisting of a brain and ventral nerve cord, coordinates sensory input, motor output, and internal regulation. It allows for complex behaviors, learning, and in some cases, social interactions.

45. How do arthropods deal with the problem of water loss in terrestrial environments?

Terrestrial arthropods have a waxy cuticle to reduce water loss, efficient excretory systems that conserve water, and behavioral adaptations like being active at night or burrowing to avoid dry conditions.

46. What is the role of arthropods in pollination?

Many arthropods, especially insects like bees, butterflies, and some flies, are crucial pollinators. They transfer pollen between flowers while feeding on nectar, enabling plant reproduction and maintaining biodiversity in ecosystems.

47. How do arthropods cope with the challenges of flight?

Flying arthropods have evolved lightweight bodies, efficient respiratory systems, and powerful flight muscles. Their exoskeletons provide attachment points for flight muscles, and many have sophisticated sensory systems for navigation and stability in flight.

48. What is the purpose of bioluminescence in some arthropods?

Bioluminescence in arthropods like fireflies or some deep-sea crustaceans can serve various purposes including attracting mates, luring prey, confusing predators, or communication within a species.

49. How do arthropods maintain balance and orientation?

Many arthropods have specialized sensory organs called statocysts that detect gravity and movement, helping with balance and orientation. Some also use visual cues and information from sensory hairs on their bodies.

50. What is the significance of eusociality in some arthropod species?

Eusociality, found in some insects like ants, bees, and termites, involves cooperative brood care, overlapping generations, and division of labor. This social structure allows for complex colony behaviors and often leads to ecological dominance.

51. How do arthropods cope with parasites and diseases?

Arthropods have innate immune systems that can recognize and respond to pathogens. They also employ behavioral defenses like grooming, and some social species even practice forms of 'social immunity' where group behaviors help prevent disease spread.

52. What is the role of pedipalps in different arthropod groups?

Pedipalps are the second pair of appendages near an arthropod's mouth. Their function varies: in spiders, they're used for sensing and mating; in scorpions, for grasping prey; and in horseshoe crabs, for moving food into the mouth.

53. How do arthropods regulate their body temperature?

Most arthropods are ectothermic, relying on external sources for body heat. They use behavioral methods like basking in sun or seeking shade, and some have physiological adaptations like producing heat through muscle contractions or possessing antifreeze proteins.

54. What is the function of the arthropod heart?

The arthropod heart, often a tube-like structure, pumps hemolymph through the body. In the open circulatory system of most arthropods, this helps distribute nutrients, hormones, and immune cells throughout the body cavity.

55. How do arthropods adapt to life in extreme environments?

Arthropods in extreme environments have specialized adaptations. For example, desert arthropods may be nocturnal and have water-conserving adaptations, while those in polar regions might produce antifreeze proteins or enter diapause during harsh conditions.

56. What is the significance of metamorphosis in the life cycles of many arthropods?

Metamorphosis allows arthropods to occupy different ecological niches at different life stages, reducing competition between juveniles and adults. It also enables specialization of body forms for specific functions like reproduction or dispersal.

57. How do arthropods contribute to the process of decomposition?

Many arthropods, especially insects and mites, are important decomposers. They break down dead organic matter, recycling nutrients back into the ecosystem. This process is crucial for soil fertility and the overall functioning of ecosystems.