Phylum Arthropoda: Characteristics, Adaptations, and Classification (USA Grade 10 , NGSS)
🦐🦞🦀🕷️Overview: Why are Arthropods successful?
- Arthropods get their name from their Jointed Appendages (Arthro = Joint, Poda = Foot). These specialized limbs are articulated with the exoskeleton, allowing them to behave like predators and move with high speed and precision.
Table of Contents
- Introduction to Phylum Arthropoda
- Key Characteristics (Segmented Body & Jointed Appendages)
- The Role of Chitinous Exoskeleton & Ecdysis
- Internal Systems (Circulatory & Respiratory)
- Sensory Organs (Statocysts & Compound Eyes)
- Ecological and Economic Importance
- Conclusion & Self-Test
- Arthropods have segmented bodies, jointed appendages and chitinous exoskeleton.
- It is the largest phylum in animal kingdom. If there are 10 animals in world than 7 are insect.k
- Arthropoda have been evolved from Annelids therefore they have more affinity with the Annelids .
- Arthropoda are segmented animal like Annelids and found in both aquatic and terrestrial conditions etc.
- Arthropods are the most diverse group of animals on Earth. In fact, 80% of all known animal species belong to this phylum.
- Chitinous exoskeleton jointed Appendages , open Vascular system etc make them successfully to survive in various habitat in water, Air and terrestrial.
Evolutionary Key concept of Arthropods:
- These features makes them so adaptable to survive in terrestrial and aquatic habitat.
- The chitinous exoskeleton is a game-changer for Arthropods. It provides structural support and protection from predators. Most importantly, it prevents desiccation (water loss), allowing Arthropods to stay hydrated and thrive in harsh desert conditions where other animals might perish."
- Arthropods have this shielded armour on both side of body. It is made up of chitin.
Do you know ? The cell wall of Fungi is also made up of Chitin. You can read here about the Fungi.
- The chitinous Exoskeleton has great evolutionary value for Arthropods because it gives support and protection to Arthropods.
- It keeps body of Arthropods hydrated and provide protection from desiccation . It means that water in bodies of Arthropods remain maintain and Arthropods survive in arid condition.
- This feature gives skill to survive on terrestrial habitat because this exoskeleton is a water tight barrier.
- This skeleton is modified as sclerites in insect and calcified in crustacean according to various habitat.
- Due to hardness of chitin, it confins the growth and promote ecdysis.
Segmented Body :
- Segmented Body allows the Arthropods to divide the entire body in to Head, Thorax and abdomen. In some, head and thorax is fused so their body is divided into Cephalothorax and abdomen.
- Segmented Body is related to tagmatization. Each segment ( head or thorax or abdomen) is able to perform special role.
- Head and thorax are for the sensory and locomotion in function respectively whereas abdomen is meant for digestion and reproduction.
- Segmentation also allow muscle movement and contraction. Prominently they allow jointed Appendages to perform various function like walking, swimming, capturing etc.
- If one segment is damaged accidentally or by the predator than other segment compensate the function of other and animals may survive at all.
- This ability makes Arthropods successful to survive in different habitat like deep sea, Air and Desert.
Jointed Appendages:
- The presence of jointed Appendages make Arthropods highly adapted for movement, feeding and reproduction.
- These jointed Appendages are articulated with chitinous exoskeleton. This articulation provides extensive mobility to animals behave like Predators .
- Arthropods can run, swim walk efficiently with the jointed Appendages.
Specialised body system of Arthropods:
Open Circulatory system:
- Arthropods possess an Open Circulatory System. Instead of a closed network of vessels, the blood (called Hemolymph) directly bathes the internal organs in a body cavity known as the Hemocoel."
- Arthropods do not have blood vessels to carry the blood.
- In open circulatory system, all body organs are bathed in blood or Hemolymph in a body cavity called hemocoel or sinus.
- A tubular heart pumps blood into sinus . From sinus it returns back to heart through minute pore called ostia.
 |
| Open Circulatory system |
Respiratory system:
- They have various respiratory organs for the gaseous exchange due to adaptation of Various habitats.
- Insects use tracheal system, Arachnida uses Book lungs and Crustaceans use Book gills for the gaseous exchanges.
- Tracheal system in insect includes a network of tubes called tracheae. These tubes deliver oxygen to body tissue through external opening called Spiracles. Example: Cockroach Grasshopper .
 |
| Respiratory system in Grasshopper |
- Book lungs are leaf like structure in Arachnida which look like pages of books and exchanges of gases for the respiration . Example : Spider
- Book gills are horseshoe in shape and perform respiration in aquatic arthropoda , Crustaceans. Example : Crab
Eyes in Arthropods
- Arthropods have both compound and simple eyes.
- Compound eyes have mosaic vision which gives high resolution in dim light in nocturnal insects .
- Compound eye have 2000 units called ommatidia.
- Simple eyes are called ocelli which detect light intensity and movements.
Antennae in Arthropods:
- These are one pair jointed structure on head of insects and two pair in Crustaceans.
- These are prominent sensory meant for touch , smell, taste and detect sound and temprature.
Statocysts (For Balance)
- Many aquatic arthropods (like Crustaceans) possess a specialized sensory organ called a Statocyst.
- Its primary function is to maintain balance and equilibrium by sensing the direction of gravity.
- This allows the animal to orient its body correctly while swimming."
Malpighian Tubules (For Waste Removal)
- Terrestrial arthropods (Insects and Arachnids) have a unique excretory system consisting of Malpighian Tubules.
- These tubes filter metabolic waste from the hemolymph and discharge it into the intestine.
- This system is highly efficient at conserving water, which is crucial for survival on land."
💡Know It Also!
- 🚀 Fast Growth: Arthropods must shed their old exoskeleton to grow. This process, called Ecdysis (Molting), leaves them vulnerable for a short time until the new shell hardens.
- 🩸 Blue Blood: Some arthropods like Horseshoe Crabs have blue blood because it contains Hemocyanin (which has copper) instead of Hemoglobin (which has iron).
- 👁️ Super Vision: Their Compound Eyes are made of thousands of tiny lenses called Ommatidia, allowing them to see in almost every direction at once!
- 🐜 Strength: An ant can lift 50 times its own body weight! This is possible because their muscles are attached to the inside of their hard exoskeleton.
Scientific classification:
Class Insects :
- Body is divided into Head , thorax and abdomen.
- They have three pairs of jointed Appendages and one pair of antennae.
- Body is covered by the chitinous exoskeleton.
- Wings are one or two pairs located on thorax.
- Respiration Occurs through tracheal system.
- Examples : Ants , Buterfly Mosquitoes
Class Arachnida :
- Body is divided into cephalothorax ( fused head and thorax) and abdomen.
- They have four pairs of jointed Appendages and antennae are absent.
- Body is covered by the chitinous exoskeleton.
- Wings are absents.
- Respiration Occurs through Book gills .
- They have special Appendages pedipalp for feeding and reproduction.
- Examples : Spider , Scorpion mites.
Class Crustacea:
- Body is divided into cephalothorax ( fused head and thorax) and abdomen.
- They have five pairs of jointed Appendages and two pair of antennae are present in front of mouth.
- Body is covered by the chitinous exoskeleton.
- They are aquatic and found in both marine and fresh water.
- Respiration Occurs through Book gills or Book lungs .
- They have two compound eye.
- Examples : Prawn , crab Shrimps.
Class Myriapoda :
- Body is divided into distinct head and trunk.
- They have many pairs of jointed Appendages and one pair of antennae are present on head.
- Body is covered by the chitinous exoskeleton.
- Respiration Occurs through tracheal system.
- They have simple and compound eye.
- Examples : Centipede, Millipede.
| Feature | Description |
|---|---|
| Body Structure | Segmented (Head, Thorax, Abdomen) |
| Exoskeleton | Made of Chitin (Prevents Desiccation) |
| Appendages | Jointed legs for specialized movement |
| Circulation | Open Circulatory System (Hemocoel) |
| Excretion | Malpighian Tubules |
| Sensory Organs | Statocysts (Balance) & Compound Eyes |
Ecological & Economic Importance
- Arthropods play a vital role in our ecosystem and economy. From pollinating our crops to producing silk, they are indispensable to human life.
- Pollinators: Essential for global food security. Insects like Bees and Butterflies pollinate about 75% of the world's food crops.
- Decomposers: Arthropods like millipedes and beetles break down dead organic matter, helping in nutrient cycling and keeping the soil fertile.
- Pests & Vectors: Some arthropods are harmful. Mosquitoes and ticks act as vectors that spread diseases like Malaria, Zika, and Lyme disease.
- Economic Products: Many insects provide valuable products:
- Honey Bees: Honey and Beeswax.
- Silkworms: Natural Silk fibers.
- Lac Insects: Shellac/Lac used in industries.
Test Paper 1: Foundations of Arthropoda (Grade 10 Level)
Time: 30 Minutes | Total Marks: 20
Section A: Multiple Choice (10 Marks)
-
Arthropods use which process to shed their rigid exoskeleton as they grow?
- A) Osmosis
- B) Moulting (Ecdysis)
- C) Fragmentation
- D) Metamorphosis
-
Which feature is unique to the subphylum Chelicerata (Spiders & Scorpions)?
- A) Three body segments
- B) Presence of antennae
- C) Absence of antennae and 4 pairs of legs
- D) Tracheal respiratory system
-
The "Open Circulatory System" in arthropods means the blood flows into:
- A) Closed capillaries
- B) A fluid-filled cavity called Haemocoel
- C) The digestive tract
- D) The nervous system
Section B: Critical Thinking (10 Marks)
4. Environmental Adaptation: Explain how the Chitinous Exoskeleton acted as a "pre-adaptation" that allowed arthropods to colonize land successfully. (4 Marks)
5. Structural Analysis: Contrast the respiratory systems of a Crab (Aquatic) and a Grasshopper (Terrestrial). Why can't a grasshopper breathe through its skin? (6 Marks)
Test Paper 2: Advanced Evolutionary Biology (NEET/AP Level)
Time: 40 Minutes | Total Marks: 30
Section A: MCQ & Assertion-Reason (15 Marks)
-
Malpighian tubules help in excretion by removing nitrogenous waste from the:
- A) Gut
- B) Haemolymph
- C) Coelom
- D) Nephridia
-
Assertion: Arthropods are the most successful group of animals on Earth.
Reason: They possess jointed appendages and a protective exoskeleton.
- A) Both are true and Reason is correct explanation.
- B) Both true but Reason is NOT correct explanation.
- C) Assertion is true, Reason is false.
Section B: Data & Diagrams (15 Marks)
3. Labeling Task: (Imagine a diagram of a Grasshopper) Identify the Spiracles and explain their role in the Tracheal system. (5 Marks)
4. Evolutionary Success: List three sensory advancements in Arthropoda (e.g., Compound eyes, Statocysts) and explain how they provide a survival advantage over Annelids. (10 Marks)
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