Among the following organisms, which one does not belong to the class of other three ?
Contents12
- ACrab
- BMite
- CScorpion
- DSpider
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Answer: (A) Crab
This is a classification question.
Mite, Scorpion, and Spider all belong to Class Arachnida (8 legs, no antennae, body divided into cephalothorax and abdomen).
Crab belongs to Class Crustacea (10 legs, two pairs of antennae, hard exoskeleton).
So Crab is the odd one out.
Quick identification trick:
- Count the legs —
- Arachnids = 8 legs,
- Insects = 6 legs,
- Crustaceans = usually 10 legs.
But beyond leg count, crabs breathe through gills and live in water/moist environments, while arachnids are mostly terrestrial and breathe through book lungs or tracheae.
Arthropods form the largest phylum in the animal kingdom, making up over 80% of all known animal species.
The question tests whether students can distinguish between the two major arthropod classes - Arachnida (spiders, scorpions, mites with 8 legs) versus Crustacea (crabs, lobsters with 10 legs and gills).
Class Arachnida
Environment Mite Scorpion Spider
Class Arachnida: Spiders, Scorpions & Mites
Arachnids have 8 legs and no antennae — key identification feature
Body divided into cephalothorax (fused head-chest) and abdomen
Include spiders, scorpions, mites, ticks — all terrestrial predators mostly
Breathe through book lungs or tracheae, not gills
What Are Arachnids
Class Arachnida belongs to phylum Arthropoda and includes terrestrial predators like spiders, scorpions, and mites. The 8-leg count is the quickest identification trick for UPSC questions.
Major Arachnid Groups
Group | Examples | Key Features | Habitat |
|---|---|---|---|
Spiders | House spider, tarantula | Silk glands, venomous fangs | Webs, burrows, vegetation |
Scorpions | Desert scorpion | Pincers, venomous tail sting | Deserts, rocks, soil |
Mites & Ticks | Dust mite, dog tick | Microscopic to small, parasitic | Soil, plants, on animals |
Harvestmen | Daddy longlegs | Extremely long legs, no venom | Gardens, forests |
Arachnid Body Plan
Cephalothorax: fused head and thorax region containing eyes and mouthparts
Abdomen: contains digestive and reproductive organs, silk glands in spiders
Chelicerae: fang-like mouthparts for injecting venom (spiders) or crushing prey
Pedipalps: sensory appendages, enlarged into pincers in scorpions
Leg count confusion: Arachnids = 8 legs, Insects = 6 legs, Crustaceans = 10+ legs
Antenna trap: Arachnids have no antennae unlike insects and crustaceans
Spider vs Scorpion: Both are arachnids despite very different appearance
Mite classification: Tiny mites are arachnids, not insects — count the legs
Class Crustacea
Environment Crab
Class Crustacea: Crabs, Lobsters & Aquatic Arthropods
Crustaceans have 10 legs (5 pairs) and 2 pairs of antennae
Mostly aquatic — breathe through gills, not lungs
Include crabs, lobsters, shrimps, barnacles
Hard calcareous exoskeleton that molts as they grow
Crustacean Characteristics
Class Crustacea includes mostly aquatic arthropods with a hard exoskeleton. Unlike arachnids, they have 10 legs and breathe through gills, making them adapted for water life.
Major Crustacean Groups
Group | Examples | Legs | Habitat | Economic Importance |
|---|---|---|---|---|
Decapods | Crab, lobster, shrimp | 10 legs | Marine, freshwater | Major seafood source |
Copepods | Water fleas | 10+ legs | Plankton in water | Base of aquatic food web |
Barnacles | Ship barnacles | Modified legs | Attached to rocks, ships | Marine fouling organisms |
Isopods | Pill bugs, woodlice | 14 legs | Some terrestrial | Soil decomposers |
Crab-Specific Features
Sideways locomotion: crabs walk sideways due to leg joint structure
Chelae: large front claws for defense, food capture, and communication
Molting: shed hard shell periodically to grow, vulnerable during soft-shell phase
Gill breathing: extract oxygen from water, some can survive briefly on land
Crab vs Spider: Crab = 10 legs + aquatic, Spider = 8 legs + terrestrial
Breathing confusion: Crabs use gills, arachnids use book lungs/tracheae
Antenna presence: Crabs have 2 pairs antennae, arachnids have none
Arthropod Classification System
Environment
Phylum Arthropoda: Classification by Leg Count & Body Plan
Largest animal phylum — insects, arachnids, crustaceans, myriapods
Jointed legs and segmented body are defining features
Leg count is the quickest way to identify arthropod classes
Arthropod Diversity
Phylum Arthropoda is the most successful animal group, with jointed legs and segmented bodies. UPSC often tests classification within this phylum using leg count and body structure.
Major Arthropod Classes
Class | Legs | Antennae | Body Segments | Examples | Habitat |
|---|---|---|---|---|---|
Insecta | 6 legs | 1 pair | Head, thorax, abdomen | Beetle, butterfly, ant | Terrestrial, some aquatic |
Arachnida | 8 legs | None | Cephalothorax, abdomen | Spider, scorpion, mite | Mostly terrestrial |
Crustacea | 10+ legs | 2 pairs | Cephalothorax, abdomen | Crab, lobster, shrimp | Mostly aquatic |
Myriapoda | Many legs | 1 pair | Head + many segments | Centipede, millipede | Terrestrial, moist soil |
UPSC Classification Approach
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Count the legs**
6 legs = insect, 8 legs = arachnid, 10+ legs = crustacean/myriapod`"]
s2["`**Check antennae**
Present or absent? How many pairs?`"]
s3["`**Observe habitat**
Aquatic suggests crustacean, terrestrial suggests arachnid/insect`"]
s4["`**Note body plan**
Fused segments vs separate head/thorax/abdomen`"]
s1 --> s2
s2 --> s3
s3 --> s4Don't memorize only names — focus on leg count + antenna presence
Size confusion: Tiny mites are arachnids, large crabs are crustaceans — size doesn't determine class
Habitat assumption: Some crustaceans live on land (pill bugs), some arachnids near water — habitat isn't foolproof