Question map
Among the following organisms, which one does not belong to the class of other three?
Explanation
Crabs are crustaceans[1], while mites, scorpions, and spiders are all arachnids[4]. These are different classes within the phylum Arthropoda. Arthropods include crustaceans, insects, and arachnids[6], making them distinct taxonomic groups.
Arachnids do not have antennae and have 2 body parts and 4 pairs of legs[3], whereas crustaceans have a hard, external shell which protects their body[1] and possess different anatomical features. The key distinction is that crab belongs to class Crustacea while mite, scorpion, and spider all belong to class Arachnida. Therefore, the crab is the organism that does not belong to the class of the other three.
Sources- [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Crustaceans > p. 155
- [2] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Arachnids > p. 156
- [3] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Arachnids > p. 156
- [4] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Arachnids > p. 156
- [5] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Arthropods > p. 155
- [6] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Arthropods > p. 155
PROVENANCE & STUDY PATTERN
Full viewThis is a classic 'General Science meets Environment' question. It rewards basic observation over rote memorization. If you read the introductory pages of the Biodiversity chapter in standard texts (like Shankar IAS), the distinction between Arachnids and Crustaceans is explicitly listed. Do not skip the 'Basics of Life' section thinking it is too elementary.
This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.
- Explicitly lists crab alongside lobster and barnacle as the most commonly known 'crustaceans'.
- Directly links the common name 'crab' to the group label 'crustaceans', supporting identification of crabs as crustaceans.
- Pairs the words 'Crabs, crustaceans' in describing seashore fauna, reinforcing that crabs are members of the crustacean group.
- Describes ecological role of crabs as 'crustaceans', providing contextual support for their taxonomic grouping.
- Explains the broader category 'invertebrates' (animals without backbones), into which crustaceans — and thus crabs — fall.
- Places crabs within larger taxonomic/functional groupings used in the snippets, supporting classification context.
- Explicitly lists mites among 'common arachnids', directly placing mites in Arachnida.
- The same snippet provides defining arachnid traits (no antennae; two body parts; four pairs of legs), which justify class-level assignment.
- Directly lists scorpions as examples of 'arachnids', identifying their class-level group.
- Gives characteristic features of arachnids (no antennae; two body parts; four pairs of legs) that match scorpion morphology.
- Places arachnids as a subgroup within Arthropoda, corroborating the higher-level taxonomy.
- Reinforces that arachnids (hence scorpions) are distinct from insects and crustaceans within arthropods.
- Explicitly lists spiders as common arachnids, directly linking spiders to the group Arachnida.
- Provides diagnostic arachnid features (no antennae; two body parts; four pairs of legs) that match spider morphology.
- Places arachnids within the larger arthropod phylum, situating spiders taxonomically among jointed-limb, exoskeleton-bearing animals.
- Describes arthropod characteristics (jointed limbs, exoskeleton) that are consistent with arachnid/spider anatomy.
- [THE VERDICT]: Sitter. Directly available in standard Environment reference books (Shankar IAS, Chapter 9) under the classification of Invertebrates.
- [THE CONCEPTUAL TRIGGER]: Phylum Arthropoda classification. The examiner is testing if you can distinguish between the major classes: Insecta vs. Arachnida vs. Crustacea.
- [THE HORIZONTAL EXPANSION]: Memorize the 'Leg Count' rule: Insects (3 pairs/6 legs, e.g., Ants, Mosquitoes); Arachnids (4 pairs/8 legs, e.g., Spiders, Scorpions, Ticks, Mites); Crustaceans (5 pairs/10 legs, e.g., Crabs, Lobsters, Prawns, Barnacles); Myriapods (Many legs, e.g., Centipedes).
- [THE STRATEGIC METACOGNITION]: Visualise the organism. UPSC often groups animals by morphology or habitat. If three are terrestrial 'creepy crawlies' (Spider, Scorpion, Mite) and one is a marine 'shellfish' (Crab), the odd one out is usually the one with a different habitat or respiratory system (Gills vs Lungs).
References explicitly identify crabs as examples of 'crustaceans' (crab, lobster, barnacle).
High-yield for biodiversity and ecology questions: knowing that crabs are crustaceans links them to arthropod/invertebrate discussions, coastal ecology, and conservation contexts. Master by memorizing major invertebrate groups and common examples; useful for classification and ecosystem role questions.
- Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Crustaceans > p. 155
- Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 3: MAJOR BIOMES > Classifcation of marine ecosystems > p. 31
Evidence describes invertebrates as animals without backbones and situates crustaceans within invertebrate fauna.
Core concept for UPSC biology/ecology sections: differentiating vertebrates vs invertebrates is often tested and underpins questions on biodiversity, habitats, and conservation. Study via classification charts and examples across phyla.
- Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > 9.2.2. Invertebrates > p. 154
- Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Crustaceans > p. 155
References note crabs' ecological roles (burrowing in sand, being prey, present in mangroves/marine biosphere reserves).
Useful for environment/ecology mains and prelims: links taxonomy with ecosystem function, conservation (e.g., biosphere reserves, CRZ areas). Prepare by relating species groups to habitats and ecosystem services; practice applied questions on coastal ecology.
- Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 3: MAJOR BIOMES > Classifcation of marine ecosystems > p. 31
- Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 2: Physiography > Islands of the Bay of Bengal > p. 68
- Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 4: Aquatic Ecosystem > CRZ.I > p. 54
The provided text gives key diagnostic features of arachnids (no antennae; 2 body parts; 4 pairs of legs), which explain why mites are assigned to this class.
High-yield for biodiversity/ecology questions: distinguishing arthropod classes (Arachnida vs Insecta) is commonly tested. Master by memorizing diagnostic morphological markers and representative taxa to eliminate distractors in MCQs and short-answer questions.
- Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Arachnids > p. 156
Other references identify mites as biological pollutants/allergens and mention varroa mites as parasites, highlighting their ecological and public-health relevance.
Useful for environment and ecology sections (pollution, public health, agricultural pests). Questions often link taxonomy with impact (e.g., allergen sources, parasitism in bees). Prepare by connecting taxonomic identity to ecological/health roles and case examples.
- Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > Pollutants > p. 66
- Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 6: Environmental Issues > Varroa mite - parasites > p. 119
Reference [1] explicitly names scorpions as arachnids, so recognizing 'Arachnida' as the class is central to the statement.
Knowing common examples of taxonomic classes is high-yield for biodiversity and classification questions. It connects to taxonomy-based MCQs and short-answer prompts; revise lists of representative organisms per class and practice classification questions.
- Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Arachnids > p. 156
Reference [6] groups arachnids with insects and crustaceans under Arthropoda, showing hierarchical taxonomy.
Understanding phylum-level divisions and their constituent classes helps answer comparative and hierarchical taxonomy questions in GS science and environment topics. Study the defining features of each arthropod subgroup and their examples.
- Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 9: Indian Biodiversity Diverse Landscape > Arthropods > p. 155
The 'Horseshoe Crab' (Living Fossil). Despite the name, it is taxonomically closer to Spiders and Scorpions (Chelicerata) than to true Crabs (Crustacea). This is a classic future trap.
Use the 'Habitat/Menu' Heuristic. Crabs are 'Seafood' (Crustaceans, aquatic/semi-aquatic). Spiders and Scorpions are never on a seafood menu; they are terrestrial predators often found in dry corners of a house. Even if you don't know what a 'Mite' is, the Crab is the only one that lives in the ocean/water and has gills. Mark [A] and move on.
Link Mites to Agriculture & Economy (Mains GS-3). 'Varroa Mites' are a primary cause of Colony Collapse Disorder in bees, threatening global food security and pollination services. This connects a tiny taxonomy fact to a massive economic crisis.