UPSC Prelims 2014 · Question 54 of 100
UPSC Prelims 2014 question on Arachnid vs Crustacean
- ExamUPSC CSE
- Year2014
- PaperGeneral Studies Paper I
- SubjectEnvironment and Ecology
- TopicBiodiversity
- DifficultyMedium
- TypeDirect
Among the following organisms, which one does not belong to the class of other three?
Show answer
Answer: A. Crab
Verdict
The answer is crab. Mite, scorpion and spider are arachnids, while the crab is a crustacean.
Analysis
All four are arthropods, which is what makes the question look harder than it is, but the class differs. Arachnids have four pairs of walking legs, two body regions and no antennae, and mites, scorpions and spiders all fit that description. Crabs belong to Crustacea, which typically have two pairs of antennae, biramous appendages and, in decapods, five pairs of legs, and they are overwhelmingly aquatic while arachnids are overwhelmingly terrestrial.
Source
Merriam-Webster medical entry on Arachnida.
How to crack it
Odd one out questions in zoology are decided by one diagnostic character, so pick a character and apply it to all four rather than reasoning about each animal separately. Antennae work well here, since arachnids have none and crustaceans have two pairs. Habitat is a useful cross check: three land dwellers and one aquatic animal is a strong hint before you reach for taxonomy at all.
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