UPSC Prelims 2014 Q74: Science & Technology – Biology
Among the following organisms, which one does not belong to the class of other three?
- (a) Crab
- (b) Mite
- (c) Scorpion
- (d) Spider
Answer: (a) Crab
Explanation
Which organism does not belong with the other three?
All four belong to the phylum Arthropoda, but to different classes.
| Organism | Class | Legs |
|---|---|---|
| (a) Crab | Crustacea | Ten (five pairs) |
| (b) Mite | Arachnida | Eight |
| (c) Scorpion | Arachnida | Eight |
| (d) Spider | Arachnida | Eight |
Classes of Arthropoda:
| Class | Features | Examples |
|---|---|---|
| Crustacea | Mostly aquatic; gills; two pairs of antennae | Crab, prawn, lobster |
| Arachnida | Four pairs of legs; no antennae | Spider, scorpion, tick, mite |
| Insecta | Three pairs of legs; wings | Mosquito, butterfly |
| Myriapoda | Many legs | Centipede, millipede |
Answer: (a) Crab.
Why each option is right or wrong
- (a) CORRECT. The crab is a crustacean; the other three are arachnids.
- (b) Incorrect. The mite is an arachnid, like the scorpion and spider.
- (c) Incorrect. The scorpion is an arachnid.
- (d) Incorrect. The spider is an arachnid.
Key point
Arachnida (eight legs): spider, scorpion, mite, tick. Crustacea: crab, prawn, lobster. Insecta: six legs. All are arthropods.
Remember
Crab = crustacean; spider, scorpion, mite = arachnids.
Common mistakes
- Calling spiders and scorpions insects.
- Not knowing that mites and ticks are arachnids.
- Missing the word 'not' in the question.
Exam tip
Count the legs: six = insect, eight = arachnid, ten = crab.
Sources
- NCERT Class XI — Biology, Ch 4 (Animal Kingdom: Phylum Arthropoda)
- NCERT Class IX — Science, Ch 7 (Diversity in Living Organisms)