Question 30 from UPSC CSE 2014 - Science and Technology

Question

Among the following organisms, which one does not belong to the class of the other three?

IASCODEX · Accepted Answer

Correct Option: A. Crab

Strategy: Identify the organism with differing morphological features. Mite, scorpion, and spider are arachnids, characterized by two body segments and eight legs. The crab is a crustacean, with a carapace, ten legs, and antennae. This structural mismatch identifies the crab as the outlier.

Key Info: **Crabs**
Crabs are crustaceans belonging to the class Malacostraca, order Decapoda. They typically have a short projecting tail, a broad carapace, two pincers (chelae), and five pairs of legs, and are mostly aquatic. Crustaceans commonly possess antennae and gills for respiration.

**Arachnids**
Arachnids are a class (Arachnida) within the phylum Arthropoda. This class includes spiders, scorpions, mites, and ticks. Arachnids characteristically have two main body segments (cephalothorax and abdomen), four pairs of legs (total eight), no antennae, and most breathe air through book lungs or tracheae.
- **Spiders** (order Araneae) spin silk and are known for their eight-legged structure.
- **Scorpions** (order Scorpiones) are recognized by their segmented tails with a stinger.
- **Mites** (subclass Acari) are tiny, often microscopic, and diverse, inhabiting various environments.

**Conclusion**
Despite distinct lifestyles, spiders, scorpions, and mites share the key body plan features of Arachnida. Crabs, being crustaceans, are fundamentally different morphologically and taxonomically from members of this class.

IASCODEX · Answer

Option B: Mite

IASCODEX · Answer

Option C: Scorpion

IASCODEX · Answer

Option D: Spider