Johnston's organ occurs in

Options

A. Spiders
B. Centipedes
C. Crabs
D. Insects.

Answer

**Step-by-Step Solution:**

1. **Understanding Johnston's Organ**:
- Johnston's organ is a collection of sensory cells.
- It is located in the pedicel of the antenna.

2. **Location of the Pedicel**:
- The pedicel is found in the second segment of the antenna.

3. **Classification**:
- Johnston's organ is specifically associated with the class Insecta (insects).

4. **Function of Johnston's Organ**:
- The primary function is to detect motion, particularly the motion of the flagellum, which is the third segment of the antenna.

5. **Unique Feature**:
- The presence of Johnston's organ is a defining characteristic that differentiates insects from other hexapods.

6. **Historical Context**:
- The organ is named after Christopher Johnston, a physician.

7. **Analyzing the Options**:
- The options provided were:
- Spiders: Incorrect, as Johnston's organ is not found in spiders.
- Centipedes: Incorrect, as it is also not present in centipedes.
- Crabs: Incorrect, as crabs do not have Johnston's organ.
- Insects: Correct, as Johnston's organ is found in insects.

8. **Conclusion**:
- Therefore, the correct answer is that Johnston's organ occurs in insects.

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