Ans- nodulus
Each setae consists of 3 parts: Upperparts is neck, Middle swollen part is nodulus and inner part-is root or body, which is attached with setal sac along with muscles. The clitellar segment possess setae when the worms are immature but setae are shed off before the clitellum is formed at maturity.

Explanation:
The middle part of setae in an earthworm is called the nodulus. This structure plays a crucial role in the locomotion of earthworms. Let's explore this in more detail:

Earthworm Setae:
- Setae are bristle-like structures found on the ventral surface of an earthworm.
- These setae help the earthworm in anchoring and gripping the soil during burrowing and locomotion.
- The setae are present in multiple rows along the length of the earthworm's body.

The Nodulus:
- The nodulus is the middle part of each seta.
- It is a small swelling or bulge present in the seta.
- The nodulus is located between the pointed tip (acicular part) and the base (basal part) of the seta.

Functions of the Nodulus:
1. Flexibility: The nodulus provides flexibility to the seta, allowing it to move and bend during the earthworm's locomotion.
2. Anchoring: The nodulus helps to anchor the seta firmly in the epidermal cell, providing stability during movement through the soil.
3. Sensory Function: The nodulus contains sensory receptors that allow the earthworm to detect changes in its external environment.
4. Secretion: The nodulus is responsible for the secretion of mucus, which aids in the earthworm's movement through the soil.

Other Parts of Setae:
- Acicular Part: It is the pointed tip of the seta that helps in piercing the soil.
- Basal Part: It is the part of the seta that originates from the epidermal cell.

Conclusion:
In conclusion, the middle part of setae in earthworms is called the nodulus. This structure provides flexibility, anchoring, sensory function, and secretion capabilities to the setae, enabling the earthworm to efficiently move through the soil.