Introduction:
The largest tergal part in a cockroach refers to the largest dorsal (upper) plate on the insect's thorax. In cockroaches, there are three tergal parts: mesonotum, metanotum, and pronotum. Among these three, the largest tergal part is the mesonotum.

Explanation:
To understand why the mesonotum is the largest tergal part in cockroaches, let's take a closer look at each of these tergal parts:

1. Pronotum:
The pronotum is the anterior-most tergal plate located on the thorax of a cockroach. It covers the prothorax and serves as a protective shield for the head. However, it is relatively smaller in size compared to the other two tergal parts.

2. Mesonotum:
The mesonotum is the middle tergal plate on the thorax of a cockroach. It covers the mesothorax, which is responsible for the movement of the middle pair of legs. The mesonotum is the largest tergal part in cockroaches because it needs to provide a larger surface area for muscle attachment and support the movement of the powerful middle legs.

3. Metanotum:
The metanotum is the posterior-most tergal plate located on the thorax of a cockroach. It covers the metathorax, which is responsible for the movement of the hind pair of legs. The metanotum is relatively smaller in size compared to the mesonotum because the hind legs are not as strong or powerful as the middle legs.

Conclusion:
In conclusion, the largest tergal part in a cockroach is the mesonotum. It covers the middle thoracic segment and provides a larger surface area for muscle attachment, especially for the powerful middle legs. The mesonotum plays a crucial role in facilitating the cockroach's locomotion and overall movement.