To determine which statement is true for cockroaches, let's analyze each of the provided statements step by step:
### Step 1: Analyze the First Statement
**Statement:** There are three ganglia in the thorax and nine in the abdomen.
- **Analysis:** Cockroaches have a total of 9 ganglia in their nervous system. These are arranged segmentally, with 3 ganglia located in the thorax and 6 in the abdomen. Therefore, this statement is **incorrect**.
### Step 2: Analyze the Second Statement
**Statement:** The eye consists of 6 hexagonal ommatidia.
- **Analysis:** Cockroaches possess compound eyes made up of thousands of ommatidia. Specifically, they have around 2000 ommatidia, not 6000 or 6. Thus, this statement is also **incorrect**.
### Step 3: Analyze the Third Statement
**Statement:** The fat body, nephrocyte, and uricose gland help in excretion.
- **Analysis:** This statement is **correct**. Cockroaches are uricotelic organisms, meaning they excrete nitrogenous waste in the form of uric acid. The fat body, nephrocytes, and uricose glands play significant roles in this excretion process, along with Malpighian tubules.
### Step 4: Analyze the Fourth Statement
**Statement:** The respiratory system consists of a network of trachea that opens through eight pairs of spiracles.
- **Analysis:** Cockroaches have a respiratory system that includes a network of tracheae, but they have **ten pairs** of spiracles, not eight. Therefore, this statement is **incorrect**.
### Conclusion
After analyzing all four statements, the only true statement regarding cockroaches is the third one: "The fat body, nephrocyte, and uricose gland help in excretion."
### Final Answer
The correct answer is: **The fat body, nephrocyte, and uricose gland help in excretion.**
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