Arrange the following structures according to their decreasing number.
Hepatic or gastric caeca, Malpighian tubules, Ommatidia

Options

A. Malpighian tubule gt Ommatidia gt Hepatic caeca
B. Ommatidia gt Malpighian tubule gt Hepatic caeca
C. Hepatic caeca gt Ommatidia gt Malpighian tubule
D. Malpighian tubule gt Ommatidia gt Hepatic caeca

Answer

To solve the problem of arranging the structures according to their decreasing number, we need to identify the number of each structure in a cockroach. The structures in question are Malpighian tubules, Ommatidia, and Hepatic (or gastric) caeca.

### Step-by-Step Solution:

1. **Identify the Number of Ommatidia**:
- Each compound eye of a cockroach contains approximately 2000 ommatidia. Since a cockroach has two eyes, the total number of ommatidia is:
\[ 2000 \text{ (per eye)} \times 2 \text{ (eyes)} = 4000 \text{ ommatidia} \]

2. **Identify the Number of Malpighian Tubules**:
- A cockroach typically has between 100 to 150 Malpighian tubules. For the sake of this arrangement, we can take an average number, but we will note the range:
\[ 100 \text{ to } 150 \text{ Malpighian tubules} \]

3. **Identify the Number of Hepatic Caeca**:
- The number of hepatic (or gastric) caeca in a cockroach is generally between 6 to 8. Again, we can note the range:
\[ 6 \text{ to } 8 \text{ hepatic caeca} \]

4. **Arrange the Structures in Decreasing Order**:
- Now that we have the numbers:
- Ommatidia: 4000
- Malpighian tubules: 100 to 150
- Hepatic caeca: 6 to 8
- We can arrange them in decreasing order:
1. Ommatidia (4000)
2. Malpighian tubules (100-150)
3. Hepatic caeca (6-8)

### Final Arrangement:
- The final arrangement according to their decreasing number is:
1. Ommatidia
2. Malpighian tubules
3. Hepatic caeca

### Answer:
Ommatidia > Malpighian tubules > Hepatic caeca