Arrange the following structures according to their decreasing number .
1. Hepatic or gastric caeca
2. Malpighian tubules
3. 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 will analyze the number of each structure mentioned in the question.

### Step 1: Identify the number of each structure
1. **Hepatic or Gastric Caeca**: These structures are found in cockroaches and are typically 6 to 8 in number.
2. **Malpighian Tubules**: These are also found in cockroaches and are present in a number ranging from 100 to 150.
3. **Ommatidia**: These are the units of the compound eyes in cockroaches, and there are approximately 2000 ommatidia per eye.

### Step 2: Compare the numbers
- **Ommatidia**: Approximately 2000
- **Malpighian Tubules**: 100 to 150
- **Hepatic or Gastric Caeca**: 6 to 8

### Step 3: Arrange in decreasing order
Based on the numbers we identified:
1. Ommatidia (approximately 2000)
2. Malpighian Tubules (100 to 150)
3. Hepatic or Gastric Caeca (6 to 8)

### Final Arrangement
Thus, the final arrangement of the structures according to their decreasing number is:
1. Ommatidia
2. Malpighian Tubules
3. Hepatic or Gastric Caeca