Question

Which of the following are De Man’s ratios used for nematode taxonomy?
(A). P
(B). c'
(C). K'
(D). A Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A) and (D) only.
• C. (A) and (B) only.
• D. (B), (C) and (D) only.

Hint:

Remember that the classic de Man indices are \( a, b, c, \) and \( V \). Modified ratios like \( c' \) (tail ratio) are also considered part of the expanded de Man system.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Nematode taxonomy heavily relies on morphometric measurements to differentiate genera and species.
To account for differences in body size due to age or environmental conditions, early nematologist J.G. de Man introduced a set of standardized ratios (commonly known as the de Man formula or de Man indices).
These ratios express the relative proportions of various body parts compared to total body length or other landmark anatomical measurements.

Step 2: Detailed Explanation:
The classic de Man formula consists of several primary indices:
- The ratio \( a \) represents the total body length divided by the maximum body width.
- The ratio \( b \) represents the total body length divided by the esophageal length.
- The ratio \( c \) represents the total body length divided by the tail length.
Over time, nematologists introduced modified ratios to improve taxonomic accuracy, especially for traits that vary with life stage or specimen preparation.
Among these, the ratio \( c' \) is widely used.
It is defined as the tail length divided by the body width at the anus or cloaca.
This ratio is highly useful for characterizing tail shape, which is a key diagnostic feature in many nematode families.
In standard nematology databases and question banks, the ratios \( a, b, c, c', \) and \( V \) (the distance from the anterior end to the vulva as a percentage of body length) are grouped under the de Man system.
In this question, the options represent these standardized symbols, where \( P \) (often representing the position of the excretory pore as a ratio) and \( c' \) are valid morphometric ratios.
Therefore, (A) and (B) are the correct taxonomic ratios under the modified de Man system.

Step 3: Final Answer:
The ratios \( P \) and \( c' \) are used in nematode morphometrics.
Thus, the correct option is 3.