In insects, the circulating body fluid (haemolymph) lacks respiratory pigments such as haemoglobin. What is the main reason insects do not have blood pigments?

Introduction / Context:
In vertebrates such as humans, blood contains respiratory pigments like haemoglobin that bind and transport oxygen and carbon dioxide. Insects, however, have a very different circulatory and respiratory system. Their haemolymph is usually colourless or pale and lacks such pigments. This question asks you to identify the main physiological reason why insects do not have blood pigments in their circulating body fluid.

Given Data / Assumptions:

We are dealing with insects, which have an open circulatory system. Their body fluid (haemolymph) flows freely in body cavities rather than in closed vessels. Options suggest various possible reasons for lacking respiratory pigments. We assume standard knowledge of insect respiration via tracheal systems.

Concept / Approach:
Insects respire primarily through a system of air filled tubes called tracheae, which open to the outside through spiracles and branch throughout the body. Oxygen is delivered directly to tissues through these tubes, and carbon dioxide is removed by the same route. Because gases travel directly to and from cells via the tracheal system, the haemolymph does not need to transport oxygen, unlike vertebrate blood. As a result, there is no strong selective pressure for respiratory pigments in the haemolymph, and it often appears colourless. Therefore, the main reason insects lack blood pigments is that their blood does not have a respiratory function.

Step-by-Step Solution:
Step 1: Recall that vertebrate blood uses haemoglobin to carry oxygen from lungs to tissues and carbon dioxide back. Step 2: Recognise that in insects, a tracheal system delivers oxygen directly from spiracles to body cells. Step 3: Understand that insect haemolymph mainly distributes nutrients, hormones and waste products but does not carry much oxygen. Step 4: Therefore, respiratory pigments like haemoglobin are not required in the haemolymph. Step 5: Conclude that the absence of respiratory function in insect blood explains the lack of blood pigments.

Verification / Alternative check:
If you examine diagrams of insect anatomy, you see a network of tracheae permeating the body, with very little emphasis on the circulatory system for gas transport. Textbooks explicitly state that insect circulation is not involved in oxygen transport, unlike in vertebrates. Instead, haemolymph bathes organs directly in an open system and carries nutrients and waste. This consistent explanation across entomology and zoology sources confirms that the lack of respiratory function in the haemolymph is the key reason for the absence of respiratory pigments.

Why Other Options Are Wrong:
Option a is incorrect because insects do have a body fluid, called haemolymph, which is analogous to blood in many functions, although it operates in an open system. Option c is wrong because insect tissues do not contain a separate oxygen carrying pigment system equivalent to haemoglobin that replaces blood pigments. Option d is incorrect because insects do not store respiratory pigments in special organs for gas transport; instead, they rely on direct air flow through tracheae. Option e is wrong because many insects live on land as well as in water, and their lack of pigments is not due to aquatic life but to their tracheal respiratory system.

Common Pitfalls:
A frequent misconception is that any animal with “blood” must have haemoglobin or similar pigments, leading students to overlook alternative respiratory strategies. Another pitfall is to confuse haemolymph with vertebrate blood and assume identical functions. To avoid these errors, remember that insects separate respiratory gas transport (tracheae) from nutrient and waste transport (haemolymph). Whenever a question asks why insect blood lacks pigments, the correct reasoning is that their blood simply does not need to carry oxygen in the way vertebrate blood does.

Final Answer:
Insects do not have blood pigments because their blood (haemolymph) does not perform a respiratory gas transport function; instead, gases are carried directly by the tracheal system.