Insect of family Chironomidae, specifically Chironomous larvae, are commonly known as bloodworms due to their characteristic red coloration. The correct answer to why they are called bloodworms is option 'A' - they contain hemoglobin as their respiratory pigment.

Explanation:
1. Chironomidae:
Chironomidae is a family of insects commonly known as non-biting midges or chironomids. They are found in various aquatic habitats, including freshwater lakes, ponds, and rivers. The larvae of chironomids are often called bloodworms.

2. Hemoglobin as respiratory pigment:
Hemoglobin is a protein found in the red blood cells of vertebrates that plays a crucial role in carrying oxygen. However, chironomid larvae are not vertebrates, but they have a similar respiratory pigment called hemoglobin. Hemoglobin in chironomid larvae serves the same function of oxygen transport as it does in vertebrates.

3. Red coloration:
The presence of hemoglobin gives chironomid larvae their characteristic red coloration. Hemoglobin contains iron, which gives the larvae a reddish appearance. This is why they are often referred to as bloodworms.

4. Oxygen transport:
Chironomid larvae live in aquatic environments, where they require oxygen for respiration. Hemoglobin enables the larvae to efficiently transport oxygen from the surrounding water to their tissues. This adaptation allows them to survive in low oxygen environments.

5. Hemoglobin vs. Hemocyanin:
Hemoglobin is the respiratory pigment found in most animals, including humans. It binds with oxygen and transports it throughout the body. On the other hand, hemocyanin is a respiratory pigment found in some invertebrates, such as certain crustaceans and mollusks. It contains copper instead of iron and gives a blue coloration.

In conclusion, chironomid larvae are called bloodworms because they contain hemoglobin as their respiratory pigment, which gives them their characteristic red coloration. This adaptation allows them to efficiently transport oxygen in their aquatic environments.