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Haemolymph is the transport medium in insects, playing a critical role similar to that of blood in vertebrates. Unlike blood, which circulates within blood vessels, haemolymph flows freely within the body cavity of insects. It serves several vital functions, including the transport of nutrients, hormones, and waste products, and assisting in the immune response and the distribution of heat within the insect's body.

This fluid is crucial for maintaining internal conditions and ensuring that various physiological processes occur efficiently in an environment that can change rapidly. Haemolymph contains cells called hemocytes, which are involved in immune responses, and it is composed of a liquid portion known as plasma, along with these cellular components.

The other options do not accurately describe haemolymph: it is not a type of blood in mammals, nor is it a hormone or a nutrient storage substance. This distinction is essential for understanding the physiological differences between vertebrates and invertebrates, particularly in their circulatory systems.