At body sizes where simple diffusion is inadequate, systems for compartmentalizing functions develop. No longer is it feasible for food molecules to diffuse through an entire body's surface, so a gut with intake controlled by the head, manages intake of fuel. A kidney, or in our bee's case, her Malphigian tubules, take over excreting wastes, and a gill, lung or tracheal system, exchanges respiratory gases. So, at last, a circulatory system begins transporting substances around the body.

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