Many people assume all animals possess red blood, similar to humans. While red blood is prevalent, especially among vertebrates, flies and most other insects have a surprisingly different internal fluid.
The Truth About Fly “Blood”
Instead of blood, flies circulate a fluid called hemolymph. This substance is typically clear, though it can sometimes appear yellowish or greenish. Unlike the closed circulatory system in humans, where blood is confined within vessels, a fly’s hemolymph flows freely throughout its body cavity, directly bathing its internal organs. This open circulatory system facilitates the exchange of substances. A primary distinction is that hemolymph’s role is not oxygen transport.
Why It’s Not Red
The red color of human blood comes from hemoglobin, an iron-rich protein in red blood cells that transports oxygen. Flies and most insects do not possess hemoglobin for oxygen transport. Therefore, their “blood” lacks the red pigment associated with this protein.
Instead of relying on circulatory fluid for oxygen delivery, flies utilize a highly efficient respiratory system called the tracheal system. This system consists of a network of tubes that open to the outside through small pores called spiracles, located along the insect’s body. Oxygen enters these spiracles and travels directly through the tracheal tubes, branching into smaller tracheoles that deliver oxygen directly to cells and tissues. This direct delivery bypasses the need for an oxygen-carrying pigment in the hemolymph, explaining its lack of red color.
The Many Roles of Hemolymph
While hemolymph does not carry oxygen, it performs many other functions within the fly’s body. It acts as a comprehensive transport system, distributing absorbed nutrients like sugars, amino acids, and lipids from the digestive system to all cells and organs for energy, growth, and repair. Hemolymph also collects metabolic waste products from tissues and transports them to excretory organs for removal.
Hemolymph is an integral part of the fly’s immune defense system. It contains specialized cells called hemocytes, which can engulf pathogens and foreign particles, protecting the insect from infections. Hemolymph also circulates hormones throughout the body, regulating various physiological processes, including growth, development, and reproduction. Beyond transport, hemolymph contributes to maintaining the insect’s body shape and facilitates movements, such as wing expansion after emergence, by exerting hydraulic pressure.