Flies do not “bleed” in the same manner as humans or other vertebrates. When a fly sustains an injury or is crushed, the fluid that emerges is not blood, but a substance called hemolymph. This fluid is the insect equivalent of blood, yet it circulates through a fundamentally different system than the closed circulatory system found in many animals. The reddish marks sometimes seen after a fly is crushed are often pigments from its compound eyes, not from its internal fluid.
Understanding Insect “Blood”: Hemolymph and Open Circulation
Hemolymph is the body fluid that circulates within the internal cavities of insects and other arthropods. It functions similarly to blood by transporting nutrients, hormones, and waste products throughout the body. The fluid is primarily composed of water and contains various dissolved substances.
Hemolymph contains inorganic salts and organic compounds like carbohydrates (including trehalose), proteins, lipids, and amino acids. It also carries hormones and metabolic waste products. Unlike vertebrate blood, which is red due to iron-based hemoglobin that carries oxygen, insect hemolymph is typically clear, yellowish, or greenish because it generally does not transport oxygen.
The circulatory system of flies is an open system. This means that hemolymph flows freely within the body cavity, known as the hemocoel, directly bathing all internal tissues and organs. This contrasts with the closed circulatory system of vertebrates, where blood is always contained within a network of vessels.
Hemolymph circulation is driven by a single, tube-like organ called the dorsal vessel. The dorsal vessel is divided into a heart section in the abdomen and an aorta extending towards the head. The heart contracts rhythmically, pumping hemolymph forward through valved chambers and ostia into the hemocoel. Accessory pulsatile organs assist in circulating hemolymph into appendages.
How Flies Heal: Responding to Injury
When a fly sustains an injury that breaks its exoskeleton, hemolymph will leak out. Insects have developed efficient clotting mechanisms to prevent excessive fluid loss and to protect against invading pathogens. The leaking hemolymph quickly forms a plug or clot to seal the wound.
Specialized immune cells called hemocytes circulate within the hemolymph and are key to healing. Upon injury, these hemocytes rapidly aggregate at the wound site, playing a central role in coagulation and wound repair. Granulocytes can degranulate, releasing substances that contribute to the formation of an extracellular matrix, creating a soft clot.
Other hemocytes, such as plasmatocytes, contribute by aggregating and covering the forming clot. The clotting process involves various factors, including transglutaminase and prophenoloxidase (PPO). The PPO system leads to the production of melanin to harden the clot and encapsulate invading microorganisms.
The rigid structure of the exoskeleton provides a surface for the hemolymph to clot against, aiding wound closure. This clotting response is remarkably fast, with a clot becoming insoluble in under three minutes. The insect’s healing process, while effective, differs significantly from the complex clotting cascade seen in vertebrates.