When a bug is squished, the liquid that emerges is not blood like humans have. Insects lack a closed circulatory system with red blood cells and hemoglobin. This distinct biological fluid, called hemolymph, functions differently, clarifying how insect bodies operate.
The Unique Insect Circulatory System
Insects operate with an open circulatory system, unlike the closed system of humans and other vertebrates. In a closed system, blood remains contained within vessels and is continuously pumped by a heart. Conversely, an insect’s internal fluid flows freely within its body cavity, directly bathing organs and tissues.
This circulating fluid is called hemolymph, filling the insect’s body cavity, known as the hemocoel. Hemolymph consists of fluid plasma and various cells called hemocytes. Unlike vertebrate blood, hemolymph does not primarily transport oxygen. Instead, insects utilize a separate system of tubes, called tracheae, which directly deliver oxygen to their tissues.
Hemolymph performs several functions, including transporting nutrients, hormones, and waste products. It also plays a role in immune responses, with hemocytes engulfing foreign particles and aiding in wound healing through clotting. The dorsal vessel, a muscular tube along the insect’s back, acts as a pump, moving hemolymph from the abdomen towards the head.
What You See When a Bug is Squished
The fluid that appears when an insect is squished is indeed its hemolymph. This liquid is typically clear, yellowish, or sometimes greenish. The absence of red blood cells and oxygen-carrying pigments like hemoglobin, which gives vertebrate blood its characteristic red color, explains this appearance.
The composition of hemolymph includes water, inorganic salts, amino acids, proteins, and various organic compounds. Hemocytes, which are insect immune cells, are also suspended within this fluid. When an insect is injured, these hemocytes contribute to a clotting reaction, similar to how blood clots in vertebrates, helping to seal wounds and prevent fluid loss. Therefore, the liquid observed is a functional circulatory fluid, albeit one that differs significantly from human blood.
Why Colors Vary
While hemolymph is often clear or pale, its color can sometimes vary, appearing green, blue, or even orange. These variations are generally not due to oxygen transport, but to other factors. For example, a greenish tint can result from the insect’s diet, particularly in herbivores that consume plant pigments like chlorophyll, or from a mixture of blue and yellow chromoproteins.
Some insects possess hemocyanin, a copper-based protein that can transport some oxygen and may give the hemolymph a bluish or greenish hue when oxygenated. However, hemocyanin is not the primary oxygen carrier in most insects. Other pigments, metabolic byproducts, or even the presence of certain immune cells can also influence the color of hemolymph, providing a range of appearances beyond the typical clear or yellow.