If you have ever squished an insect, you have probably noticed the greenish-yellow fluid that comes out. It looks kind of like blood, but is it actually blood?
The answer is no. Insects do not have blood, or at least not blood like vertebrates and mammals have. Insect “blood” is called hemolymph, and, while it is analogous to blood and plays a similar role as blood, it is different in a few key respects.
Unlike vertebrates such as mammals and reptiles, insects have an open circulatory system that lacks veins and arteries. Hemolymph flows freely through an insect’s bodily cavity (called the hemocoel) and bathes its internal organs.
Like blood, hemolymph is made mostly out of water and carries important nutrients to the insect’s organs such as ions, carbohydrates, lipids, glycerol, and amino acids. Insect hemolymph also transports waste products and hormones.
However, unlike blood, hemolymph does not contain any respiratory compounds such as hemoglobin that carry oxygen. Instead, oxygen exchange for insects occurs via direct diffusion through bodily tissues and small tracts called “trachea” that are located all over the exoskeleton. These small openings take in oxygen and also remove carbon dioxide from the insect’s hemocoel.
This is the main reason why hemolymph has the color it does. Our blood appears red because of the hemoglobin it contains. Since hemolymph does not contain hemoglobin, it does not appear red. Instead, hemolymph has a yellowish-green tinge.
How Is Hemolymph Circulated Through the Body?
Insect hemolymph flows freely through an insect’s bodily cavity. But how is hemolymph directed to where it needs to go?
Funnily enough, insects do have bodily structures that can be called hearts. The insect heart is contained in the dorsal section of the abdomen and contains muscles and small openings called ostia that allow the hemolymph to flow in and out.
As the heart pumps, it pushes hemolymph out of the ostia and through the body towards the head. As it moves, it comes into contact with the organs and gives them vital nutrients.
Insect hearts are different from mammalian hearts in several respects, but the genes that encode for insect hearts are very similar to those that encode for the heart in mammals.
What Else Does Hemolymph Do?
Aside from transporting nutrients, hemolymph plays an important role in hatching, molting, reproduction, and movement in insects. For example, hemolymph fluid creates hydrostatic pressure that expands a newly molted butterfly’s wings. Hydrostatic pressure from hemolymph also explains how crickets can jump extremely far distances with their legs.
Hemolymph also plays an important role in the insect immune system. Hemolymph contains cells called hemocytes, which engulf harmful bacteria and pathogens and then break them down so they are harmless. In that sense, hemocytes can be considered analogous to white blood cells (phagocytes) in mammalian blood.
So, while insects do not have blood properly speaking, they have a similar fluid that is analogous in many respects and plays a similar role as blood does in vertebrates.