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A Question About Insect Blood

A question came in this week that was out of the ordinary and I thought I’d share it here.

Question:  “I have a crazy question and figured that you are the best person to answer it.  On Wednesday, I am leading a class about the skeletal system, and I am anticipating a question from one of the younger kids.  I don’t know how to answer it.  How do insects make their blood?  In humans, blood is manufactured in the bone marrow.  Is the exoskeleton of an insect also responsible for blood production?”

Answer:  An insect's blood is called hemolymph (or sometimes haemolymph), and it circulates around the interior body cavity, between the exoskeleton and the inner organs. It is a yellow, greenish or pale-colored fluid. The hemolymph is moved about by the insect’s hearts and by the movement of muscles, but the whole system is much more open than that of vertebrates. The hemolymph is not carried in closed channels like the arteries and veins of humans; it flows freely.

The liquid part of the hemolymph, or plasma, is about 90% water. The water comes from the insect’s food and what it drinks. The water enters the body cavity through the cells of the digestive tract. Right before an insect sheds its exoskeleton or molts, it increases the volume of liquid inside its body, and thus pressure inside. The insect does this both by excreting less (its "kidneys" are called malphigian tubules) and also by drinking more. The increased pressure is used to expand the new, soft exoskeleton while the insect is molting. After the insect has finished molting and its exoskeleton has hardened, it excretes the excess water to reduce the pressure to normal again.

Within the plasma are cells called hemocytes, which carry out some of the same functions as our white blood cells, such as capturing foreign particles (phagocytosis) and wound healing. The hemocytes are derived from the mesoderm in the embryo (which one of the embryonic tissues). It is thought that the hemocytes formed in the embryo give rise to all the new hemocytes through cellular divisions. Insects don't have anything analogous to human bone marrow.

For the most part, the hemolymph does not carry oxygen, which is one of the important functions of our blood. Instead tiny tubes called trachea carry oxygen in insects. (You can see the outside openings of the trachea on the sides the green June beetle larva in the previous post.) The trachea reach all the way down to the cellular level. Thus, the hemolymph system in insects is more similar to our lymphatic system.

And, there’s no such thing as a crazy question! ☺

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