Skip to content


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! ☺


This week we were grubbing around in the compost heap when we found this. We were quite excited.

green june beetle grub

It's about the size of my thumb. I can tell it is a beetle larva from the fact it has a hard head capsule (dark brown) and six distinct legs right behind its head. Those small brown circles on its sides are the opening of the airways it uses to breathe. Those are called trachea.

green june beetle grub

I knew what kind of beetle it was when it flipped onto its back and began crawling along upside down. My son said this was “Freaky!” (I think that is tween-speak for “Really cool!”) You can see the legs better. It also has bristly brown hairs. It quickly crawled off, upside down.

green june beetle grub

This interesting critter is a green June beetle grub, Cotinus mutabilis. They are a type of scarab beetle. The larvae (grubs) feed on compost and help with decomposition. They are up to two inches long when mature. Next Spring it will pupate in the soil and emerge as an adult.

The adults have beautiful metallic green wings with brownish-gold at the margins. They congregate in large numbers to feed on various types of soft fruit, which gives them their other common name, figeater beetle. Their normal food in Arizona is prickly pear or saguaro cactus fruit. They also visit our desert willow flowers for nectar.

I only have a photo of a preserved specimen. Don't worry, all of our preserved bugs died of natural causes.

green june beetle

The adults are even shinier when they are alive. Isn't it interesting such a "freaky" larva can turn into such a spectacular adult? Talk about the ugly duckling...

By the way, if this grub hadn't scooted away so quickly, it would probably have been bird food. Grackles love them.


The bug of the week is a tiny beauty with an attitude. When I first spotted this bug I thought she might be an ant.

bethylidae wasp

But then I looked again. This insect was just not acting like an ant.  She was climbing around on a plant like she was searching for prey, more like a wasp.

bethylidae wasp bethylidae wasp

She doesn’t look quite like an ant either. Her antennae are not elbowed like an ant’s would be. Bug Guide has some pictures of bethylid wasps that look fairly similar to this one, although I couldn’t find an exact match.

Edit: I guess first impressions were best. Seems this is a Pseudomyrmex ant. Check this photograph of Pseudomyrmex pallidus. Thanks to Cameron for straightening me out.

Bethylid wasps are parasites* of caterpillars and beetles larvae. Even the small ones like this have a potent sting. They use their stingers to immobilize the host larva, and then lay their eggs on it. The wasp larvae that hatch from the eggs use the caterpillar or beetle grub for food. Because the hosts are often pests of crops, these bethylid wasps are considered to be beneficial insects. Just leave them alone if you are lucky enough to see one.

*A parasite uses one host that is bigger than itself for food. A predator catches and eats many smaller prey items for food.