Midges, Hypoxia, and Hemoglobin

Note: This post may not make you a better fly angler, but it will make you a more knowledgeable one.

One of the pitfalls of living in the mud at the bottom of a wetland or lake is that there can be very little oxygen present. Oxygen diffuses much slower through water than it does in air, and even slower through the mud barrier where the decay of organic debris further depletes what little oxygen is present. Under these low oxygen (hypoxic) conditions, life is limited to organisms that are adapted to survive in these extreme environments.

Some chironomid (midge) larvae have adapted to exploit these hypoxic habitats through the use of hemoglobin. Hemoglobin is the protein component of red blood cells that circulates through our body and is responsible for delivering oxygen to tissues. It also gives blood cells their red color. Hemoglobin has a high affinity for binding to oxygen molecules making it the perfect oxygen delivery system where constant circulation is present.

There are several groups of chironomids that utilize hemoglobin to aid in survival under hypoxic conditions, but the two most common ones are in the genera Chironomus and Glyptotendipes (commonly known as blood-worms). The high concentration of hemoglobin in their body fluids (hemolymph) gives them their distinctive red color. While there can be some passive movement of the hemolymph through their body, it does not circulate like our blood does. Instead, it simply surrounds the organs and tissues.

Note that more evolved and larger insects may have an open circulatory system, but even here, the hemolymph does not circulate very fast and oxygen uptake and transport is primarily via the air-filled tracheal system.

A Chironomus larva with its typical bright red coloration.

So how exactly does the hemoglobin help blood-worms survive in hypoxic and sometimes anoxic (no oxygen) habitats? If there is little to no oxygen to bind to the hemoglobin to begin with, what advantage can there be?

Most blood-worms build extensive tube networks in and on the mud by tunneling and reinforcing the tube walls with mucus. These tubes provide protection from predators but they also allow the midge larva to circulate water through the tube by undulating its body within the tube (caddisflies also do this). This action pulls water with a slightly higher oxygen content through the tube. In some cases, the midge larva will also elevate the tube opening, even by just a few millimeters, to place the tube intake above the low oxygen boundary layer at the water-mud interface. Of course under low oxygen conditions they can't spend all of their time undulating to gain oxygen, they need to feed too. That is where the hemoglobin comes in - as the larva undulates and circulates water through the tube, the hemoglobin picks up oxygen (much like recharging a battery) - then when the larva stops undulating to feed; oxygen bound to the hemoglobin diffuses into the tissue to sustain metabolic activity. Once the hemolymph oxygen level falls to a critical point, the midge larva will once again take up its ventilation position in the tube to recharge its oxygen supply.

A Glyptotendipes larva out of its tube and foraging for food.

This species of Glyptotendipes builds its tube out of mucus and bits of organic debris. Here, it has chosen to establish its tube up in the vegetation rather than on the bottom. Note the red and bright green coloration – this is the impetus for the midge fly pattern known as a Christmas Chironomid.