The Varve Midge


One of the most common insects in certain areas of the river is a large midge that was first discovered in 1996 by UMASS researchers diving in the river. Midges are in the Order Diptera (Flies) and the Family Chironomidae, and are closely related to Mosquitos. Since they do not feed on blood (or in fact on anything in many cases) they are commonly called the non-biting midges.

The midge we are discussing here is a currently undescribed species in the genus Axarus. This is a large midge, the adult is about half an inch long, and the larvae can reach almost an inch. The larvae are bright red, and are commonly called "bloodworms". This is due to a compound called hemoglobin that is present in the hemolymph (the insect version of blood) of the larvae. Insect hemoglobin is related to human hemoglobin, and performs the same function: storage and transport of oxygen to the body tissues. Here are pictures of both the adult (on the black background) and the larva (on the gray clay) of this Axarus species:



The Axarus species in the river lives as a larva in tunnels that the worm-like immature insects burrow in exposed areas of clay known as varves, which are fairly common in the riverbottom. They range in depth from very shallow exposures that can emerge from the water, to deep exposures that are 60-70 feet below the surface. Almost all varve exposures of any size are inhabited by varve midge larvae. Many varve deposits form exposures of very fine clay that can be quite extensive, some are shown here:

Amazingly, these large larvae can inhabit these clays at densities of over 12,000 insects per square meter! In the picture of the larva above you can see some of the burrow openings, each burrow is U-shaped and has two openings to the surface of the clay. They can extend as deep as 3 inches, and can considerably weaken the clay, as shown here:



The insect lines this burrow with silk from its salivary glands, which helps it maintain a good grip in the tunnel, and perhaps also helps it procure food. In this picture you can see the multiple chimneys of silk that are often present at the openings of the tunnels:



Note also how the clay is crumbling and weakened here, the midge larvae are actively destroying their own habitat!

Because they are large and numerous insects, Axarus larvae are naturally a good source of protein for other denizens of the river such as this Largemouth Bass, here seen cruising near a varve exposure:

There are places in the river where some animal, most probably a fish of some kind, is actively digging for Axarus larvae, and can cause extensive excavation of the clay, as shown in these pictures:


This is good evidence of the importance these insects represent to the river food chain.

Another interesting thing about Axarus larvae is the fact that they have a cellular feature known as "polytene chromosomes", which allows some aspects of their genetic make-up to be easily observed using a light microscope. This is valuable because this species has a high rate of mutations called inversions, and these are proving to be a fruitful area of research in the river.

Polytene chromosomes are often formed when a cell needs to produce a large amount of a given protein. In Axarus larvae, the protein is silk, and the cells that have the polytene chromosomes are the salivary gland cells. In polytene chromosomes the homologous chromosome pairs synapse (line up alongside each other), and also each chromatid (the strand of DNA contained in a given chromosome) is replicated hundreds or even thousands of times. When such chromosomes are stained with a DNA-specific stain, the result (in an ordinary light microscope) looks like this:



The banding pattern seen on the chromosome is specific to this species and can be used to differentiate them from other, similar midges. Sometimes there are areas of a chromosome that have been inverted, or flipped around backwards. These form characteristic loops, like the one shown here:



The following diagram shows both types of chromosomes, inverted and non-inverted, and indicates why the loop is formed:



Studying these inversions in the Connecticut River Axarus population is helping us to understand more about their population dynamics and genetics.