In fact, this midge makes up a large percentage of organisms found and counted in the sediments of the Oxbow Lakes. The Glassworm, also known as the Phantom Midge due to its transparency, is a small insect from the family Chaoboridae. Closely related to the ever-ubiquitous Chironomidae, it is small and worm-like in its larval stage, inhabiting rivers and other natural water sources, such as the Oxbow Lakes along the Savannah River.
However, the glassworm has a fascinating life history — one unlike most other insects. One of the main things that sets the glassworm apart is its diel vertical migration. This transparent creature spends the daytime in the depths of the hypolimnion — the bottom layer of the lake — poking its head out of the dense sediment, feeding on zooplankton swimming by. However, after twilight the glassworm migrates from the sediment to the epilimnion — the uppermost layer of the lake.
The exchange of gases from these airsacs are actually audible, taking up much of the perceptible frequencies underwater at dusk. One of the reasons Chaoboridae exhibits this behavior is because its prey does as well. These hydrostatic organs keep the body suspended perfectly in the water. To catch the prey such as small water crustaceans like water fleas it has a prehesile antenna.
Head of midge with prehensile antenna, top left. The yellow gut ends in the middle of the photo with the jaws just visible below the compaound eye. The upper antenna traps the prey against the other antenna so that the jaws can eat the prey. By day it often lies near the bottom of the lake rising later in the day. The midge adults lay egg masses on the surface up to several hundred eggs incased in jelly. The pupa forms in the water, floating to the surface to emerge after about 4 days.
PM larvae have prehensile antennae that are modified to grasp their prey—daphnia, rotifers, small mosquito larvae, and other PM larvae—crush it, and deliver it to their mouth. They may also do a little filter feeding. They are consumed by other aquatic predators and by birds.
Larval lifestyles seem to be fairly similar across the board. Both the larvae and pupae are ambulatory, moving by wriggling the pupae are vertical migrators, too. See a picture of a PM pupa. PM larvae are found, sometimes very abundantly, in still, open waters around the globe, where their ability to live in low-oxygen conditions allows them to be tolerant of water pollution. One study in a Dutch lake found 1, to 1, individuals per square meter, with an annual biomass of more than 10 pounds of one species of PM alone.
In another European study, PM larvae were thick enough in the water that their movements could be tracked with sonar.
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