Life History of the Larva of the Glassworm

Life History of the Larva of the Glassworm (Chaoboridae)

By Liam Wolff, Phinizy Research Intern

Phantom Midge by Peter Maguire

Phantom Midges by Peter Maguire

The Phinizy Center for Water Science’s research projects often includes studying the diversity and abundance of certain macroinvertebrates. In the Oxbow Lakes left behind by the Savannah River, one type of midge larva is particularly common in the sediment samples that are collected. 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.

Phantom Midge by Peter Maguire

Phantom Midges by Peter Maguire

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. Two pairs of air sacs control the larva’s bouyancy and are used to move up or down the stratification zones. 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. Zooplankton migrate to the epilimnion at night due to changes in temperature and light.

Microscope photo of two glassworms dyed with rose bengal dye from the oxbow lake Possum Eddy. Photo by Liam Wolff.

Microscope photo of two glassworms dyed with rose bengal dye from the oxbow lake Possum Eddy. Photo by Liam Wolff.

Another motive for the Phantom Midge larva to return back to its abode in the hypolimnion at dawn is to escape predation from fish. In fact, some claim this is the primary reason that Chaboridae display diel vertical migration (DVM). One study showed that glassworms in the genus Chaoborus only partook in DVM in the presence of fish (Larson 2016). In a pond devoid of fish, no glassworms were found to demonstrate their nightly migration. How the larvae are aware of the prevalance of fish is explained by chemicals signals sent out by fish called Kairomones that the glassworms can detect. Since fish cannot tolerate the low levels of oxygen in the sediments of lakes, the glassworm hides in the hypolimnion from dawn until dusk. In fact, with the exception of zooplankton, most organisms cannot tolerate the anoxic waters of the sediment. The only way the glassworm can survive is by producing energy (ATP) through an alternate route (anaerobic malate cycle). This makes them tolerable to more polluted, less healthy water bodies.

One other reason the glassworm is unique is because it has a huge impact on the community of zooplankton. Although Chaoboridae feeds mostly on rotifers, copepods and cladocerans make up a large percentage of its diet as well. During its various instar stages, the glassworm tends to prefer different types of zooplankton. This helps regulate the abundance and diversity of zooplankton in lakes, rivers, and streams.

See more photos of the Phantom Midge by Peter Maguire in his online album.