The Outside Story: Ghost Midges, Late Night Eaters | Weekend magazine

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Ghost midges are among the most common but least seen planktonic insect larvae in lakes and ponds. These members of the genus Chaoborus earn the nickname “ghost” due to their unique appearance and unusual behaviors.

Measuring nearly an inch long, ghost midges are virtually impossible to see. Their almost transparent bodies deserve another common name: glassworms. Besides a small eye patch, their most visible structures are two pairs of air-filled sacs. These sacs facilitate the midges’ ability to move vertically through the water.

Ghost midges rise to the surface of the lake for midnight snacks. Their nocturnal tendencies are another reason humans rarely see them. Finding them, however, is relatively easy, especially if you camp or live near a lake or pond. To participate in the hunt, you need a bucket, a headlamp and a pair of nylon stockings.

Head to your nearest lake well after dark and dunk some water in your bucket. Pour the water through the nylons – and repeat. The more water you pour through the bottoms, the more plankton you accumulate. After collecting the plankton, add some water to your bucket and turn the nylons upside down to flush the concentrated plankton into your bucket. A white bucket helps plankton stand out.

Why is nighttime a good time for ghost gnats? It comes down to food and predators. Ghost midges occupy the middle of the food chain. At the base of the food chain, algae harness solar energy to chain carbon atoms into sugars and starches. Algae are eaten by small zooplankton, including water fleas and mosquito larvae. These tiny animals are eaten by ghost gnats which, in turn, are eaten by small fish.

Because the sun rules the system, most food is found near the surface where the light enters – and ghost gnats need to be up there to feed. But light poses a problem: Fish that eat ghost gnats are visual predators, so visible gnats become dead gnats. The solution for ghost midges is to dine near the surface at night and spend the day on the murky lake bottom, where it’s easier to hide and where low oxygen levels exclude most fish. Ghost midge species that do not migrate are only found in fishless lakes.

The reasons for ghost midge migration are well known, but how this feat is accomplished has remained a mystery until recently. In January 2022, Evan McKenzie of the University of British Columbia and his colleagues described a unique buoyancy control mechanism in these larvae that comes down to acidity. After a night of foraging near the surface, the larvae increase the acidity surrounding their air sacs, which causes bands of a protein called resilin to contract, compressing the gas in the sacs. Compression of the air sacs makes the organism denser than water and the larvae sink. The system reverses to alkaline at night and – presto! — the larvae rise to the surface.

Ghost midges are perfectly adapted for vertical migration and catching prey. In deep lakes, they travel hundreds of feet on a daily journey that takes them from the safety of the lake bottom to open waters densely stocked with delicious snacks.

When the larvae come closer to the surface, they hunt until dawn drives them deep. Like other true fly larvae, ghost midges do not have legs to attack their prey. Instead, their antennae developed a raptor function that would make Hugh Jackman’s Wolverine claws in X-Men look positively tame. Part grappling hooks, part impaling devices, these multi-tool antennae work with larval mandibles to pierce and crush prey that will feed the larvae to their next life stage.

When the time comes, the midges pupate until it is time to emerge as adult midges. These adult midges do not bite, but may indulge in a sip of nectar. It was during this brief part of their life cycle that they might finally be noticed by people – like clouds of little flies on a summer evening. These flies live for a few days, then lay rafts of eggs on the surface of the water to complete their life mission before succumbing and, perhaps, finally becoming fish food.

Declan McCabe teaches biology at Saint Michael’s College. His first book, “Turning Stones: Exploring Life in Freshwater,” will soon be published by McDonald and Woodward. The Outside Story is attributed to and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation.

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