Chaoborus

Glassworms are almost entirely transparent, except for pairs of black kidney-shaped structures in the front and the back of the body. These dots are the air sacs. They use these air sacs to migrate up and down in lakes. Glassworms breathe through the end of their abdomen and have two small eyes at the front of their bodies.

Chaoborus adults do not bite mammals or suck blood. Larvae live in open waters and even sediments, where there may be little oxygen for them to breathe. In some lakes they can be found as deep as 70 m (230 ft). In these deep anoxic waters they can avoid predation more easily than near the surface. They get around the fact that a normal air filled invertebrate tracheal system would fail at these depths by having it reduced to just two air sacs.[2] They are predaceous, and catch their prey with their modified prehensile antennae. They look somewhat like mosquito larvae, on which they prey and frequently destroy in large numbers. Chaoborus are considered opportunistic eaters, as their main diet consists of copepods and cladocerans, though they are believed to prefer copepods. However, they tend to select whatever prey enters its strike zone as it is not known for chasing prey. Prey selection also varies between species as head and mandible size dictate maximum prey size.[3]

Huge swarms of Chaoborus edulis, resembling distant plumes of smoke over Lake Malawi's water

Their main predator is fish, which can cause a behavioral response due to light factors influenced by their presence. This fish factor causes an increase in the average depth the individuals can be found. This vertical migration behavior is believed to be a chemical reaction, rather than a visual or mechanical.[4] In the African Great Lakes of Malawi and Victoria (but not Tanganyika where this genus is absent, perhaps because of chemical factors[5]), huge swarms of Chaoborus midges occur and are collected by local people to make kungu cakes, biscuits or burgers, a local delicacy rich in protein.[6][7][8] David Livingstone (1865) claimed that they "tasted not unlike caviare".[6]

Life cycle

Chaoborus spends the majority of its life in the larval stage. In this larval stage, there are 4 growth instars before reaching adulthood. Most Chaoborus species are univoltine, or live for only one year, though some populations have been recorded as having a two-year generation time. Timing of pupation depends on local environmental conditions, though generally occurs after any ice has thawed and temperatures in the water begin to rise again.[9]

Chaoborus punctipennis adult

The simplest way to collect glassworms is by a plankton net.[10] They can be found in almost all water, but most abound in fish-free quarry ponds during winter. Glassworms are very easy to store if the water is kept cold and aerated. They are very tolerant to bad water conditions, including chlorine. They are sometimes collected and sold as fish food.[11]

These 44 species belong to the genus Chaoborus:

Data sources: i = ITIS,[12] c = Catalogue of Life,[13] g = GBIF,[14] b = Bugguide.net[15]