Bob Copeland, an entomologist living in Kenya, provided the following explanation for this amazing phenomenon:
"...[this is a photo of] one of the entomological wonders of the world. The brown clouds rising out of Lake Victoria represent a mass emergence of "lake flies" - small bottom-feeding [flies]. There are billions in the photo. They emerge synchronously and float away in the winds. You had better run if they come ashore where you live. Babies cry and mothers grab their drying clothes. Windows are shut and people wait in horror until they pass (only a slight exaggeration). The older people still eat them, patting them into small cakes and deep frying them. Hmmmm, good."
Other related information: These swarms of synchronously-emerging flies have also been observed at other lakes in East Africa, such as Lake Malawi and Lake Nyasa. The fly larvae, which are called "glassworms" (3 images at top of linked page) due to their transparent bodies, live on the lake bottom where they feed on plankton using their antennae. Yes, you heard right! These antennae are raptorial; modified for use in feeding. When they form pupae (the stage between larva and adult), they rise from the lake bottom with the help of air sacs at the front (anterior) end of the pupa. As you might expect, many are eaten by fish on their way to the surface. They struggle free from their pupal cases at the surface and take flight where they face yet another obstacle...the likelihood of being eaten by birds. In fact, terns migrate all the way from Europe to take advantage of the easy prey.
Fish and birds are not the only predators of this dipteran delicacy. People in lakefront communities collect the midges with baskets attached to long handles. The adult flies are crushed, boiled, molded into cakes, and dried in the sun. They may also be prepared in a different way depending on the region (e.g., deep fried). These cakes are called Kungu and are said to taste similar to caviar or salted locusts. Rich in protein and calcium, these cakes are an important dietary staple for these people; providing six times as much protein as ox liver. Around Lake Nyasa, the lakeflies are called Nkhungu which is similar to the name for these cakes.
We do not have flies collected from this particular mass-emergence event and cannot say with certainty which species made up these particular "clouds." Typically, Chaoborus edulis Edwards, a phantom midge (Chaoboridae) and other chaoborids are the predominant species in such swarms. According to Pete Cranston (Professor of Entomology, University of California, Davis), a midge specialist, they often also include Cladotanytarsus lewisi Freeman, a species of non-biting or true midge (Chironomidae) which can cause an allergic response if inhaled. In fact, Bob Copeland's comment, "you had better run" is not far from the truth. These clouds are so dense that people have reportedly suffocated to death or have had severe allergic reactions. You may have noticed that one of the common names for these flies is "phantom midges." This name comes from the menacing "clouds" that are visible on the horizon from miles away. If you would like to get an idea of what a swarm of chaoborids sounds like, click here and listen. Ask your parents if they think they sound like the alien ships in Orson Welles' War of the Worlds!" (Sounds provided by Naturesongs.com 2005).
The timing of the mass emergence of chaoborid flies is controlled by circadian (circa = about + dies = one day) rhythms, or an "internal clock." This "clock" causes organisms to respond to external cues. Some aquatic insects (including Chaoborus edulis Edwards) exhibit lunar emergence rhythms. These lakeflies swarm monthly near the time of the new moon. Seems the waxing and waning of the moon affect things much smaller than the flow and ebb of the oceans' tides. Many organisms change behavior in response to environmental cues. Can you think of daily (or seasonal) cues which you respond to? For instance, does cloudy weather or the short days of winter make you feel sleepy? Do these external cues conflict with your "internal clock" which is based on approximately a 24-hour day?
For further information, see the following references: Armitage, P.D.; Cranston, P.S.; Pinder, L.C.V. (eds.). 1995. The Chironomidae: Biology and Ecology of Non-biting Midges. London: Chapman and Hall, pp. 371-372, 430-431. (Uganda: Chaoboridae) Corbet, Philip S. 1964. Temporal Patterns of Emergence in Aquatic Insects. The Canadian Entomologist 96: 264-279. Gaino, Elda & Rebora, Manuela. 1999. Larval Antennal Sensilla in Water-Living Insects. Microscopy Research and Technique 47: 440-457. Montschadsky, A.S. 1936. Les larves des moustiques (Fam. Culicidae) de l'URSS et des limitrophes. Tableux analytiques del le Faune URSS 24. Nicastro, Daniela; Smola, Ulrich; Melzer, Roland R. 1995. The Antennal Sensilla of the Carnivorous "Phantom" Larva of Chaoborus crystallinus (De Geer) (Diptera, Nematocera). Canadian Journal of Zoology 73: 15-26. Oldroyd, Harold. 1964. The Natural History of Flies. New York. W. W. Norton & Company, Inc. 324 pp. Shaxson, Annabel; Dickson, P; Walker, J. 1985. The Malawi Cookbook. Zomba, Malawi. Blantyre Printing and Publishing Co., Ltd. pp. 21-23.
Related websites: bioimages.org.uk (Great Phantom Midge images!)
Content by Jonathan Eibl and Robert Copeland
// Last Updated:02/01/2005 17:06:25