Current Issue

Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Battling bewildering botlass flies in Belize: Black flies, Simuliidae


Rushing rivers provide an ideal habitat for black fly larvae to develop.


As the winter that never quits continues to pound much of the US, it is once again time to head south to the warm tropical forests of Belize where we recently encountered nozzle-headed termites, ants tending fungus gardens and protecting their homes, chemically defended butterflies and their larvae, and large spiders keeping cool and catching prey.  Aficionados of Bug of the Week may think these travels are all sweetness and light, but this week we meet a darker side of the rainforest.

In this time lapse video that is twice the normal speed, this black fly gains several times its body weight in about 60 seconds. In its aftermath, my leg continued to ooze blood for several minutes.

While exploring a riparian forest in central Belize, several students and faculty encountered one of the most blood thirsty denizens of the jungle locally known as the botlass fly. Part of the daily preparation for expeditions into the jungle included slathering exposed skin with some of the most potent insect repellents known to mankind. This precaution was uniformly employed for sorties especially at dawn and dusk when mosquitoes were at their sanguinary best. However, several in our party learned firsthand that even the strongest insect repellents were no match for the Belizean botlass flies, which cared not whether it was dawn or dusk and happily attacked throughout the day. Here in the United States and Canada, botlass flies are familiar to many campers and fisher folk and are commonly known as black flies or buffalo gnats. These tiny flies reach incredible densities in forested areas in late spring and early summer in northern states across the US.  Due to their exceptional springtime numbers they are jokingly called the state bird of Maine. 

Strange black fly larvae live the life aquatic where they attach to rocks and feed on minute animals and plants on the surface of the substrate.

Unlike mosquitoes that insert hypodermic-like mouthparts and tap into small capillaries, black flies use jaws with serrated edges to slash flesh and severe tiny blood vessels. As blood pools in the wound, the black fly laps it up. Only the female black fly has the blood lust and she uses this rich protein source to produce as many as 800 eggs over the course of her lifetime. Males are the gentler gender and consume nectar from flowers. With her load of fully developed eggs, the female black fly visits running mountain streams or other fresh water sources and deposits her eggs on rocks, logs, emergent vegetation, or directly into the water. Eggs hatch and larvae attach to rocks and other structures in the water where they consume small plants and animals on the surface of their substrate.

Amazing reddish welts and swollen legs were the handiwork of Belizean black flies suffered by many of the students and faculty who met these vicious vampires.

The wound of the black fly is quite something to behold. While the bite itself is cloaked by anesthetics administered in the saliva of the fly, the aftermath can be quite disagreeable. For many victims on our excursion, reddish purple blood spots appeared beneath the skin at the site of each bite. These red welts were accompanied by intense itching that lasted several days. Moderate to impressive swelling occurred on the ankles and legs of several students and some complained that their skin was sore to the touch.  Fortunately, no one required medical attention and the swelling relented after a few days. The purple spots at the feeding sites were still apparent on my legs for more than a week after exposure to these tiny vampires.

The direct injury caused by black fly bites is the lesser of the evils visited unto humans by these tiny flies. In several countries in Central and South America and Africa, black flies carry nasty filarial worms capable of invading the human body. They occupy small tumors beneath the skin. In some cases these filarial worms take up residence in the eye and cause permanent sight loss known as river blindness or Robles disease. While many experienced the discomfort of the bite of the botlass fly, none were afflicted by any serious illness. Lucky for us!


We thank the hearty crew of BSCI 339M, ‘Mayan Culture and the Interface between Tropical Rainforests and Coral Reefs’, for providing the inspiration for this Bug of the Week. The wonderful reference Herms’s Medical Entomology by M.T. James and R. F. Harwood was used to prepare this episode.