Diachlorus ferrugatus
In 3 days ~30% in a week ~50%
Diachlorus ferrugatus, commonly known as the yellow fly or yellow fly of the dismal swamp in the United States or doctor fly in Belize, is a species of highly aggressive biting horse-fly of the family Tabanidae native to North and Central America to Costa Rica.
The yellow fly was described as Chrysops ferrugatus by Johan Christian Fabricius in 1805. Adult yellow flies are around 1 centimetre (0.39 in) long with yellow bodies, mid-legs, and hind-legs, and black fore-legs. The eyes are blue-green with purple bands. They fly with little sound, and the first sign of their presence noticed by humans is usually their bite.
The larvae are thin white grubs with fine yellow fur, each segment having three pairs of pseudopodia. The yellow fly ranges from the southeastern United States, where it is found from New Jersey to Texas, to Costa Rica. It is most common near bodies of water, along with trees nearby to hide in. Spanish moss (Tillandsia usneoides) is a popular hiding spot.
It is notorious for its propensity to inflict painful bites on people, having been described as the most aggressive fly species in... Wikipedia →
| # | Park | Country | Daily sighting probability |
|---|---|---|---|
| 1 | Sian Ka'an Biosphere Reserve | 🇲🇽 Mexico | |
| 2 | Big Cypress National Preserve | 🇺🇸 United States | |
| 3 | Everglades National Park | 🇺🇸 United States |
| # | City | Country | Daily sighting probability |
|---|---|---|---|
| 1 | Orlando | 🇺🇸 United States |
No species of the same family or order lives in Sian Ka'an Biosphere Reserve, so these are its closest relatives here: other insects.
With luck
About 10% of wildlife watchers report a Yellow Fly Of The Dismal Swamp sighting in Sian Ka'an Biosphere Reserve on a given day, based on 5 verified sightings from GBIF citizen-science data from 2019 to 2026. It is the 15th most-seen of the 20 insects we can rate there. Over three days the chance rises to about 30%, and over a week to about 50%, if each day is a fresh chance.
April
In that month about 30% of wildlife watchers report a Yellow Fly Of The Dismal Swamp in Sian Ka'an Biosphere Reserve on a given day, against under 5% in June–August and October–February, the lowest months. That is one of Sian Ka'an Biosphere Reserve's driest months, with about 40 mm of rain against 230 mm in June.
Between noon and 3 pm
Across all the parks we cover, that three-hour window holds the most Yellow Fly Of The Dismal Swamp sightings with a recorded time, and about 8 in 10 of them come between 6 am and 6 pm: it is a daytime animal.
Yes
Of the 3 parks where we can rate Yellow Fly Of The Dismal Swamp sightings, Sian Ka'an Biosphere Reserve has the highest daily chance: about 10%. Next come Big Cypress National Preserve (United States, about 10%) and Everglades National Park (United States, about 10%).
GBIF citizen-science records
It is estimated from how often the Yellow Fly Of The Dismal Swamp is recorded against all the insects logged in Sian Ka'an Biosphere Reserve (5 verified sightings from 2019 to 2026), calibrated against known sighting rates. Figures are rounded to the nearest 10% because citizen-science data supports honest approximations, not decimals, and months with records from fewer than five people show "—". How it works.
