A black fly is any member of the family Simuliidae of the Culicomorpha

Culicomorpha

Culicomorpha is an infraorder of Nematocera. It includes mosquitoes, black flies, and several extant and extinct families of insect.-Classification:Extant families* Superfamily Culicoidea** Dixidae -meniscus midges** Corethrellidae -frog biting midges...

 infraorder. They are related to the Ceratopogonidae

Ceratopogonidae

Ceratopogonidae, or biting midges , are a family of small flies in the order Diptera...

, Chironomidae

Chironomidae

Chironomidae are a family of nematoceran flies with a global distribution. They are closely related to the Ceratopogonidae, Simuliidae, and Thaumaleidae...

, and Thaumaleidae

Thaumaleidae

Thaumaleidae, the solitary midges or trickle midges, are a group of nematoceran flies related to the Ceratopogonidae, Chironomidae, and the Simuliidae. They are small, stocky, yellow to brown flies . There are very few species known for this family...

. There are over 1,800 known species of black flies (of which 11 are extinct). Most species belong to the immense genus Simulium

Simulium

Simulium is a genus of black flies, which may transmit diseases such as onchocerciasis . It is a large genus with several hundred species, and 41 sub-genera....

. Most black flies gain nourishment by feeding on the blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 of mammals, including humans, although the males feed mainly on nectar. They are usually small, black or gray, with short legs

Arthropod leg

The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...

, and antennae

Antenna (biology)

Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

.