Fig. 40. Newly-hatched black flies can form large swarms in May.

Only female black flies need blood, and they bites outdoors at all times of the day. Most black flies prefer other host animals; however, they bite humans if more suitable hosts are not present.

When a black fly is about to suck blood, it is not easy to chase away. However, it is easy to slap. Its mouth parts are short. It scrapes a small hole in the skin and drinks the blood from the wound. Black flies bite where the skin is thin, often around the eyes and ears. The bites hurt, and some people experience itching and swelling that can last for several days. If you have been bitten by many black flies, you can get allergic reactions that include dizziness, fever and other discomfort.

When black flies appear in large numbers, they can kill large animals such as cows and horses in a matter of hours. The animals die in a state of shock. It is not known whether this is caused by toxic effects from the black fly saliva, or an allergic reaction, but probably it is about a little bit of both.

It will hardly end as badly for humans, as mentioned, we are not the preferred prey, and, furthermore, we can bring ourselves to safety. Black flies do not bite indoors.