These new arrivals are not the summer visitors, whom we mostly love. They are the black flies, which we almost entirely hate.

Charlene Donahue, a forest entomologist at the Maine Forest Service, doesn’t like them in her eyes or mouth but isn’t otherwise bothered by them and actually sees some good in them. She knows them better than most folks.

Here’s why they plague us from May until around the Fourth of July: The females lay their eggs in the summer in the vegetation along rivers and streams. In the spring run-off, the rising water washes them off and carries them downstream until they hatch and become worm-like larvae, attaching themselves to rocks by hooking onto a stickum that they exude and paste onto the rocks. They feed on the passing flow of tinier organisms. The warm days of May come, and the larvae become pupae, form their wings and emerge as what look like house flies except that they are only about one-eighth inch long. The females then start looking for any mammal that they can bite to get blood to supply protein for a new batch of eggs. Unlike mosquitoes, which stick, black flies actually bite and chew, often leaving a wound that bleeds and can fester.

They annoy dogs and cats as well as people, and they can drive deer and moose crazy. The buzzing and biting can reduce a cow’s milk production. Once really warm weather arrives, with temperatures that reach 90, most of the black flies disappear until the next spring.

One nasty minority, a species named for the Penobscot River, goes through the egg cycle all summer and into fall, producing one generation after another along the Penobscot and other rivers. This species flourished starting with the clean-up of the Penobscot in the 1970s. In fact, this bug is valued as a “bio-indicator.” Inspectors find it easier and cheaper to sample the blackflies than to analyze the water to check river purity: the more black flies the better is the river’s health. The larvae also are near the bottom of the river food chain, providing excellent trout food. So they’re not all bad.

Ms. Donahue sprays the brim of her baseball hat, wears long sleeves, and tucks her pants into her socks. Some authorities advise light-colored clothing and say blue, brown and purple are the worst. Lots of luck.