Aquatic at birth and through the larval stage, black flies don't establish themselves on land until they reach adulthood. As adults, the males feed on nectar and thus pose no problems for humans. Mature females, however, are energetic parasites with a decided preference for warm-blooded hosts- as many hikers, campers, boaters and outdoorsmen know all too well! And, though the black fly itself is tiny at only a tenth of an inch in length on average, its bite packs a surprisingly big punch. Bitten areas not only tend to itch badly, but typically swell up and become painful to the touch as well.