No-see-ums are tiny flies that are usually invisible to the naked eye; they look similar in form but are smaller than mosquitoes. Their size varies from 3mm to 12mm. They tend to gather in large clusters or groups near bodies of water but can be found other places as well (although most people don't try to find them.) Males are said to be 'typically' harmless because the females bite more often than the males do. The females need the blood of mammals in order produce their eggs. While the bites don't usually inflict much pain at the time of the bite, like most bug bites they tend to itch and leave red marks where the bite occurred.