Once or twice a year, it appears that I get bitten by an insect(s) that bite in clusters and result in very, very itchy skin abrasions. The itch can last for five or six days. Do you know what kind of insect this might be.

Sounds like spider bites. I get them too. They like warm beds, or get in pajamas that have been laying around, especially in the spring. And yes, the itch lasts that long and if they are in a place I can get to, I put alcohol on the bite and that disinfects immediately.

Tick