Chiggers

The thing called a chigger,
is really no bigger,
than the smaller end of a pin,
but the bump that it raises,
just itches like blazes,
and that's where the rub sets in.
-H.B. Hungerford, Entomologist

Here are a couple of a places to learn about chiggers: 
Source of chigger photo and Howstuffworks article.