Trombiculidae - (chigger mites)
The adults live on the ground and only the small larvae (chiggers) are parasitic and suck fluid from vertebrates, including man. They may transmit rickettsia diseases. The chiggers are very small (less than ½ mm) and produce a small wound on the skin with the help of digestive enzymes. They then feed on the body fluid. The wound is initially painless, but can cause strong irritation and itching after the chiggers finished feeding and drop to the ground. The nymphal and adult stages are no longer parasitic and feed on plant material or are predatory on other arthropods.
For details see the respective page in Wikipedia.
The following genera are currently entered under Trombiculidae: