Trombicula autumnalis ...

... is more commonly known as harvest mite. And recently hit the spot light as prime suspect in being involved as a vector transmitting the so called Seasonal Canine Illness, which has caused a number of fatalities in otherwise healthy young dogs. We live in a hot spot of this relatively recent disease and our practice is actively involved in helping the extensive research time finding the cause.
We haven't got proof (yet) but all dogs we have treated carried varying numbers of the larval stage of harvest mites. As you can see at this point they only have 3 pairs of legs and actually feed of skin cells causing massive irritation and impressive rashes on the host. After feasting they drop off and develop into harmless adults with 4 pairs of legs. The larval stage is only about 0.2mm in diameter hence a microscope blip today :-)