Scolytidae: Tomicus piniperda on Scots pine. The beetle has several phoretic mites on the hind end (back of the elytra) which are "hitching a ride". The mites are not harmful to the beetle but must be flown to the new tree by the beetle. The mites then disembark when inside the gallery and do whatever they do. This could be simply eating fungi or bacterial ooze in the galleries to even eating the bark beetles eggs (depending on the species of mite).

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Images © 1996 by John A. Byers, Chemical Ecology.