Trombidium mites

A close look at several butterfly species at this time of year will reveal an infestation of the mite larva Trombidium breei - clusters of tiny red blobs attached to the thorax of the butterfly.  They are easiest to spot on Marbled Whites but also appear on Meadow Brown, Gatekeeper, Small Skipper, Ringlet and, here, on the Common Blue. They often appear after a spell of poor weather when the butterflies have been roosting for longer periods. Apparently they don't affect the butterfly's mobility or longevity.