No. 51: May-Jun 1987
"Many lizards are infested by chig gers, the larvae of trombiculid mites, which feed on tissue fluid and cell debris. Surprisingly, lizards seem to go out of their way to attract the chiggers -- they have special mite pockets that provide a protected, warm and humid site. In many cases, the skin of the lizard also has smaller scales than normal and a good blood supply in the pocket, which enables the parasites to feed more readily."
There does not seem to be any advantage to the lizards providing plush accommodations for the chiggers. The chiggers can wreak havoc on their hosts in the form of skin lesions, allergic reactions, secondary infections, and the transmission of diseases. Nevertheless, some 150 species in 5 distinct lizard families possess mite pockets, which are often located in different places in different lizard species. Apparently, the mite pockets evolved separately several times. But why? (Benton, Michael J.; "The Mite Pockets of Lizards," Nature, 325:391, 1987.)
Comment. Why haven't the lizards evolved thicker skin or some sort of chemical defense instead of reducing their fitness with mite pockets? Or, are other factors operating?