This was a funny side note in my Acarology class, so I found a paper with an enlightening Abstract:
"It is impossible for trombidiform mites to defecate because they lack both an anus and a hind gut. Digestive residues remain in the mid-gut lumen and there are great differences in the capacity of species to transfer and store digestive residues in specific mid-gut lobes. The chigger mite species with the most precise pattern of faeces storage spontaneously ruptures the body wall in a narrowly defined region and extrudes a faeces-filled gut lobe through the rupture. This seems to be a normal function in the field and in healthy reproducing laboratory cultures and the process, named schizeckenosy, may be a secondarily evolved substitute for defecation."-Schizeckenosy: The substitute for defecation in chigger mites (Mitchell and Nadchatram 1969)
Chiggers are a parasitic life-stage of mites from the family Trombiculidae. Occasionally they infect people, causing quite the itch. The image below is an excellent representation of an animal .007 inches, less than half a millimeter, long.