Microthorax pusillus is a very common ciliate in my sites Ulmisried and Simmelried. There I find this ciliate in the mud or between decomposing plant masses. In my populations some specimens with 40 µm length were about 10 % larger than given by Kahl and Foissner (s. fig. 3 a-b). Although the ciliate is quite small, it is easily recognized by the bean-shaped body and the mouth opening located at the posterior end. On the right side of the body there are 3 rows of cilia, which are interrupted in the middle of the body (s. fig. 4a). Each cilium in these rows originates in a separate depression in the pellicle, creating a typical chain pattern.

Microthorax pusillus feeds on bacteria but also on rhodobacteria. Since the Simmelried is rich in rhodobacteria, I have found several times specimens here, which obviously fed preferentially on pink rhodobacteria (s. figs. 3 a-b and 4 a-b). Thus the specimens also appeared intensely pink at low magnification. At high magnifications, partially decomposed rhodobacteria were then visible in the cytoplasm due to the ongoing digestion process.