The spathidiids belong to the ciliate subclass Haptoria (Protozoa, Ciliophora), that is, they are predators using toxicysts to overwhelm the prey. Spathidiid ciliates prefer terrestrial and semisterrestrial habitats, but many occur also in freshwater, and some are marine.

Over 200 nominal spathidiid species have been described, sometimes based on seemingly minute differences. Thus, many protozoologists considered them as indeterminable and claimed for a detailed revision. The present monograph carefully revises the taxonomy, nomenclature, and ecology of all nominal species and shows that spathidiid diversity has been greatly underestimated. Based on the reinvestigation of described species with modern methods (silver impregnation, scanning electron microscopy) and the first description of over 50 new species, the family Spathidiidae is split into four families and 20 genera. Each species is described and figured in detail, making it unnecessary to go back to the original literature often difficult to obtain. Two identification keys are provided, viz., one for taxonomists and another, simple key for users not specifically trained in ciliate taxonomy.

This first part of the monograph contains the families Protospathidiidae, Arcuospathidiidae, and Apertospathulidae. The second part will contain the family Spathididae and a new family, the Pharyngospathidiidae. This monograph is part of our attempt to revise the free-living ciliates. The colpodids (Foissner 1993), the stichotrichine spirotrichs (Berger 1999, 2005), and the oligottichs (Agatha, in press) are already available or will be available soon.

The present monograph carefully revises the taxonomy, nomenclature, and ecology of all nominal species of spathidiids and shows that spathidiid diversity has been greatly underestimated. Based on the reinvestigation of described species with modern methods and the first description of over 50 new species, the family Spathidiidae is split into four families and 20 genera. Each species is described and figured in detail. Two identification keys are provided.