The discovery of the third nymph in the ontogeny of Batracarus hylarane (Acari: Ereynetidae: Lawrencarinae) gives the occasion for stressing the importance of calyptostases in the ontogeny of arthropods. The number of stases in actinotrichid mites is fixed at six, and is viewed as a major developmental constraint. A similar constraint probably applies to other - if not most - arthropods and calyptostases are considered to be indicators of such a developmental constraint. However, this constraint escapes traditional analysis for several reasons, such as the weakness of traditional terminologiy, the underuse of an appropriate methodology able to detect the presence of calyptostases, and some reluctance to take into account these regressed states.