Phylogenetic studies on several groups of arthropods, such as Acari (mites) or Collembola (springtails), make wide use of chaetotaxy. Chaetotaxic characters, besides being morphological features (setal shape), often have a binary nature, that is presence vs. absence. A seta can be variable in a population, and one may attribute a presence probability to this seta. In fact, the presence probability should be defined for each instar. One might think that a variable seta corresponds to a polymorphism (e.g., two or more alleles in a population); in fact, setal variability should be regarded as the result of a propensity intrinsic to individuals, i.e., a potentiality being expressed at random among specimens. The phenomenon known as lateral inhibition explains how an early random fluctuation is at the root of the cell fate. Probabilistic organs are likely to originate from a similar phenomenon. A tridimensional species/instars/characters table (SIC table), containing setal presence probabilities, can be built up. Two kinds of analyses may be applied to such a table. In these analyses, the problem of homeotypic setae is emphasized. Related issues are the accuracy of using setal probabilities and the problem of reducing redundancy of information. A first process leads to a bidimensional table with characters in columns, and the species divided into their instars in rows. By using multidimensional statistics we can access ontophylogenetic trajectories, and in this way, ontogenetic comparisons can be achieved. The aim of the second process is to produce a species/characters table which can be used in a cladistic analysis.