ANALYSIS OF A SPECIES INSTARS/CHARACTERS TABLE - A THEORETICAL SURVEYON THE USE OF CHAETOTAXY IN ONTOPHYLOGENETIC STUDIES/

Phylogenetic studies on several groups of arthropods, such as Acari (m ites) or Collembola (springtails), make wide use of chaetotaxy. Chaeto taxic characters, besides being morphological features (setal shape), often have a binary nature, that is presence vs. absence. A seta can b e variable in a population, acid one may attribute a presence probabil ity 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, se tal variability should be regarded as the result of a propensity intri nsic to individuals, i.e., a potentiality being expressed at random am ong specimens. The phenomenon known as lateral inhibition explains how an early random fluctuation is at the root of the cell fate. Probabil istic organs are likely to originate from a similar phenomenon. A trid imensional species/instars/characters table (SIC table), containing se tal presence probabilities, can be built up. Two kinds of analyses may be applied to such a table. In these analyses, the problem of homeoty pic setae is emphasized. Related issues are the accuracy of using seta l probabilities and the problem of reducing redundancy of information. A first process leads to a bidimensional table with characters in col umns, and the species divided into their instars in rows. By using mul tidimensional statistics we can access ontogenetic trajectories, and i n this way, ontogenetic comparisons can be achieved. The aim of the se cond process is to produce a species/characters table which can be use d in a cladistic analysis.