Chromosome behavior during gametogenesis and early cleavage is studied in two species of Thelastomatidae (Oxyurida), Hammerschmidtiella sp. and Thelastoma sp., parasites of diplopods. Both species are shown to be haplodiploid: male with 4 Thelastoma) or 5 (Hammerschmidtiella) chromosomes and double this number in the corresponding female. Haplodiploidy is now known in the 4 families of the Oxyurida and the author concludes that this form of reproduction is characteristic of the entire order. The Oxyuridan life cycle seems simple: 2 moults in the egg, no intermediate host, no extra-intestinal migration. However, several findings (some quite recent) indicate that this life cycle is often complicated by phenomena very closely adapted to host biology. The following particularities are analysed: 1) male progenesis; 2) laying of eggs in agglomerated packets; 3) peri-anal egg deposition; 4) poecilogony; 5) spanandry; 6) traumatic extra-genital insemination; 7) thelytoky. In a final chapter the author relates these adaptations to host biology and attempts to explain how they may have come to be favoured by natural selection during the course of evolution.