Sex determination
Many insects use a sex-determination system based on the number of sex chromosomes. This is called XX/XO sex determination–the O indicates the absence of the sex chromosome. All other chromosomes in these organisms are diploid, but organisms may inherit one or two X chromosomes. Infield crickets, for example, insects with a single X chromosome develop as male, while those with two develop as female. In the nematode C. elegans most worms are self-fertilizing XX hermaphrodites, but occasionally abnormalities in chromosome inheritance regularly give rise to individuals with only one X chromosome–these XO individuals are fertile males (and half their offspring are male).