A key focus has been how male investment in reproduction (particularly mate-feeding) controls the operation of sexual selection and the evolution of sexual differences in mating, ornaments, and weaponry used in sexual competition. The systems under study have included insects and spiders in which males feed their mates, particularly orthopteran insects (crickets, katydids and New Zealand weta). One focal species, the Mormon cricket, appears to show variable life history not only in sexual behaviour but also in apparent "phases" (solitary versus gregarious).