Cleptoparasitic bees lay an egg in a host cell already provisioned with pollen. The parasite larva, or in many species the female herself, destroys the host egg or larva, and the intruder eats the food that the host bee collected.
Because the parasite never forages for pollen, the branched hairs and dense scopa that define most bees are useless and have been lost. Nomada are shiny, sparsely haired, often red and yellow, and are routinely mistaken for wasps. Similar losses have happened independently in many bee lineages: cleptoparasitism has arisen dozens of times across the bees.
Entry to a host nest is the hard problem, and chemistry solves it. Some Nomada males transfer host-like compounds to females during mating, and females of several species produce cuticular chemicals resembling the host's own scent, letting them pass a guarding female. Cuckoo bees are not rare curiosities: a substantial fraction of all bee species live this way, so the presence of Nomada in a garden is usually a sign that its host bees are doing well.