PARASITIC BEES (Denudatae).—This group of parasitic bees includes fourteen European genera, of which six are British. They form a group taxonomically most unsatisfactory, the members having little in common except the negative characters of the absence of pollen-carrying apparatus. Although there is a great dearth of information as to the life-histories of parasitic bees, yet some highly interesting facts and generalisations about their relations with their hosts have already been obtained. Verhoeff has recently given the following account of the relations between the parasitic bee _Stelis minuta_ and its host _Osmia leucomelana_:—The _Osmia_ forms cells in blackberry stems, provisions them in the usual manner, and deposits an egg in each. But the _Stelis_ lays an egg in the store of provisions before the _Osmia_ does, and thus its egg is placed lower down in the mass of food than that of the legitimate owner, which is in fact at the top. The _Stelis_ larva emerges from the egg somewhat earlier than the _Osmia_ larva does. For a considerable time the two larvae so disclosed consume together the stock of provisions, the _Osmia_ at the upper, the _Stelis_ at the lower, end thereof. By the consumption of the provisions the two larvae are brought into proximity, and by this time the _Stelis_ larva, being about twice the size of the _Osmia_ larva, kills and eats it. Verhoeff witnessed the struggle between the two larvae, and states further that the operation of eating the _Osmia_ larva after it has been killed lasts one or two days. He adds that parasitic larvae are less numerous than the host larvae, it being well known that parasitic bees produce fewer offspring than host bees. Verhoeff further states that he has observed similar relations to obtain between the larvae of other parasitic bees and their hosts, but warns us against concluding that the facts are analogous in all cases. {30}[Illustration: Fig. 15.—_Nomada sex-fasciata_ ♀. Britain.]