BEES DENUDATAE 29 



tion of the integument, for this last exuvium is thick and ri<dd. 



o o 



Although it voids no excrement till much later the union of the 

 stomach and hind-intestine is accomplished when the larva is 

 half-grown. A larva, from which Miiller took away a portion of 

 its unconsumed food -store, began directly afterwards to emit 

 excrement. The pupa has greater power of movement than 

 the resting larva ; when it has completed its metamorphosis 

 and become a perfect Insect, it, if it be a female, commences 

 almost immediately after its emergence to form burrows by the 

 complex and perfect series of actions we have described. 



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 generalisa- 

 tions about their relations with their hosts have already been 



t/ 



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 pro- 

 visions 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 con- 

 siderable 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. 



