The name of this genus may be derived from _στελὶς_, _a sort of parasitical plant_, perhaps _mistletoe_, if we could be sure that Panzer imposed it after being aware of the parasitical nature of these bees. It is true his book (the ‘Revision’) was published in 1805, and Kirby, who first intimated a suspicion of such cuckoo-like habits in some of the bees, published his in 1802; therefore it might have been given in allusion to that peculiarity of their economy, but it may also be from _στηλὶς_, _a little column_, in application to their cylindrical form. In but few of the parasitical bees do we know the precise nature of their transformations, I have therefore been obliged to be silent upon this point of their natural history, and I have nothing to state of its nature in these, although I expect there is much uniformity with but slight modifications in all. The species of this genus are parasitical upon the _Osmiæ_; thus the _S. phæoptera_ is found to infest the _O. fulviventris_, and the _S. octomaculata_ intrudes itself into the nests of _O. leucomelana_, both of which occur tolerably abundantly near Bristol. I have no doubt that the south-west and west of England, if well searched, would yield many choice insects. It is singular that bee-parasitism does not prevail throughout all the genera of bees, some being subject to it and others not. Thus the genera _Colletes_, _Andrena_, _Halictus_, _Panurgus_, _Eucera_, _Anthophora_, _Saropoda_, _Megachile_, _Osmia_, and _Bombus_ have all parasites, whereas the genera _Cilissa_, _Macropis_, _Dasypoda_, _Ceratina_, _Anthidium_, _Chelostoma_, _Heriades_, _Anthocopa_, and _Apis_ have none, as far as we yet know; and some of the genera of parasites frequent two or more genera indifferently, whilst others are restricted to a single one; also some of the species of the parasitical genera infest indifferently several of the species of the genus to which their parasitism is mainly limited; other species have a more circumscribed range and do not visit the nests of more than a single species. What law may control all these seeming anomalies we cannot discover,—it may possibly be scent that guides them, and this may control their parasitism by indicating the species they are taught by their instinct to be most suitable from the quality of the pollen with which it supplies its own nest, to be that which is best adapted for the nurture of their young. It is not likely that we shall very speedily lift the veil from these mysteries, but they are suggestive of observation which in seeking one thing may fall upon another equally interesting.