A division might very consistently be established in the genus by the separation of those which have subclavate antennæ, and the segments of whose abdomen are slightly constricted; these also are more essentially midsummer insects, and usually frequent the Ragwort. This is the only genus of parasites amongst the true bees whose parasitism is directed exclusively upwards in the scientific arrangement; the parasitism of all the rest of the genera of _Nudipedes_ bears upon the genera below them in the series. Some of the species of the _Nomadæ_ attack more than one species or one genus, but the majority are strictly limited to but one genus and one species. The genera obnoxious to this annoyance are _Andrena_, _Halictus_, _Panurgus_, and _Eucera_; the latter two have but one of these enemies each, the _Nomada Fabriciana_ infesting the _Panurgus Banksianus_, and the _N. sexfasciata_ frequenting the _Eucera longicornis_. Under _Panurgus_ I have alluded to the relative abundance of the parasite at the metropolis of its sitos. As far as known, the other species are thus distributed. Those frequenting several indifferently are the _Nomada alternata_, _Lathburiana_, _succincta_, and _ruficornis_, which are found to infest _Andrena Trimmerana_, _tibialis_, _Afzeliella_, and _fulva_, without displaying any choice; whereas others confine themselves to one sitos exclusively: thus _Nomada ochrostoma_ limits itself to _Andrena labialis_; _N. Germanica_ to _A. fulvescens_; _N. lateralis_ to _A. longipes_; _N. baccata_ to _A. argentata_; _N. borealis_ to _A. Clarkella_; _N. Fabriciana_ to _Panurgus Banksianus_; and _N. sexfasciata_ to _Eucera longicornis_. Observation has not yet fully determined whither each species of _Nomada_ conveys its parasitism; several infest the _Halicti_, especially the smaller species; the association of these it is difficult to determine; I have usually found several of the small _Halicti_ burrowing together in the vertical surface of an enclosure bank, and several of the small _Nomada_ hovering cautiously opposite, now alighting and entering a burrow, then retreating backwards and winging off. I lost patience in endeavouring to combine the species by the aid of blades of grass or slight straws thrust into the aperture, but the crumbling nature of the soil frustrated my wishes, and I abandoned the attempt. This field of observation is widely open to the exertions of observing naturalists, and the novelty of their discoveries would well reward the toil of the undertaking, for it would not be long before they gathered fruit.