254 BRITISH BEES. 



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 

 Nomada 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 annoy- 

 ance are Andrena, Halictus, Panurgus, and Eucera ; the 

 latter two have but one of these enemies each, the No- 

 mada Fabriciana infesting the Panurgus Banksianus, 

 and the N. sexfasciata frequenting the Eucera longicornis. 

 Under Panurgus I have alluded to the relative abun- 

 dance 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, succinct a, and ruficornis, which 

 are found to infest Andrena Trimmer ana, tibialis, Afze- 

 liella, an&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. later alls to A. lon- 

 gipes ; N. baccata to A. argentata ; N. borealis to A. 

 Clarkella ; N. Fabriciana to Panurgus Banksianus ; and 

 N. sexfasdata to Eucera longicornis. Observation has 

 not yet fully determined whither each species of Nomada 

 conveys its parasitism ; several infest the Halicti, espe- 

 cially 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 wing- 

 ing off. I lost patience in endeavouring to combine the 



