WHEELER— THE PARASITIC ACULEATA. 13 



Nomadine series is by no means clear. Friese in his earlier work 

 (1889) could not decide whether it was to be derived from Cocli- 

 oxys, as Perez suggested, or from a form like Eucera. To-day 

 even such an alternative seems too simple, for in all probability the 

 long series of " Nomadine " genera now known consists of several 

 heterophyletic groups. M electa and Epeoliis are derived from A)i- 

 tJwphora by Robertson and others, and Saunders and Robertson 

 would derive Nomada from Andrena, whereas such genera as Am- 

 mobates, Biastes, Pasites and Phiariis are now supposed by Friese 

 ( 1916) to be connected with Megachilc through genera like Cccsarea 

 and Paraccclioxys, the last being also the source of Ccclioxys and 

 of Dioxys and Paradioxys through the genus Prodioxys. Among 

 the exotic parasitic genera it seems clear that some have arisen from 

 host genera very dififerent from those above mentioned. Thus there 

 is every reason to suppose that Thalestria has arisen from Oxcea, 

 Agla'e and Exccrete from Euglossa, Eucondylops from Allodape, 

 Peresia from Osmia. 



It will be seen, therefore, that even if we make all due allow- 

 ance for dubious cases there still remain a number in which the 

 closest morphological affinity of the parasitic is with its host genus. 

 This is evident from the accompanying table (Table I.) in which 

 the most clearly established cases (fully 50 per cent.) are marked 

 with an asterisk. In constructing this table I have profited by a 

 number of valuable suggestions kindly communicated by Professor 

 Cockerell. We must assume, I believe, that in some cases the 

 primitive host genera are now extinct, that in some cases, there- 

 fore, the parasites have come to infest species of genera to which 

 they have no morphological affinity, that many parasites are directly 

 derived from other parasitic genera and that in some cases the 

 phenomena of parasitic convergence are so pronounced and oblit- 

 erate or obscure the generic affinities to such a degree that they can 

 be elucidated only by the most painstaking study. For my imme- 

 diate purposes, however, the present results will suffice, since they 

 agree with the conditions in other groups of Aculeata, as will be 

 shown in the sequel. 



In marked contrast with the bees, the solitary wasps comprise 

 few parasitic species, if we exclude the Mutillidae, which I am not 



