82 R. W. CROSSKEY 



results if the phoretic forms on mayflies (like those on crabs) are separated as a 

 discrete subgenus — despite the obvious resemblance in the form of the male hypo- 

 pygium : the new resultant subgenus Phoretomyia here described is distinct from 

 Meilloniellum, apart from the immature stages, on the large size and quite different 

 abdominal vestiture, by the tapering subconical shape of the styles, and by the 

 enlarged flattened tarsi occurring in the majority of forms. These same characters 

 also distinguish the adults of Phoretomyia from those of the subgenus Lewisellum in 

 which the immature stages live in phoretic association with crabs. 



The constituent taxa in the subgenus Phoretomyia, despite the diversity of the 

 immature stages, are considered to be monophyletic because of the clear homo- 

 geneity of the adult stage. But the widely differing larvae and pupae make it 

 convenient to recognize three small species-groups within the subgenus ; these are 

 clearly natural groups, defined ecologically by phoretic association with particular 

 genera of mayflies as well as by morphological differences. 



The species-groups are keyed and defined below. 



Key to the Species-Groups of the Subgenus PHORETOMYIA 



Immature Stages 



Note: in the following key the genus of mayflies to which attachment occurs is shown in 

 brackets. 



i Larval hypostomium with a very large median tooth flanked by five blunt teeth on 

 each side (Text-fig 267). Larval cuticle covered with colourless hairs. Cocoon 

 normal, covering pupal thorax and abdomen. [Attached to Elassoneuria] 



berneri- group (p. 83) 



- Larval hypostomium not of this form. Larval cuticle bare. Cocoon reduced, 



covering only pupal abdomen or abdomen and posterior part of thorax. [At- 

 tached to other mayfly genera] ......... 2 



2 Larval cephalic fan in form of short flat brush, sides of larval head strongly convex 

 and cephalic apotome broadest near middle (Text-fig. 286). Larval antenna con- 

 spicuous, almost as long as stem of cephalic fan. Pupal abdomen bare on segments 

 5-8 (except at most for two or three small spiny hooklets each side on 5). [At- 

 tached to Afronurus, Baetis and unidentified Baetidae] . copleyi- group (p. 84) 



- Larval cephalic fan when open forming usual hemispherical brush, sides of larval 



head not convex and cephalic apotome broadest posteriorly. Larval antenna in- 

 conspicuous and colourless, only about half as long as stem of cephalic fan. Pupal 

 abdomen girdled with numerous blunt hooks on segments 5-7 and with similar 

 hooks laterally on segment 8 (Text-figs. 170 & 173). [Attached to Afronurus] 



lumbwanum- group (p. 83) 



Adults 



1 o* hypopygium without properly formed parameral hooks, these represented by an 



irregular rudimentary process on end of each paramere (except in diceros). Post- 

 notum with patch of yellow or golden scales on each side (except in diceros) 



copley i- group (p. 84) 



- o* hypopygium with one long strong parameral hook on each side, sometimes with 



small supernumerary hook at base of main hook. Postnotum bare ... 2 



2 Scutum with black-brown scales on a broad median longitudinal area and with 



