ON MACKAYIA DIMORPHA 257 



The types of M. dimorpha are a pair in the writer's 

 collection. Two $ $ and six $ 2 were taken by Mr Hugh 

 Mackay on a specimen of the Manx Shearwater (Ptiffijtus 

 anglorum), from the island of Eigg (Scotland). The host 

 was sent to Mr Mackay by the Rev. J. M'William, on 

 30th May 1910. 



Key to the genera Giebelia, Mackayia, 

 Philoceanus. 



Gen. characters. — Philopteridae with broad transverse 

 laterally projecting membranous flap on under side of 

 forehead. 

 A. Antennae differing in the sexes. A.A. 



Antennae simple in both sexes. Giebelia. 



A.A. Form slender, nirmoid ; metathorax with distinct back- 

 ward-projecting postero-lateral angles. 



Philoceanus. 



Form stout, Giebelia-likc ; metathorax continuously 



rounded behind. Mackayia. 



It remains to note some points raised by this and the allied 

 species. 



1. Mackayia, Giebelia, and Philoceanus agree in having a well- 

 marked labral collar or transverse lobe. The function of this 

 laterally folded outgrowth of the under side of the clypeus 

 may be to hold the food in position during detrition. This 

 peculiar structure is possibly not strictly comparable with the 

 similar outgrowth in the Liotheid Physostomum. 



2. Systematically Mackayia falls between Giebelia and 

 Philoceanus. With the former it agrees in general 

 fades, structure, genus of host, but the antenna: connect 

 it with Philoceanus. We have here, then, a true link 

 between two somewhat isolated forms, which is precisely 

 what one would expect in view of the marked tendency which 

 the genera of this order (Mallophaga) show to run into one 

 another. 



3. Giebelia, up to the present, is a New World form oiPuffinus- 

 parasite. The range of the genus may be wider, but if not, 

 Mackayia may be the Old World representative. The Shear- 

 waters, however, are notorious wanderers, and without further 

 evidence it would be rash to venture an opinion. 



II 2 K 



