ECTOPARASITES 115 



is a heterogeneous one, the species composing it 

 belonging to so many genera, and the genera 

 usually belonging to different families or orders. 

 How far this community of habitat leads to 

 structural convergence is not clear, because the 

 anatomical characters of the associated forms 

 have not been worked out in sufficient detail 

 from this point of view. A case in point is the 

 overlapping of head and pronotum in some 

 ectoparasitic insects to which attention has been 

 drawn recently by Dr K. Jordan. 1 In Arixenia, 

 an apterous earwig found in the nursing pouch of 

 the naked bat of the Sunda Islands, the occipital 

 margin of the head is slightly concave, without 

 a sharp edge, and is not closely applied to the 

 pronotum. In Hemimerus, another apterous ear- 

 wig bearing a superficial resemblance to some 

 Blattidse, from the African murine genus Crice- 

 tomys, the hind edge of the head projects back- 

 wards, overlapping the pronotum to a slight 

 extent. Dr Jordan adds that this overlapping, 

 which is exceptional among insects, is best 

 known in fleas and in some Hemiptera parasitic 

 on bats. In the beaver parasite, Platypsyllus 

 castoris, the head and pronotum fit well together, 

 and there is a comb of spines extending from the 

 edge of the head on to the thorax, bridging over 

 the gap which might be formed when the head 



1 K. Jordan, " New Apterous Earwig (Arixenia),' Novitat. 

 xvi., 1909, p. 318. 



