No. 555] SPECIES-FORMING OF ECTO-PARASITES 133 



sitic insects, but only rarely can this annoyance reach 

 such a pitch as to interfere much with the bird's feed- 

 ing or resting or sleeping. And the loss of minute bits 

 of feather can certainly be of no consequence at all. The 

 Mallophaga then may be called benignant parasites, as 

 contrasted with the malevolent, blood-sucking fleas and 

 true lice affecting the same hosts. 



Ill 



With this fleeting acquaintanceship with the various 

 principal structural and physiological characters of our 

 group of insects, we may attend now to some of the spe- 

 cial facts of their distribution and their inter-ordinal re- 

 lationships, and to the problems which these facts pose 

 to us. 



First, with regard to the taxonomic conditions within 

 the group. I have divided the order into two sub-orders, 

 sharply distinguished by certain structural differences 

 whose physiological or ethologic significance, however, 

 is not at all plainly apparent. The most convenient rec- 

 ognition character for this sub-ordinal separation is the 

 condition of the antennae, short, broad, capitate, three- 

 segmented and concealed in special antennary fossae in 

 one group; longer, slender, five-segmented, projecting 

 and without receiving fossae, in the other. 



Each suborder contains two families, one of which, with 

 two-clawed members, occurs exclusively (with the ex- 

 ception of one or two species in each of four genera in 

 one case) on birds, and the other, with one-clawed mem- 

 bers, exclusively on mammals. Both of the mammal- 

 infesting famil ies include but one genus each; while the 

 two bird-infesting families include fifteen and ten genera, 

 respectively. 



The whole order then, with its one thousand five hun- 

 dred species, comprises but twenty-seven genera, 

 grouped into four families, arranged in two suborders. 

 The disposition of the species among the genera has a 

 rather extraordinary aspect. Nine, or one third, of all 



