189 



couiparisoii iiiiiijj;iiu! the dcvt'lopiiicut of a lljKinatopiuus liom a tonn 

 similar to that of Acanthia, and siuce in Acanthia, as well as a imiiihcr 

 of other Ilemiptera, the blood-suckiiij;' liabitis already i)reseiit, there, is 

 no ditticulty in understanding the transition of a non-parasitic or a semi- 

 parasitic form to the parasitic one. It must consist simply in the more 

 and more constant attachment to the host. 



In both groups the evidence seems clear that evolution has been from 

 non-parasitic forms by Avay of semi-parasitic ones quite directly to the 

 condition of constant i>arasitism, and taking the groups at large, and 

 comparing them with the ui^arest non-parasitic forms we may infer that 

 the Mallophaga are a comparatively modern grouj), while the Pedicu- 

 lidie give evidence of greater auti([uity. 



Leaving this matter, which must of necessity be somewhat specula- 

 tive, we may now i)ass to a consideration of those structural changes 

 which have been entailed by assumption of the parasitic habit. 



Abortion of wings is one of the most common results of parasitism, 

 and in these groups the reduction has been complete, no trace of 

 these organs appearing, but since the wings are rudimental in 

 Atropos and Clothilla, neither of which are parasitic, we can readily 

 conceive that the immediate non-parasitic progenitors of Mallophaga 

 were wingless also, and we are hardly warranted in accrediting the wing- 

 atrophy to the assumption of the parasitic habit. In fact we must look 

 for wing structure for all these forms to some winged Psocid. This is 

 true in part of the Pediculidte, for we have too many instances of wing 

 abortion without parasitic habit to affirm that this form of degradation 

 results from parasitism. It would be more i)roper to say that the wing 

 abortion is a result of disuse and that in these groups wings are absent 

 not because the insects are parasites, but because their habits under 

 present conditions and probably under the conditions of their non- 

 parasitic ancestors rendered Avings of little use and they became non- 

 functional and then atrophied. 



In the general form of body these groups have become adapted to 

 their peculiar environment, and flattened bodies, sometimes greatly 

 elongated and slender almost invariably smooth, permit of great freedom 

 of movement within the feathers or hairy covering of their hosts. 



The eyes are in many cases quite rudimental, never composed of more 

 than a few facets, and show, as compared with free forms, constant tend- 

 ency to reduction in number and but little to increase in size. 



The antennse present some interesting modifications, and in respect of 

 number of joints there is a quite remarkable constancy in number (five) 

 in both Mallophaga and Pediculidic, the exceptions to this being very 

 few, and in such cases almost all are three, but in one genus four. 



Many present striking structures of sensory or clasping function. 

 Sensory pits are prominent usually in the terminal joint and while the 

 reduction for Mallophaga may be looked u])on as from tifteen in Atropos 

 to five (three in Trichodectes), the more perfect sensory api^aratus is to 



