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comparison imagine the development of a Hematopinus from a form 
similar to that of Acanthia, and since in Acanthia, as well as a number 
of other Hemiptera, the blood-sucking habit is already present, there is 
no difficulty 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 fon 
non-parasitic forms by way of semi-parasitic ones quite directly to the 
condition of constant parasitism, and taking the groups at large and 
comparing them with the nearest non-parasitic forms we may infer that 
the Mallophaga are a comparatively modern group, while the Pedicu- 
lide give evidence of greater antiquity. 
Leaving this matter, which must of necessity be somewhat specula- 
tive, we may now pass 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 
eonceive 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 Pediculide, 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 proper 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 wings 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 fren forms, constant tend- 
ency to reduction in number and but little to increase in size. 
‘The antenne present some interesting modifications, and in respect of 
number of joints there is a quite remarkable constancy in number (five) 
in both Mallophaga and Pediculide, 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 upon as from fifteen in Atropos 
to five (three in Trichodectes), the more perfect sensory apparatus is to 
