191 
also marked, the Liotheidz being much less dependent upon their hosts 
than the Philopteride. Most of them travel freely and live for some 
time away from their hosts, while the Philopteridz cling fast to the 
host even after its death. In both groups, however, we have special 
structures resulting from their environment, such as the palettes upon 
the basal joint in Lemobothrium and allied genera, and ridged or ser- 
rated surfaces, to add to the rigidity of hold. 
In Pediculidz the tarsal structure presents some remarkable resem- 
blances to that of Philopteridz in the large basal joint and more slen- 
der claw-bearing joint. Special roughened, serrated, or ridged surfaces 
are also common on the second joint, and the claw is often serrated or 
roughened on the inner face. <A peculiar structure which I have de- 
seribed in Hematopinus suis* consists of an extensible pad at the end 
of the tibia, which is so located as by its extension to press upon the 
hair surrounded by the claw, and would seem a most useful organ in 
strengthening the hold upon the hair and adapting the clasping struc- 
ture to variations in the diameter of the hair at different points. In 
some species the claw becomes flattened into a nail-like organ, and a 
certain amount of rotation of tibial parts is observed. 
Reviewing now hastily the characters which have been sketehed and, 
I think, that for the Mallophaga, with the exception of wings, the loss of 
which has been stated as occurring before the assumption of the para- 
sitic habit, we must admit that parasitism has resulted in specialization 
and progressive evolution, not retrogression or degradation. The 
short antennee of five or three joints present certainly as high a degree 
of development as the fifteen-jointed but simple antennae of their free 
relations. The tarsi and claws present in every case specialization, and 
in some cases development of highly organized modifications of the 
simple feet. The mouth parts show specialization of mandibles and 
reduction, it is true, in labial and maxillar structure, but very little re- 
duction from the Psocid type, and a reduction that need not be looked 
upon as rendering them inferior in any way in the use of these parts 
or the purposes to which they may be put. 
It seems to me, therefore, that while in many cases parasitism un- 
doubtedly results in degradation, the results, excepting for wings, in 
these insects have been specialization without degradation. It would 
Seem more proper to consider that we have specialization of those organs 
the use of which is retained, and degradation in organs that fall into 
disuse, while the quite common expression that parasitism entails de- 
gradation appears to me to be in this respect somewhat inexact. 
* Bull. 7, Div. Entomology U. 8. Dept. Agriculture, p. 20. 
12201—No. 5 3 
