222 Prof. J. C. Schjédte on Phthiriasis, and 
the skin? Why should they, now and then at least, undertake 
the difficult task of entirely burying themselves in the skin? 
And, again, supposing even that they might thus more easily 
get at their food afterwards, it does not appear why they should 
do this only occasionally and in particular circumscribed places. 
The more we think of this theory, the more confused does it 
appear. 
It is evidently high time to put aside as insufficient our pre- 
paration as well as the statements and illustrations of Erichson, 
Simon, and Landois, which are based on that preparation. 
Let us make only one short experiment more—one that requires 
but very little arrangement. Let us take a fresh head and 
examine it from beneath, but this time with a lower magnifying- 
power, by reflected light, and without the intervention of a thin 
glass. The first result is sufficiently surprising ; for the “ man- 
dibles” are gone. By and by, however, we find them by slowly 
raising and depressing the compound body of the microscope ; 
but hereby we discover at the same time that they are situated 
underneath the skin. And now there is an end of the “ mandible” 
theory ; for one need not be a naturalist to perceive that neither 
man nor beast can work forceps which cannot be opened. We 
must try some other way of finding the truth, and we will try 
the method pursued by honest painstaking Swammerdam. 
Suppose, then, a sufficient number of P. vestimenti* to be 
* Linneus would not recognize more than one species of Pediculus in 
man (P. humanus), though he acknowledged two varieties or races—one 
of the head, and the other of the body. De Geer considered them two 
distinet species, and described them as P. humanus capitis and P. humanus 
corporis, which latter name Nitzsch proposed to change into P. vestimenti, 
which is now generally used. De Geer’s statements as to their specific 
differences have since made the round of all later works, whether general 
manuals or special treatises, without any essential change. It is therefore 
high time to point out that they are not only insufficient, but erroneous, 
as nobody would be able to distinguish them by the words of Latreille, 
Simon, Burmeister, Gervais, &c.; for it is by no means the case that the 
dark colourmg on the edges of the thoracic and abdominal segments is 
peculiar to P. capitis, and absent in P. vestimenti; the examiation of a 
number of specimens will, on the contrary, show that P. capitis is occa- 
sionally without any dark colouring at all, and that P. vestimenti is not 
even generally without some, but, on the contrary, often as darkly tinged 
as P. capitis. In both species these extremities are connected by a gradual 
series of intermediate varieties. The two species are, upon the whole, so 
very closely allied, that their discrimmation demands the greatest attention, 
and it is very difficult to fix reliable marks of distinction. The best means 
of distinction are, in my opinion, afforded by the abdominal segments, 
particularly the two last but one, which in P. capitis are more sharply 
separated towards the sides, whilst in P. vestimentt they join one another 
more evenly; but this point of difference (mentioned also by De Geer, 
though not made use of in his diagnoses) cannot be made appreciable ex- 
cept by means of good drawings. Further, it is necessary, for the proper 
