# 
(x. H. F. Nuttall 145 
case of corporis because of its habit of gorging (see Nuttall, Parasitology , 
xi. 343-344): Sikora found capitis when unfed measured 2-84, fully fed 2-92; 
corporis when starving measured 3-2 by 1-4, fully gorged 3-84 by 1-56 mm. 
Finally, owing to the lack of sharply defined structural demarcations, it 
is difficult to measure the length of the head, thorax and abdomen (especially 
its segments) more than approximately. Therefore little value necessarily 
attaches to small differences in size, and seeing that Pediculus varies con¬ 
siderably in size, measurements can have but small importance unless applied to 
a large number of full distended specimens assuming that these are obtainable. 
With regard to the width of the abdomen, Fahrenholz states that it is 
greater in his “P. capitis marginatus ” than in “P. capitis angustus ” whose 
abdomen is elongated, and that the greatest width is attained at the “ 7th 
segment” in assiwilis and at the “5th segment” in friedenthali 1 . 
The value of Fahrenholz’s statements bearing on the width of the abdomen 
is nil, because I have often detected elongated, broad, and intermediate forms 
of abdomen in both sexes where I have examined large lots of capitis and 
corporis that have been collected from single individuals in different parts 
of the world. Pure strains of corporis , raised in the laboratory, have yielded 
a like result. Moreover, as with the length, the width of the abdomen in both 
sexes varies according to the state of engorgement, and in females according to 
the number and size of the ova they contain. Thus in laboratory strains of 
corporis I find that the greatest width ranges forward and backward from 
the 4th, 4th—5th and 5th abdominal segments (the 5th = Fahrenholz’s “ 7th ”) 
in the <£, whilst in the $ the greatest width alternates between the 3rd—4th, 
4th, and 5th abdominal segment, the latter most frequently attaining the 
greatest width (abdominal segment 3 = Fahrenholz’s “5th”). 
For the reasons previously given, no value can be attached to Fahrenholz’s 
statement that the last abdominal segment in “P. assimilis ” $ is equally long 
and broad, such measurements being fallacious especially in balsam-mounted 
specimens. 
Following the lead of many previous authors, Fahrenholz lays stress on 
the relative size of capitis and corporis and of his purported new forms. Ex- 
: amined critically, however, in the light of the subjoined table, these differences 
| should be seen once and for all not to hold good. I have stated this repeatedly 
in a general way but the appended table will perhaps carry more conviction. 
Dismissing the measurements of capitis and corporis given by older writers, 
as perhaps wanting in accuracy, I have arranged the following table of measure¬ 
ments by recent authors and newer measurements of my own in accordance 
The reader is liable to be confused by Fahrenholz’s enumeration of the segments. He does 
not state that he includes segments 1—3 in the thorax. What appears to him and most observers 
to be the 1st abdominal segment is numbered 4 by Fahrenholz, but it actually consists of two 
fused segments (abdominal segments 1 and 2) which bear the first abdominal spiracle. Fahrenholz 
when he refers to segments 5-6-7 must therefore, in the light of our present knowledge, be under¬ 
stood to refer to abdominal segments 3-4-5 which bear correspondingly numbered abdominal 
spiracles. 
10—2 
