82 
Parasitic Arthropods 
Three species commonly attack man. Strangely enough, there 
are very few accurate data regarding their life history. 
Pediculus humanus (fig. 65), the head louse, is the most widely 
distributed. It is usually referred to in medical literature as Pedi¬ 
culus capitis, but the Linnean specific name has priority. In color 
it is of a pale gray, blackish on the margins. It is claimed by some 
authors that the color varies according to the color of the skin of the 
host. The abdomen is 
composed of seven dis¬ 
tinct segments, bearing 
spiracles laterally. 
There is considerable 
variation in size. The 
males average 1.8 mm. 
and the females 2.7 mm. 
in length. 
The eggs, fifty to 
sixty in number, stick 
firmly to the hairs of 
the host and are known 
as nits. They are large 
and conspicuous, especi¬ 
ally on dark hair and 
are provided with an 
operculum, or cap, at 
the free end, where the 
nymphs emerge. They 
hatch in about six days 
and about the eigh¬ 
teenth day the young 
lice are sexually mature. 
The head lice live by preference on the scalp of their host but 
occasionally they are found on the eyelashes and beard, or in the 
pubic region. They may also occur elsewhere on the body The 
penetration of the rostrum into the skin and the discharge of an irritat¬ 
ing saliva produce a severe itching, accompanied by the formation 
of an eczema-like eruption (fig. 66). When the infestation is severe, 
the discharge from the pustules mats down the hair, and scabs are 
formed, under which the insects swarm. “ If allowed to run, a regular 
carapace may form, called trichoma, and the head exudes a foetid 
66 . 
Pediculosis of the head. The illustration shows the 
characteristic indications of the presence of lice, viz: 
the occipital eczema gluing the hairs together, the 
swollen cervical glands, and the porrigo, or erup¬ 
tion of contagious pustules upon the neck. After 
Fox. 
