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 yoting 

 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, tmder which the insects swarm. " If allowed to run, a regular 

 carapace may form, called trichoma, and the head exudes a foetid 



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. 



