HEXAPODE DERMATOSES 



220I 



The bites of these insects are either considered in pp. 223-226, or 

 in the references given above, and need not be further considered, 

 except with regard to the PedicuUdae. which cause the dermatosis 

 called Pediculosis. 



Pediculosis. 



Synonyms. — Phthiriasis, Vagabond's disease. 



Definition. — Pediculosis is a term applied to the various lesions,, 

 primary and secondary, induced by the bites of Pediculus humanus 

 Linnseus, ly^S, Pediculus corporis de Geer,ji778, snidPhthirius pubis 

 Linnaeus, 1758 (pp. 755-757)- 



Remarks, — Pediculosis is extremely common in the tropics, 

 where it is an everyday sight to see the lower-class natives busily 

 employed in killing the lice in their friends' heads. It is also 

 commonly present in all armies on active service, and as lice are 

 carriers of such diseases as typhus, relapsing fever, and trench 

 fever, etc., they have assumed a very important position in the 

 recent war. 



etiology. — Pediculosis is due to the irritation caused by the 

 venom injected during the bites of the three species of lice mentioned 

 in the definition. 



Pathology. — The mouth parts of a louse consist of two tubes 

 one inside the other ; the outer chitinous tube, called the proboscis, 

 is composed of the fused labrum and labium, and is armed with a 

 collar of minute curved booklets, while the inner membranous tube 

 is composed of the maxillae and mandibles, and is called the suctorial 

 tube. When about to bite, the louse inserts its hard proboscis into 

 a sweat duct, everts its small booklets, and thus obtains a hold 

 upon the skin. It then protrudes the suctorial tube, which it 

 drives deep into the skin of the host until it reaches the blood. It 

 is during this process that it probably injects the venom from its 

 salivary glands, but the nature of this poison is quite unknown, 

 although its presence is with reason suspected owing to the pruritus 

 caused by the bites of the lice. 



Having reached the blood, it proceeds to fill itself to repletion, 

 pumping the blood into its alimentary canal by means of the 

 chitinous pharyngeal pump. After feeding, it withdraws its pro- 

 boscis, and the blood fills up the orifice and coagulates, forming a 

 minute red papule. Considerable pruritus is now felt, and the 

 victim scratches vigorously to relieve this sensation, and often 

 produces marked excoriations, which may become secondarily 

 infected with the common pyogenic cocci, causing purulent lesions. 

 Repeated biting, associated with injection of the venom, and 

 constant scratching, leads to pigmentation of the skin, causing 

 the so-called pityriasis nigra of Willan, or melanoderma pediculis. 

 If this process is continued, the skin becomes thickened and deeply 

 pigmented, and forms the so-called ' vagabond's disease.' The pig- 

 mentation is naturally not observable in very dark skins, and may 



