THYSANOURA, PARASITA, SUCTORIA. 159 



days after, when they begin to lay in their turn. According 

 to these observations, and the calculations to which they 

 have given rise, two female lice may have eighteen thousand 

 descendants in the course of two months. 



Linnagus considered the louse, which remains constantly on 

 the human head, as a variety of the common louse. It is, 

 however, from its characters, as described in the text, and also 

 from its undeviating locality, proved to be a distinct species. 

 This same great naturalist also informs us, that the largest 

 lice he ever met with, are to be found in the warm caverns 

 of Fahlun, in Sweden. 



Children, and persons who totally neglect their hair, and 

 are otherwise of filthy habits, are generally infested by the 

 head-lice. When powder was fashionable, and preposter- 

 ously esteemed an embellishment to the hair, it frequently 

 proved a productive cause of these vermin. Those who left 

 too long upon their head the dandriff produced by the use of 

 powder, or who employed adulterated powder for the orna- 

 ment of their hair, were very liable to the intrusion of these 

 amiable guests. The other species, which is found on all 

 parts of the body, excepting those of generation, comes 

 generally in consequence of the neglect of personal cleanli- 

 ness, and the not changing the linen and internal garments 

 sufficiently often. It is from this same species that those 

 persons suffer who are afflicted with that most dreadful and 

 disgusting malady phthiriasis. It may be observed that 

 their appearance, though in fewer numbers, is sometimes in- 

 dicative of the approach, or even of the crisis of other diseases, 

 and that where there is no neglect of personal cleanliness. 



As for the third species, we believe, that its appearance 

 is invariably the result of indiscriminate debauchery, ac- 

 companied of course with personal neglect. 



Dirt attracts these insects, and prepares for them a nidus 

 favourable to the reproduction of their posterity. This is the 



