THE MICROSCOPE IN .ETIOLOGY. 339 



in, fleshy beak, and piercing bristles, or with rudimentary 

 biting mouth. Body has usually nine articulations. 



Pediculus Capitis. Head-louse. 



P. Pubis or Phthirius Ingidnalis. Crabs. 



P. Vestimenti. Clothes -louse. 



Sub-order Hemiptera. 



Cimex Lectularius. Bed-bugs. 



Order Diptera. Insects with mouths for piercing or 

 sucking. Inarticulate thorax, with cuticular anterior 

 wings. Swing-bats for posterior wings. Complete meta- 

 morphosis. 



Palex Irritans. Flea. 



Pulex or Dermatophilus Penetrans. Sand-flea. Native 

 of South America. Breeds under the cutis, and the ova 

 develop in the sand. 



(Estrus Hominis. Gad-fly. May deposit ova in skin of 

 man, producing boils. 



Musca Vomitoria. Large, blue-bottle fly. 



M. Sarcophaga. Common flesh-fly. 



M. Domestica. House-fly. 



All may deposit ova or fully formed larva in cavities 

 and wounds. 



V. DISEASE GERMS. 



The germ-theory of disease ascribes disease, particularly 

 infectious disease, to the introduction of minute parasitic 

 organisms into the tissues of the body, and their subse- 

 quent multiplication there. Many of the early natural- 

 ists entertained substantially this view, as Vallisneri, 

 Reaumur, and Linnaeus. It was considered, however, but 

 a mere hypothesis, until recent microscopic observations 

 have revived an interest in this direction. Liebermeister, 

 in his recent monograph on typhoid fever, says, " Within 

 the last ten years a great revolution has taken place with 

 regard to the popular signification of a contagium vivum. 

 New investigations on the appearance, mode of propaga- 



