MOSQUITOES AND TSETSE-FLIES. 17 



showing a complete gradation between birds of different colour- 

 ing, which have been regarded as different species. Both 

 these examples may by some naturalists be considered instances, 

 not of crossing of distinct species, but of " dimorphism," or the 

 occurrence of a single species in nature under two different 

 garbs; but from whatever point of view they may be 

 regarded, they illustrate the difficulty of defining and limiting 

 the meaning of the term "species," of such constant use in 

 natural history. 



In the middle liue of the hall is placed a case containing Mosquitoes 

 greatly enlarged models of Mosquitoes or Gnats (figs. 3 and 4), y^^'****' 

 and of an African Tsetse-My (fig. 2) ; also still more enlarged 

 gelatine models of mammalian blood-corpuscles, showing the 

 parasites by which they are infested in the diseases respec- 

 tively communicated by means of Mosquitoes and Tsetse. 

 Models of the parasites themselves are also shown (figs. 5 

 and 6). 



Malaria, or ague, is a disease peculiar to man. It is caused 

 by extremely minute parasites which live in the red corpuscles 

 of the blood. Formerly malaria was believed to be contracted 

 by merely breathing the air of marshy districts, but it is now 

 proved that the parasites are transmitted from man to man 

 by the " bite," or rather " stab," of a Mosquito or Gnat. The 

 Common Mosquito or Stabbing Gnat {Gulex pipiens), fig. 3, 

 does not transmit the malaria-parasite ; the Spot-winged Mos- 

 quitoes (fig. 4), of the genus Anopheles, abundant in England and 

 nearly all parts of the world, being the carriers. This parasite 

 multiplies not only in the human blood, but in the stomach 

 and tissues of the Gnat — as shown in the models (fig. 5). 



Tsetses (fig. 2) are African blood-sucking Flies, with the 

 mouth-parts adapted for piercing the skin of mammals such as 

 Antelopes and Zebras. The blood of some of these animals is 

 infected with a parasite (fig. 6), which, when carried adherent 

 to the proboscis of the Fly, and introduced by its bite into the 

 blood of domesticated animals such as the Horse and Ox, 

 produces the fatal Tsetse-Disease. The model of one of these 

 flies exhibited is 28 times (linear) natural size; the Tsetse- 

 parasites and the red blood-corpuscles are enlarged 6,000 

 diameters. 



c 



