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 line of the hall is placed a case containing Mosquitoes 
greatly enlarged models of Mosquitoes or Gnats (figs. 3 and 4), p^gs 
and of an African Tsetse-Fly (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 (Culex 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 
