INSECTS AND HUMAN DISEASE 105 



Insects do not breathe by means of lungs, like the higher 

 animals, nor even, in the adult stages, at any rate, by means 

 of internal gills, like the fishes, or external ones, like the 

 tadpoles, but by means of a network of fine tubes called 

 tracheae, which ramify within their bodies. The siphon 

 tube of a mosquito larva is composed of a cylindrical piece 

 of chitin, a very tough and resistant substance, of which 

 the hard parts of insects are composed; within the tube 

 are the starting points of the two main tracheae. The 

 tracheae, as we have said, are tubes, and they have no 

 power of opening and closing ; how then do the larvae, which 

 always forsake the surface of the water when disturbed, 

 prevent themselves from being choked ? When a seal or 

 sea-lion is about to go beneath the surface of the water, 

 its nostrils are always carefully closed, to prevent the 

 ingress of any liquid to its respiratory organs ; in a similar 

 manner, when the mosquito larva leaves the surface, flaps 

 of chitin close up the end of the siphon tube, thus serving 

 the same end. Though the functions of the chitin flaps 

 are similar to those of the specially modified nostrils of the 

 seal, anatomically, of course, they bear no relation to one 

 another whatever. 



We have said that malaria can only be contracted as the 

 result of a bite by an infected mosquito. Certain Italian 

 physicians, by name Bignami, Bastianelii, and Grassi, had 

 shown this, but, from one reason and another, their findings 

 did not carry conviction to the world at large. In 1900, 

 at the instigation of Sir Patrick Manson, and with the 

 assistance of the Colonial Office, two strictly scientific 

 experiments were carried out, with the object of proving 

 the truth of the mosquito malaria theory, in the most 

 striking manner possible. In brief, for one experiment, 

 two men who had never suffered from malaria were to 

 live in a severely malarious region, throughout the fever 

 season, taking no measures against the disease, except 

 protection from mosquito bites. For the other experiment, 



