104 RESPIRATION 



supply of oxygen together with the rapid removal of carbon 

 dioxide enables certain insects to contract and expand their 

 muscles, and so to vibrate their wings, no less than 880 times 

 a second. 



In insects, centipedes, and spiders, instead of the blood being 

 brought to a special organ, gill or lung, and there receiving 

 its oxygen and then transmitting it to the innumerable cells 

 of the body which it bathes, the air is taken straight to the 

 tissues and cells of the insect or spider without the inter- 

 vention of a respiratory fluid, "blood." Openings occur 

 at the side of the body called stigmata or spiracles. These 

 lead into ducts which, dividing and subdividing, spread in 

 their final branches to every cell of the tissue. These tubes 

 or tracheae, as they are called, are kept from collapsing by 

 means of a spiral thickening, which resembles the metal wire 

 in a garden hose or the coil-like thickening of the spiral- 

 vessels in certain plants. 



The direct supply of oxygen to each cell, \\dthout the 

 cumbersome intervention of the blood, may to some extent 

 explain the dominance of this class not only in variety of 

 species but in number of individuals. 



A very careful calculation of the number of species of ani- 

 mals in the different large groups, made in 1881, gave the 

 following results : 



Here the total number of species of insects amounts to 220,150, 

 whilst the remaining animals amount only to 91,503. I have 

 recently had occasion to consult the authorities of the British 

 Museum as to the number of known species. They estimate 

 that mammals number 10,000; birds 16,000; reptiles and am- 

 phibia 9,000; fish 20,000; mollusca 60,000; Crustacea 12,000 

 — probably an underestimate — whilst the number of insects 

 is now put at 470,000, a little under half a million. These 



