6 SPIDERS [CH. 



any fairly minute animal is still an insect, just 

 as any insect is popularly a "fly' -or, in the 

 United States, a "bug." Scientifically the use of 

 the term Insect is now much restricted, though still 

 extensive enough in all conscience, since it includes 

 many more than a quarter of a million known species. 

 Zoologists recognise a large group of animals charac- 

 terised by having no internal skeleton but a more or 

 less firm external coating of a peculiar substance called 

 chitin, often strengthened by calcareous deposits, 

 which necessitates the presence of joints in their 

 bodies, and especially in their limbs if they are to 

 move freely, just as medieval suits of armour required 

 to be jointed. These are the Arthropoda. One sub- 

 division of this group consists of aquatic animals, 

 breathing by gills, and known as Crustacea. Crabs, 

 lobsters, shrimps and " water-fleas " are familiar 

 examples, and with the exception of the so-called 

 land-crabs the only Crustaceans habitually found on 

 land are wood-lice. 



The other Arthropoda are air-breathing, and 

 since their characteristic breathing organs are 

 branching tubes known as tracheae, the term 

 Tracheata is sometimes used to include them all. 

 They fall naturally into three divisions, the Myriapoda, 

 the Insecta and the Arachnida, and it is in this 

 last-named division that we shall find the spiders. 



The Myriapoda are the centipedes and millipedes, 



