The Structure and Special Physiology of Insects 3 
but they are not all birds. The phylum Chordata is subdivided into or 
composed of the various classes Pisces (fishes), Aves (birds), etc. And 
similarly the phylum Arthropoda is composed of several distinct classes, 
viz.: the Crustacea, including the crayfishes, crabs, shrimps, lobsters, 
water-fleas, and barnacles; the Onychophora, containing a single genus 
(Peripatus) of worm-like creatures; the Myriapoda, including the thousand¬ 
legged worms and centipeds; the Arachnida, including the scorpions, spiders, 
mites, and ticks; and finally the class Insecta (or Hexapoda, as it is some¬ 
times called), whose members are distinguished from the other Arthro- 
antennas 
pods by having the body-rings or segments grouped into three regions, called 
head, thorax, and abdomen, by having jointed appendages only on the body- 
rings composing the head and thorax (one or two pairs of appendages may 
occur on the terminal segments of the abdomen), and by breathing by means 
of air-tubes (trachese) which ramify the whole interior of the body and 
open on its surface through paired openings (spiracles). The insects also 
have three pairs of legs, never more, and less only in cases of degeneration, 
and by this obvious character can be readily distinguished from the Myria¬ 
pods, which have many pairs, and the Arachnids, which have four pairs. 
Centipeds are not insects, nor are spiders and mites and ticks. What 
are insects most of this book is given to showing. 
To proceed to the classifying of insects into orders and families and 
genera and species inside of the all-including class is the next work of the 
collector and classifier. And for this—if for no other reason—some further 
knowledge of insect structure is indispensable. The classification rests 
