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By nearly
any measure, the most successful animals on the planet
are the arthropods. They have conquered land, sea and
air, and make up over three-fourths of all currently
known living and fossil organisms, or over one million
species in all. Since many arthropod species remain
undocumented or undiscovered, especially in tropical
rain forests, the true number of living arthropod species
is probably in the tens of millions. One recent conservative
estimate puts the number of arthropod species in tropical
forests at 6 to 9 million species (Thomas, 1990).
Arthropods range
in distribution from the deep sea to mountain peaks,
in size from the king crab with its 12-foot armspan
to microscopic insects and crustaceans, and in taste
from chocolate covered ants to crawfish jambalaya and
lobster Newburg. Despite this unbelievable diversity,
the basic body plan of arthropods is fairly constant.
Arthropods have a stiff cuticle made largely of chitin
and proteins, forming an exoskeleton that may or may
not be further stiffened with calcium carbonate. They
have segmented bodies and show various patterns of segment
fusion (tagmosis) to form integrated units (heads, abdomens,
and so on). The phylum takes its name from its distinctive
jointed appendages, which may be modified in a number
of ways to form antennae, mouthparts, and reproductive
organs.
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