Bruce Heyer 11/4/07
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Animals 3: Terrestrial Arthropods
Biology 6A / Bruce Heyer & Brian McCauley
November 5 & 6, 2007
As described in the previous general discussion of Phylum Arthropoda, this extremely successful
taxon is characterized by cephalized, bilateral symmetry; a segmented body typically with a pair
of specialized jointed appendages on each
(segment); fusion of multiple somites into a
functional body region called a
; and a chitinous true exoskeleton that provides both
protection of the internal organs and attachment of muscle bands. Whereas the arthropod class
Crustacea dominate the marine environments — and a few crustacean groups such as the pillbug
isopods do inhabit the land — the terrestrial world has been thoroughly colonized by three other
classes of this phylum:
Arachnida — spiders, scorpions, mites, ticks. Body in two tagmata:
the prosoma (“forebody”), or
, with several simple eyes (ocelli), no
antenna, and six pairs of appendages (one pair of fanged chelicera, one pair of
tactile pedipalps, and four pairs of legs).
the opisthosoma (“hind body”), or
, often without appendages other than
ovipositors on females, and web spinnerets on spiders. Scorpion abdomen ends with
a segmented tail.
Myriapoda — centipedes, millipedes. Elongated body in two tagmata: the
with a pair of legs on each segment.
Insecta — the insects. We will consider this most diverse group of organisms in more detail.
Read the description of insects. Carefully observe the pill bugs and spiders. Note
at least three observable features from the description that clearly show why
these both are not insects.
As measured by diversity, distribution and abundance, insects are considered the most successful
group of organisms living on Earth. Almost one million species have been described — more than
all other animal species combined — and there may be at least ten times that many yet to be
identified. Insects are extraordinarily adaptable creatures, with the physical and behavioral
plasticity to live successfully in almost every terrestrial environment on earth, from the desert to
the Antarctic. Millions of insects may exist in a single acre of land. They are the main consumers
of land plants and constitute a major food source for many other animals.
Insects are directly beneficial to humans by producing honey, silk, wax, and other products.
Indirectly, they are important as pollinators of crops, natural enemies of pests, scavengers, and
food for other creatures. At the same time, some insects are major pests of humans and
domesticated animals because they destroy crops and vector diseases. In reality, less than one
percent of insect species are pests, and only a few hundred of these are consistently a problem.