11ArthropodsF07 - Animals 3: Terrestrial Arthropods Biology...

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Bruce Heyer 11/4/07 Page 1 of 8 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 somite (segment); fusion of multiple somites into a functional body region called a tagma ; 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: I. Arachnida — spiders, scorpions, mites, ticks. Body in two tagmata: a. the prosoma (“forebody”), or cephalothorax , 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). b. the opisthosoma (“hind body”), or abdomen , often without appendages other than ovipositors on females, and web spinnerets on spiders. Scorpion abdomen ends with a segmented tail. II. Myriapoda — centipedes, millipedes. Elongated body in two tagmata: the head and the highly segmented trunk with a pair of legs on each segment. III. 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. 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.
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11ArthropodsF07 - Animals 3: Terrestrial Arthropods Biology...

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