why does a spider have 8 legs
was correct in citing the encyclopedia and Wikipedia about the definitions. To give a perhaps more digestible and cursory explanation, think of a millipede and a grasshopper. Metameres are simply a series of linear repeating segments. In a millipede, each segment with its pair of appendages is repeated, quite dramatically in number, but they don t differ from each other very much (homonomous). They all basically operate the same way for the same function in unison for the purpose of locomotion. Now think of a grasshopper. The grasshopper s thorax (middle section) has some repeating segments each with a pair of appendages just like the millipede. However specialization of metameric units allows the first pair to be specialized for grasping terrain or food or even a mate while the last pair of appendages are the strong jumping legs that are so easily recognized (heteronomous).
In this case of variation on a theme, the appendage-bearing segment is the theme that gets elaborated upon according to position in the thorax. The thorax, then, is a tagma used primarily for locomotion, but with local specialization. The number of tagmata are a major characteristic that differentiates between the major groups of anrthropods. For example arachnids like the spider in OP s question have 2 tagmata, a prosoma and opisthosoma, whereas insects have 3 tagmata, a head, thorax, and abdomen. And yes, even the head is a fused number of repeating segments! Unfortunately I don t think I m qualified to conjecture as to how tagmatization or specialization of repeated segmental units called metameres, might have influenced the evolution of appendage numbers between spiders and insects!
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