First R4, L3, R2, L1, then L4, R3, L2, R1: this is the “alternating tetrapods” walking pattern, long familiar in many arachnids, in which the eight legs are moved as two groups of four. Here R and L stand for the right and left sides of the body, and the indices 1 to 4, for the first (fore-) to fourth (hind-) legs. As in insects, with their six legs, the locomotor pattern of spiders is a diagonal rhythm: the legs diagonally opposed to one another on the two sides of the body are moved synchronously — not truly in synchrony, but one shortly after the other. The result is that the legs on a given side step in a wavelike forward progression, which can easily be observed especially during slow walking; when the walk is quite fast, the synchronicity of the movements within one of the two leg pairs is greater. Andreas Brüssel (1987) found out, while working on his dissertation, that when Cupiennius salei is walking at a mean speed of 10 cm/s it uses the following patterns, in order of decreasing frequency: 4-2-3-1 (68.1%); 4-1-3-2 (14.8%); 4-3-1-2 (12.8%); 4-2-1-3 (4.3%). By far the most common ipsilateral step sequence was thus the 4-2-3-1 rhythm, and after the spider has been walking for a short time it is almost the only one (91%) (see also Seyfarth and Bohnenberger 1980). Figure 1 shows it very clearly. However, a number of observers have stressed that the exact time at which a leg is moved can depart considerably from the strict pattern of alternating tetrapods, and spiders can quickly adjust to the loss of one or even two legs (Kaestner 1924, Wilson 1967, Seyfarth and Bohnenberger 1980, Seyfarth 1985).
KeywordsStrain Amplitude Ground Reaction Force Stance Phase Flexor Muscle Step Cycle
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