Referee 1's review (Fritz Vollrath):

Interesting, well written and argued paper, potentially important if/when the arguments are supported independently.

I would suggest revising points where cause and effect are confused. Moreover, the discussion might be shortened to reflect the results. Alternatively the many purportedly supporting ideas might be phrased as hypotheses that can (and should) be independently tested.

Specifics

Properties of a normal pendulum and, in an ideal case, requires no muscle to move the body center of masses (BCM) steadily forward [9]. Mechanical power of upside-down
Q: really: NO muscle is required??


(Fig. 1). Therefore, since body mass does not constrain as much the evolution of leg
traits, selection can act on leg traits, such as stride frequency (leg diameter) and stride length (leg length). Thus, if animals have evolved following the physics of
Q: I don’t see how leg diameter determines stride frequency


Mechanics explains the adaptive evolution of spider morphology. Indeed, our results suggest that leg length has been directly favoured by natural selection, since larger spiders that hang from their webs have disproportionately longer forelegs relative to smaller spiders; i.e., positive allometry and this effect is significantly stronger in these
Q: I don’t see how your results have shown that natural selection has directly favoured leg length, although you have shown some correlation

“posture mode x body size” interaction, F1,101 = 0.91; P = 0.608). Thus, consistent with the mechanics of pendulum motion, both standing and hanging spiders have evolved disproportionately longer legs, and hanging spiders have done so in a higher degree.
Q: disproportionately to WHAT? And: what was driving the postulated increase in leg length in the ‘standers’ as opposed to the ‘hangers’ for which you are making such a strong case.

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N.B. These are the comments made by the referee when reviewing an earlier version of this paper. Prior to publication the manuscript has been revised in light of these comments and to address other editorial requirements.