For communication systems using airborne sound, the close relationship between body size and frequency has shaped the evolution of signal function and diversity (Morton, 1977; Gerhardt and Huber, 2002). Here we asked if a similar relationship exists for the most widespread form of mechanical signalling, substrate-borne vibratory communication. We examined the relationship of body size to signal frequency within populations and across species.

We found rather different patterns in the within-population comparison than in the between-species comparisons. In the two species of insects for which we examined within-population variation (the membracid treehoppers U. crassicornis and E. binotata), there was no correlation between the size of the signaller and the frequency of the signal. In contrast, there was a negative relationship between body size and measurements of signal frequency among 51 species in the family Membracidae, when using species as independent data points and when using phylogenetically independent contrasts. When we expanded our comparison to variation in size and signal frequency across various insect orders, the results depended on whether or not we

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