We investigated the relationship between body size (weight) and hearing sensitivity in response to a high-frequency, bat-like stimulus in a number of phaneropterine katydids on BCI, Panama. These phaneropterines are nocturnal flying species and thus potential prey of various insectivorous bats on the island. We tested the prediction that larger species compensate for the disadvantage — of producing stronger echoes for searching bats — by being more sensitive to bat calls, thereby increasing safety margins towards this predator. Contrary to this prediction, larger katydids were not more sensitive. This was corroborated in neurophysiological experiments in the nocturnal rainforest, where simultaneous recordings of the T-fibre activity in response to searching bats revealed no substantial difference between small and large katydids. We offer three explanations for the lack of correlation between body size and high-frequency sensitivity in these species.