Across vertebrates, internal arousal states sharpen what an animal can perceive, coupling brain-wide neuromodulation to the gain of sensory neurons. Whether this principle extends to invertebrate nervous systems, with their orders-of-magnitude smaller scale and very different architecture, has remained an open question, in part because the behavioral tools to ask it have not been applied to insects in ecologically meaningful contexts. Here I show that arousal lowers visual detection thresholds in a paper wasp (Polistes fuscatus). Using a four-monitor stimulus arena that allowed foundresses to remain on their nests during testing, I measured psychometric functions for the detection of small moving stimuli under baseline conditions and after a brief vibratory cue that mimicked natural nest disturbance. Vibratory stimulation lowered the smallest reliably detected stimulus from 1.71° to 1.40° of visual angle, an 18% improvement in sensitivity, and elevated wing angle, a postural marker of arousal, from 41° to 59°. The shift was concentrated at sizes near the threshold, with an unchanged upper asymptote and a stable or slightly steeper slope, a signature consistent with sensory gain modulation rather than changed response criteria. Wing angle continuously predicted detection probability across the validation dataset of 15 colonies, suggesting it tracks an internal state related to perception. Across 15 colonies, baseline vigilance scaled with reproductive investment, with foundresses guarding pupae more responsive than those guarding eggs. Together these results demonstrate that the coupling between internal state and perception, long studied in mammals and flies, operates in a social insect performing ecologically natural defensive behavior. The mechanism appears to be implementable with around a million neurons, suggesting that flexible perception is not a luxury of large brains but a basic organizational principle of nervous systems.