Insect activity often tracks moisture gradients. We studied ant activity, size, and diversity along three moisture gradients in a Panamanian rain forest. Ant activity at baits increased by 25 percent from the dry to the wet season, and > 200 percent on a topographical gradient from a ravine to an exposed plateau. Activity varied little from day to night. Three microhabitats—tree trunk, shrub, and litter—showed different responses to these three gradients. The size distribution of the species pool (N = 63) was right skewed, but the average size of ants at a bait was strongly bimodal. Ants active in moister times and places were not significantly smaller. We suggest that gradients of desiccation risk and food availability were the two most likely causes of these patterns. Two temporal niche axes (daily and seasonal) showed little species specialization, but half of the common species could be categorized as litter or plant microhabitat specialists.