This study was designed to determine the effect of predation by the zodariid, obligate myrmecophagous spider Zodarion elegans (Simon, 1873) on the activity pattern and behavior of the Mediterranean harvester ant Messor wasmanni Krausse, 1910. Previous field studies suggested that the activity patterns of M. wasmanni colonies are modified not only by microclimatic conditions, but also by biological factors such as the presence of predators. The present study clearly demonstrates that M. wasmanni colonies respond in two different ways to spider predation. The ants 1) attack the invader, and 2) temporarily stop all foraging activities and close nest entrances with pebbles and other materials. This causes obligate myrmecophagous spiders such as Z. elegans to leave such inactive colonies and move to active ones nearby. Moreover, the laboratory experiments show that the capture of a single ant is in every case sufficient to trigger cessation of foraging activities and nest closure. After the spider attacks, entrances remained closed for up to six days. Perturbation experiments show that events that directly affect workers engaged in one task outside the nest also alter the activity intensity of task performance inside the nest. All exterior workers remain inside the nest when entrances are closed. Inside the nest, only a small proportion of marked foragers switch to food processing, while the majority was inactive or performed grooming during the inactive phase outside the nest.