Minor workers of Pheidole cephalica respond to small quantities of water placed in the nest entrance by making alarm runs through the nest, often ending at alternate entrances. They use odor trails to lead nestmates into the entrance galleries and sometimes away from the nest altogether. With this procedure one or two workers are able to mobilize a large fraction of the colony in 30 secs or less. Standing water by itself is enough to induce the response, but moving water, which provides both chemical and tactile stimuli, is still more effective. Purely tactile stimulation does not evoke the response, even when vigorous enough to knock workers over. On the other hand, invasion by a single fire ant (Solenopsis invicta) often causes alarm runs, which are combined with alarm-recruitment to produce a composite pattern of defense and retreat distinct from flood evacuation. The responses to flooding by 25 species of Pheidole in addition to P. cephalica have been tested. Some species react with both alarm runs and alarm waves, in which short loops generate broader and more slowly advancing fronts of excitement. Others respond with alarm waves alone. Alarm runs occur more frequently in species that nest in pieces of rotting wood, and hence typically excavate a linear array of nest galleries and chambers. They tend to be absent in species that nest in soil and hence to excavate a broader spread of cavities. The correlation proved significant in both the ' liberal ' test, in which all species were counted equally, and the ' conservative ' test, in which sets of closely related species were treated as single taxonomic units.
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