Communication, raiding behavior and prey storage in Cerapachys (Hymenoptera: Formicidae)

Raiding expeditions in Cerapachys turneri are organized by individual scout ants, that return to the colony after having discovered a nest of the prey species. The scout lays a chemical trail with secretions from the poison gland, which serve as recruitment and orientation signals for the nestmates. Circumstantial evidence suggests that in addition the scout releases a stimulating chemical recruitment signal from the pygidial gland. This occurs probably when the scouts move with their gaster held slightly upwards in a calling position. Wilson (1958) reports the field notes made by H. Potter on the cerapachyine species Phyracaces potteri, which contain the only available description of the early stages of a complete raid observed in the field. Before the raid started Potter noted a few workers moving rapidly about, 'each with its abdomen raised upwards'. These observations match closely my findings in the laboratory and lend further support to the hypothesis that in addition to the trails laid with poison gland secretions, anotherstimulating signal is discharged, presumably from the pygidial gland of the recruiting ants. Wilson (1958) observed groups of Phyracaces moving along a raiding trail laid down by a raiding party on the previous day. In this case no individual leadership was involved and the foragers seemed to emerge from the nest randomly without a special recruitment stimulation by scout ants. Obviously these ants were following an established foraging trail, leading to a previously raided Pheidole nest which appeared to be vacated this time. Small exploratory parties conducted brief excursions to the side, but in most cases they turned back to the main trail. No nest suitable for raiding was found during these explorations. These observations strongly suggest that chemical trails laid during raiding expeditions might still function as orientation cues one day later and that foraging parties can follow these established trails without the leadership of a recruiting scout ant. Indeed, my laboratory experiments with Cerapachys have demonstrated that artificial trails drawn with poison gland material are effective as orientation cues at least for several hours. Although the raiding cerapachyine ants are usually enourmously outnumbered by the worker force of the prey species, not one Cerapachys worker was lost during all the raiding experiments in the laboratory. As can be seen from Fig.2, Cerapachys and Sphinctomyrmex are excellently protected by a heavily sclerotized cuticle. The intersegmental joints, that is, the joints between head and thorax and between thorax, petiole and gaster, are covered by cuticular projections so that no intersegmental membrane is exposed, even if the ant is twisted and bent to an extreme degree. In addition, Cerapachys and probably all the other cerapachyine ants have a most powerful sting that immobilizes the opponents within seconds. Not only the adults of the raided colony, but also the captured larvae and pupae are stung by the raiders before they are retrieved to the Cerapachys nest. Observations and experiments demonstrated that the prey larvae are kept in a stage of metabolic stasis and can thereby be stored for a period of more than two months. This food storage system enables Cerapachys to adjust the raiding activities to food requirement and supply. From the laboratory experiments we can conclude that Cerapachys does not conduct daily or periodic raiding expeditions. The frequency of raiding expeditions depends on the food supply stored inside the Cerapachys nest. I was unable to demonstrate periodic nomadic behavior in cerapachys in the laboratory. I assume that nest emigrations might occur relatively frequently in this species, but that they do not follow a periodic pattern. instead, environmental factors such as food supply or physical conditions of the nest site are likely to play the important role in inducing a Cerapachys colony to emigrate.