R. Visicchio

Colony founding in Polyergus rufescens: the role of the Dufour's gland.

Summary: In the European slave-making ant Polyergus rufescens, the occurrence of chemical strategies during the initial phase of dependent colony foundation or usurpation was investigated. To test this idea, we analysed the effect of the secretion of different glands (Dufours, poison, pygidial, rectal, and mandibular) on the behaviour of workers of its common host species, Formica cunicularia (subgenus Serviformica). Workers of another species, Formica rufibarbis (Serviformica), were daubed with these extracts, and introduced into colony fragments of F. cunicularia. The results of a set of laboratory aggression test showed that the secretion of the mandibular, pygidial, rectal, and poison glands do not alter the characteristic aggressive reactions generally performed by resident workers against alien ants. By contrast, the Dufours gland seems to play a crucial role in the appeasement of residents of the target host colony. In fact, its secretion drastically lowers the degree of overt attacks shown by F. cunicularia workers against the intruders. This chemical strategy probably allows an easier invasion and usurpation of host colonies by newly mated females of P. rufescens.

Behavioural assays testing the appeasement allomone of Polyergus rufescens queens during host-colony usurpation.

The role of decyl butyrate (the main component of the secretion of Dufours gland in newly-mated queens) during host-colony usurpation was investigated in the European amazon ant Polyergus rufescens. The effect of this ester on the behaviour of workers of the common host species Formica cunicularia was analysed. We used a laboratory aggression test during which workers of the species Camponotus ligniperda, masked with synthetic decyl butyrate, were introduced into colony fragments of F. cunicularia. The results showed that this compound drastically lowers the degree of overt aggression against the introduced ants, and we suggest that it acts as an appeasement allomone. Moreover, pieces of filter-paper soaked with decyl butyrate were more attractive than pieces of paper treated with solvent. The results are discussed in relation to a chemical strategy that likely allows an easier invasion and usurpation of host colonies by queens of this obligatory slave-making ant.

Mandible associated glands in queens of the slave-making ant Polyergus rufescens (Hymenoptera, Formicidae)

SummaryA first description of the structural organization of two exocrine glands associated with the mandibles of queens of the obligatory slave-making ant Polyergus rufescens is reported. The mandibular gland consists of clustered bicellular secretory units connected by means of cuticular ducts to a big reservoir. The reservoir continues in a duct that opens proximally on the mandible cuticle. Intramandibular glands are isolated bicellular secretory units connected to the external pores through a cuticular duct. No reservoir has been observed. In both cases the secretory cells belong to the 3rd class. Functional aspects of the investigated glands are discussed taking also into account for the parasitic habit of this slave-making species.

Dufour’s gland contents of queens of the slave-making ant Polyergus rufescens and its host species Formica cunicularia

Dufour’s gland contents of the queens of the slave-making ant species Polyergus rufescens and its host, Formica cunicularia, were analysed by gas chromatography-mass spectrometry. In P. rufescens, the gland contains a mixture of several esters, one of which, decyl isobutyrate, was consistently present in large quantities (constituting more than 80% of glandular contents). Glandular contents in the host species, were found to be different, containing, in addition to two substances also found in P. rufescens, a mixture of acetates and several hydrocarbons. Decyl isobutyrate was not present. We discuss the possible role of these substances in the life history of P. rufescens.

Glandular sources of recruitment, trail, and propaganda semiochemicals in the slave-making ant Polyergus rufescens

The chemical sources involved in raiding activities of the slave-making ant Polyergus rufescens were investigated in the laboratory. In particular, we identified the sources of substances the raiding workers employ to convey information to nestmates and to disrupt reactions by defending workers during host nest invasion. Different secretions were offered to the dulotic ants on the tip of a blotting paper strip inserted a few centimetres into the nest entrance. The Dufours gland secretion induced the characteristic mass exit from the nest and is likely the source of a recruitment signal. Moreover, laboratory tests with secretions from different anatomical structures revealed that the trail signals for homing raiding workers are conveyed by their hindgut contents released during the outbound trip. Finally, to ascertain if P. rufescens raiding workers use “propaganda” pheromones, the behavioural response of the Formica (Serviformica) cunicularia host workers to different secretions of the slave-makers was investigated. Only mandibular secretions caused the typical panic reactions shown by residents at the moment of nest invasion. This probably facilitates both nest invasion and brood sacking by raiding workers.

The mandibular glands as a source of sexual pheromones in virgin queens of Polyergus rufescens (Hymenoptera, Formicidae)

Abstract> ‘Female‐calling syndrome’ is the reproductive strategy adopted by the European slave‐making ant Polyergus rufescens, where a winged female disperses a short distance from the nest, ascends to an elevated position (at the top of a blade of grass) and is joined shortly thereafter by conspecific males. This suggests the probable use of very effective sexual calling pheromones by females. Here, the role of the mandibular glands of winged females is demonstrated to be a source of sexual pheromones. The secretion of these glands (among the structures tested: the mandibular, Dufour, poison and pygidial glands and the head without mandibular glands) appears to have a strong attractive effect on males and also elicits male copulatory behaviour.

Behavioral bioassays testing the methyl 6-methylsalicylate as a component of the female sex pheromone in the slave-making ant Polyergus rufescens (Hymenoptera, Formicidae)

The mating behavior of the slave-making ant Polyergus rufescens is an example of “Female-calling Syndrome” as defined by Hölldobler and Bartz (1985). In this species, in fact, nuptial flights are inconspicuous and not well synchronized between sexes. Moreover, females typically call for males, do not disperse widely, and remain near their natal nest or return to a dulotic colony after mating. Lastly, the virgin queen usually mates only with the first male that is able to locate her and after mating she cuts off her wings and hides in the vegetation surrounding the nest (Mori et al., 2001). Two main aspects of P. rufescens ecology could explain the mating strategy adopted by this species: the relative low density of both sexes and its parasitic nature. As suggested by Bourke and Franks (1995), female calling could be advantageous for species that do not produce a large amount of sexuals: through this mating strategy, females probably maximize their chances of finding a mate. Moreover, female calling is suitable for scattered colonies, like those

How raiders of the slave-making ant Polyergus rufescens (Hymenoptera Formicidae) evaluate a target host nest

Raiding activity is a crucial task of the slave-making ant Polyergus rufescens, during which colonies of the host species are attacked in order to pillage the resident brood and obtain a slave work force. The features of the host colony influencing the organization of P. rufescens raids were investigated by laboratory experiments which allow the analysis of behavioural patterns unlikely to be observed in the field. In particular, the mechanism of evaluation of the target nest was studied. By providing P. rufescens the possibility to sack different host nests, we found that a greater number of raiders was recruited when a greater amount of brood was present inside the attacked nest. The hypotheses that raiding activity is performed in accordance with the “optimal foraging theory” and that raiders prefer to sack colonies where the residents are more aggressive are discussed.

The defensive behaviour of two Formica slave‐ant species: coevolutive implications with their parasite Polyergus rufescens (Hymenoptera, Formicidae)†

The cooperative defence in ant societies is a suitable system that can keep the integrity of the colonies from threats represented by competitors, predators and parasites. This is due to the presence of a well‐organized alarm communication system that induces defensive behaviours. Alarm reaction of Formica cunicularia and F. rufibarbis ant species was investigated by means of laboratory tests presenting different kinds of threatening stimuli (ants from alien colonies, homospecific and heterospecific) to colony fragments. Results show that both species react to a potential threat to the colony with an aggressive alarm instead of a panic alarm, characterized by a prompt exit of workers from the nest, attraction towards the stimulus and aggressive behaviours. Moreover they seem to feature the phenomenon of the ‘Enemy Specification’ towards their obligate parasite Polyergus rufescens. This could be considered as a result of a reciprocal adaptation between host and parasite. Furthermore, the comparison between the alarm reaction of the two studied species shows that F. rufibarbis seems to be more aggressive than F. cunicularia towards the parasite; this result could add a further clarification about the preference of the parasite towards F. cunicularia as host species.

Laboratory observations on raiding behaviour of the slave‐making ant, Polyergus rufescens

Abstract In this research, the necessary conditions to induce raiding activity of the slave‐making ant Polyergus rufescens were re‐created in the laboratory. This, the first description of such behaviour observed under controlled situations, facilitates the study of some behavioural patterns of this crucial event that are unlikely observable in the field.