Knocking and Scraping as Alarm Signals in Dolichoderinae Ants from the Malay Peninsula (Hymenoptera: Formicidae: Dolichoderinae)

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The defence signaling and behaviour in Dolichoderinae ant species from the Malay Peninsula are observed in the field and brought home to the lab for measurements. The results are discussed in comparison with the corresponding behaviour in the European ants Camponotus ligniperda (Latreille 1802) and C herculeanus (Linnaeus 1758). 6 out of the 7 investigated tropical ant species from the Malay Peninsula were found to produce vibration signals with the exception of Dolichoderus tuberifer Emery 1887. The vibrations were generated in 2 different manners, namely by knocking and scraping. A high speed camera (500 pictures / s) was employed to scrutinise the production of the signals. To achieve the generation of vibrations 5 species (Dolichoderus thoracicus (Smith 1860), D burmanicus Bingham 1903, D sulcaticeps (Mayr 1870) and 2 non-described species) scratch over the ground using their mandibels whilst standing. The signals reveal a distinct pattern of three clearly distinguishable sequences and are characterised by a repetivity of 3-5 times in general However, the signals can occur up to 15 times. One element of the signal lasts 0.04-0.14 s (depending on temperature). The time between the elements lasts 0.22-0.36 s The frequency domain reveals a range from 1-19.5 kHz. In contrast, the sixth species (Dolichoderus patens (Mayr 1870)) uses its gaster to generate the signals. It hits the ground. It can move slowly during vibration production. The signal only lasts 0.012-0.019 s with breaks of 0.058-0.162 s accordingly. The signal is repeated up to 30 times and even more frequently in a few cases, with frequencies detected between 0.5-22 kHz. In general, only the workers of the various species seem to generate vibration signals, never the sexuals. Behavioural patterns were examined in D thoracicus and D patens. Workers of D thoracicus reacted in different ways depending on their level of excitement while perceiving the vibration signal. If excitement was low they froze, motionless. When more excited they also produced signals or increased their running speed manifoldly and showed a lower threshold of aggression towards visually perceivable moving objects. Threatening behaviour (i.e. straddling of the mandibles) was observed when objects were obscured. The behaviour takes place outside the nest. In contrast, the workers of D patens did neither move vivdly nor reacted aggressively, but gravitated slowly towards the nest instead. At low excitement levels a waving of their gaster was recorded, whereas a knocking motion onto the ground could be observed when the ants were stimulated excessively. On perception of these signals the sexuals of either species were prompted swiftly to move into the nest.

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