Stridulation-like behaviour in the Red Wood ant (<i>Formica rufa</i>)

Insects are known for communicating via sounds created by rubbing body parts, a behaviour referred to as stridulation. Red Wood ants (<i>Formica rufa</i> group) are not known to possess stridulatory organs and have historically been categorised as a non-stridulating species. In this exploratory study, we report that Red Wood ants generate stridulation-like sounds, being about 0.7 ± 0.2 seconds-long (mean ± standard deviation), rattling sounds that were repeatedly generated in our laboratory setting (about 0.6 productions h-1 individual-1). In addition, we assess Red Wood ant behavioural responses to playbacks of this stridulation-like sound. Our playback experiments show that the stridulation-like sound initiates a reduced locomotory speed among conspecifics, an effect not seen when the ants were exposed to silence. However, this response was not different from that generated by an artificial pure tone, making the use of this stridulation-like sound uncertain. We hypothesise that the stridulation-like sound is produced by rubbing the leg against a ridge structure located on the anterior margin of the pronotum, a structure that we describe using X-ray micro-tomography. Our exploratory study suggests that the recorded sound may be part of <i>Formica</i> ant communication that is hard to detect and easily missed in behavioural assays.