Turning lances into shields: flower mantids stretch their raptorial forelegs to avert and deflect predator attack.

Evolutionary co-option, in which existing traits acquire novel adaptive functions, is a key strategy by which organisms adapt to new environmental challenges. Although such co-option has been widely documented at the genetic and morphological levels, its incidence at the behavioural level remains largely unknown. Mantids stretch their forelegs to capture prey; however, some flower mantids also perform foreleg stretches in the absence of prey. The current study tested whether this behaviour represents a novel function of the foreleg stretch, thus representing a case of behavioural co-option. Predator encounter behaviour assays revealed that foreleg stretching facilitates the escape of flower mantids from large predatory mantids by delaying predator approach or deflecting their attack towards less vulnerable body parts. Phylogenetic analysis suggested that the ancestral function of foreleg stretching involves prey capture, with the anti-predator function subsequently acquired in the flower mantid clade, coinciding with the diversification of large-sized mantids, the most likely invertebrate predators of flower mantids. This study provides a case of behavioural co-option, where a predator uses its predatory organ as a defensive implement to cope with its own predators. These findings further suggest that behavioural co-option may be common in nature, meriting more comprehensive studies.