Moulting stage is not defenceless: antipredator strategy of gregarious locusts during moulting

In arthropods, moulting is characterized by immobility and physical vulnerability, making individuals undergoing it sensitive to predation. Here, we examined how gregarious nymphs of desert locusts, Schistocerca gregaria, avoid predation during moulting, in the Sahara Desert. In the field, premoulting, gregarious, last-instar nymphs stopped marching and remained on a roosting bush. In response to an approaching simulated vertebrate predator, both pre- and postmoulting nymphs quickly dropped into the bush and hid themselves under branches. Although middle-moulting nymphs that were shedding an old exocuticle were immobile for approximately 10 min, once the femurs were out of the old exocuticle, the insects started to exhibit dropping/hiding escape behaviour. Behavioural experiments confirmed that pre-, middle- and postmoulting individuals were able to extend their wings by climbing after dropping. However, if dropped locusts were not allowed to climb and stayed on the ground, they failed to extend their wings. Hence, our results suggest that the integration of a physically protected moulting site choice before entering the moulting state, a dropping/hiding escape behaviour, which does not require dynamic locomotion, and flexible movement during the soft-bodied state could act to reduce moulting-associated predation. This is the first report of active behavioural and physical antipredator strategies in moulting gregarious juvenile arthropods.