The Elicitation of the Predatory Labial Strike of Dragonfly Larvae in Response to a Purely Mechanical Stimulus

The predatory labial strike of dragonfly (Aeschna) larvae could be elicited by an artificial water jet stimulus. The larvae showed fair prey-catching ability even when visually deprived, whereas when deprived of mechanosensory and visual information, the ability decreased sharply. Also, immobilized prey were unable to elicit a strike even from intact larvae. The percentage of labial strike (PLS) increased in proportion to the logarithm of the water velocity at the body surface. Animals starved for 1 day or more showed higher PLS than satiated ones. In addition to the water velocity, the movement of the jet nozzle (temporary change in the velocity) was also important in eliciting the labial strike. Although repetitive stimuli resulted in a rapid habituation of the labial strike, the second stimulus in a series was always the most effective: an arousal state to the newness was observed. The anterior parts of the body surface (including the legs) showed higher sensitivity of PLS to the water jet stimulus, whereas the posterior parts elicited responses other than the labial strike: turning, head orientation and the setting of a posture. These results explain nocturnal predation of larvae and predation in turbid pond beds where visual cues will be unavailable.