Defensive responses of larval Manduca sexta and their sensitization by noxious stimuli in the laboratory and field.

Sensitization of defensive responses following noxious stimulation occurs in diverse species, but no demonstration of nociceptive sensitization in insects has been reported. A set of defensive behavior patterns in larval Manduca sexta is described and shown to undergo sensitization following noxious mechanical stimulation. The striking response is a rapid bending that accurately propels the head towards sharply poking or pinching stimuli applied to most abdominal segments. The strike is accompanied by opening of the mandibles and, sometimes, regurgitation. The strike may function to dislodge small attackers and startle larger predators. When the same stimuli are applied to anterior segments, the head is pulled away in a withdrawal response. Noxious stimuli to anterior or posterior segments can evoke a transient withdrawal (cocking) that precedes a strike towards the source of stimulation and may function to maximize the velocity of the strike. More intense noxious stimuli evoke faster, larger strikes and may also elicit thrashing, which consists of large, cyclic, side-to-side movements that are not directed at any target. These are sometimes also associated with low-amplitude quivering cycles. Striking and thrashing sequences elicited by obvious wounding are sometimes followed by grooming-like behavior. Very young larvae also show locomotor responses to noxious stimuli. Observations in the field of attacks on M. sexta larvae by Cardinalis cardinalis, an avian predator, suggest that thrashing decreases the success of a bird in biting a larva. In the laboratory, noxious stimulation was found to produce two forms of sensitization. Repeated pinching of prolegs produces incremental sensitization, with later pinches evoking more strikes than the first pinch. Brisk pinching or poking of prolegs also produces conventional sensitization, in which weak test stimuli delivered to another site evoke more strikes following noxious stimulation. The degree and duration of sensitization increase with more intense noxious stimulation. The most intense stimulus sequences were found to enhance strike frequency for approximately 60 min. Nociceptive sensitization generalizes to sites distant from sites of noxious stimulation, suggesting that it involves a general, but transient, arousal of defensive responses. http://www.biologists.com/JEB/movies/jeb3271.html