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 i n diverse species, but no demonstration of nociceptive sensitization in ins ects has been reported, A set of defensive behavior patterns in larval Mand uca sexta is described and shown to undergo sensitization following noxious mechanical stimulation, The striking response is a rapid bending that accu rately propels the head towards sharply poking or pinching stimuli applied to most abdominal segments, The strike is accompanied by opening of the man dibles and, sometimes, regurgitation, The strike may function to dislodge s mall attackers and startle larger predators, When the same stimuli are appl ied to anterior segments, the head is pulled away in a withdrawal response, Noxious stimuli to anterior or posterior segments can evoke a transient wi thdrawal (cocking) that precedes a strike towards the source of stimulation and may function to maximize the velocity of the strike, More intense noxi ous stimuli evoke faster, larger strikes and may also elicit thrashing, whi ch consists of large, cyclic, side-to-side movements that are not directed at any target, These are sometimes also associated with low-amplitude quive ring cycles, Striking and thrashing sequences elicited by obvious wounding are sometimes followed by grooming-like behavior. Very young larvae also sh ow locomotor responses to noxious stimuli, Observations in the field of att acks on M. sexta larvae by Cardinalis cardinalis, an avian predator, sugges t that thrashing decreases the success of a bird in biting a larva, In the laboratory, noxious stimulation was found to produce two forms of sensitiza tion. 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 i ntense noxious stimulation, The most intense stimulus sequences were found to enhance strike frequency for approximately 60 min, Nociceptive sensitiza tion generalizes to sites distant from sites of noxious stimulation, sugges ting that it involves a general, but transient, arousal of defensive respon ses.