Effect of thermal and chemical components of bombardier beetle chemical defense: Glossopharyngeal response in two species of toads (Bufo americanus, B. marinus)

Abstract

The effect of temperature in the chemical defense of bombardier beetles is examined on the basis of toad glossopharyngeal nerve responses to tongue stimulation with either bombardier beetle defensive discharges or artificial stimuli of variable temperature and quinone content. The results indicate the following: 1. Responses to bombardier beetle discharges (Fig. 1) are characterized by short latency (11–23 ms) and rapid attainment of maximum amplitude (within 100 ms). Response amplitude diminishes sharply within 400 to 700 ms, leaving a long-lasting (greater than 1.6 s) low level response. Repeated stimulation destroys general tongue sensitivity. 2a. Responses to artificial stimuli all begin with a brief mechanoreceptive response to the impact of the stimulus droplet (Figs. 2, 3 and 4). The latency of this initial response component is less than 8 ms; maximum response amplitude is reached between 12 and 15 ms after stimulus onset. 2b. Responses to unheated water aerosols contain only this initial response component (Figs. 3 and 4a); they end within 50 ms of stimulus onset. 2c. Responses to hot water aerosols show a second component in the interval between 30 and 200 ms after stimulus onset (Figs. 2 and 4). The maximum amplitude and breadth of this response — but not the time of maximum amplitude — change with increasing stimulus temperature (Fig. 4). Repeated stimulation does not measurably diminish tongue sensitivity. 2d. For unheated quinone stimuli (Fig. 3), the second response component begins sooner and lasts longer (i.e., more than 1.6 s) than it does for hot water stimuli, but there is no difference between the two stimuli in the time at which maximum amplitude is reached. Repeated stimulation with quinone destroys tongue sensitivity. 2e. Responses to hot quinone stimuli show an increase in the amplitude of the first 50 ms of the second response component which exceeds that to be expected from a summation of independent thermal and chemical responses (Fig. 3). A synergistic action of temperature in facilitating sublimation and spread of quinone is believed to explain this effect. 2f. Synergism of temperature and quinone is no longer evident after the first 50 ms of the second response component. On the contrary, the maximum amplitude of the second peak in responses to hot quinone approaches the sum of responses to hot water and unheated quinone in only one toad species (B. marinus, Fig. 3c and d). InB. americanus (Fig. 3a and b), maximum response amplitude is similar for hot quinone, hot water, and unheated quinone. 2g. Responses to hot quinone aerosols (Fig. 3) are similar to responses to bombardier beetle defensive discharges (Fig. 1) except for the presence of the initial mechanoreceptive response. 3. It is concluded that the evolutionary advantages of the hot defensive discharge rest primarily on its synergism of quinone effects and on concomitant advantages of the discharge mechanism. The role of heat as a primary deterrent is thought to be of secondary importance.