Power and heat measurement by direct calorimetry of individual insect response to allelo- and toxic compounds

A microcalorimeter was constructed for the individual insect in order to measure the insect's power output as a function of time (the thermogram). The instrument has a figure of merit of 7 μW/μV. It includes a Peltier pumped thermoelectric control loop which protects from intruding ambient thermals, and holds baseline drift to less than 2 μV/day. The design is a conventional twin, or differential heat conduction calorimeter, with two insect-holding vessels of glass. The vessels are connected by a stopcock, to give the insect the option of crawling from the sample chamber to the reference chamber. Continuous, metered air flow is provided. A small pulse of compound may be born in, as vapor with the air flow, for the study of attractants, toxic compounds, anesthetics and allelochemicals. The insect's reaction to such compounds, appears in the thermogram. Cabbage looper larvae were examined in their irritative exothermic reaction to microgram amounts of benzene, and in their neutral behavior toward similar amounts of aliphatic hydrocarbons delivered as a single pulse of vapor. The male northern corn rootworm was monitored in his attraction to female rootworm extract ('pheromone') and his tendency to move upwind, or up airflow, toward the pheromone. The instrument enables discrimination of the transient metabolism of muscle use, from that of the resting state power which is probably the true basal metabolic rate power.