Effects of temperature and humidity on transpiration in adults of the lesser mealworm, Alphitobius diaperinus (Coleoptera: Tenebrionidae)

Abstract

Measurements of water loss were made on adults of the lesser mealworm, Alphitobius diaperinus, using a recording micro-electrobalance and a programmable heat circulator bath. This species originates in tropical regions and infests poultry houses in temperate countries. Two routes of water loss were examined: the general cuticle and via the spiracles. Temperature and relative humidity of the ambient air substantially affect the cuticular transpiration in adults (fresh body weights from 12 mg to 22 mg). At near 0% R.H., between 20 and 40°C the rate of body water loss gradually increased; on the other hand, the insects gained weight in an atmosphere close to saturation. Above 40°C transpiration flow abruptly increased coinciding with the start of vigorous locomotor activity. This critical point corresponds to the opening of the spiracles from which the water is expelled from the tracheal system. In dead specimens, killed by cyanide or solvent, the water vapour slowly diffused out of the spiracles and, as in atracheate insects, the transpiration curves did not show a peak as the air temperature was increased. The thermostupor point (TSP) occurred as the insects became motionless; the corresponding temperature is significantly affected by atmospheric relative humidity (TSP=47.4±0.6°C at c. 0% R.H.; TSP=46.6±0.7°C at c. 100% R.H.). The transpiration flow was about four times as fast in specimens treated with solvent as in the individuals (live or cyanide-killed) that had undamaged water-proof cuticle. This species has to cope with a double challenge: (i) to adapt its physiology and ecology to poultry-house conditions which constitutes an extension of its primary habitats, and (ii) to survive over winter; high drought resistance and heat tolerance may constitute a pre-adaptation to conquer anthropogenic air-conditioned sites.