Thermovoltaic properties of hornet silk

In silk from the larval silk caps of the Oriental hornet Vespa orientalis (Hymenoptera, Vespinae), temperature-dependent changes in the electric voltage have been recorded, with rise in the voltage occurring mainly upon rise in the temperature between 10–36°C. The peak voltage was measured between 32–38°C and attained 240–360 mV, but with further increase in temperature, the voltage decreased, dropping to 0 mV at about 45–50°C. Upon second measurement (of same silk specimen), the voltage peak usually occurred later (by 8–9°C) and at higher temperature than in the first measurement. Continuous measurements during warming up to 30°C followed by cooling down to 15°C yielded an hysteresis between the warming “line” and the cooling “line”, the former often straight and the latter usually curved. Maintaining the silk specimen at a fixed temperature for a prolonged period (hours) initially causes the voltage to rise, then remain steady, and finally drop. Boiling the silk caps in tap water for 7–10 min exerts some changes in the silk properties, mainly a decrease in voltage level. The general behavior of the silk suggests that it is a polymer endowed with the qualities of an organic semiconductor. The various properties of the larval silk are discussed in great detail. Microsc. Res. Tech., 2006. © 2006 Wiley-Liss, Inc.