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 degrees C. The peak voltage was measured between 32-38 degrees C and attained 240-360 mV, but with further increase in temperature, the voltage decreased, dropping to 0 mV at about 45-50 degrees C. Upon second measurement (of same silk specimen), the voltage peak usually occurred later (by 8-9 degrees C) and at higher temperature than in the first measurement. Continuous measurements during warming up to 30 degrees C followed by cooling down to 15 degrees 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.