Natural Thermoelectric Heat Pump in Social Wasps

Photographs of wasps or hornets, taken with different temperature sensitive infrared cameras, reveal body temperatures that are sometimes significantly lower than the ambient temperature. This suggests that the hornets possess an intrinsic biological heat pump mechanism which can be used to achieve such cooling. Evidence is presented to substantiate this novel suggestion and to argue that the heat pump is most likely implemented by exploiting a thermoelectric effect in the hornet cuticle. Such a natural heat pump can conceivably also serve to cool the active hornet, engaged in daytime activities outside the nest at ambient temperatures exceeding $40°\mathrm{C}$, to a body temperature that is low enough to allow its survival in extreme thermal conditions. It might also function as a means of raising the body temperature up to a level that enables the hornet to remain active even when the ambient temperature is as low as $10°\mathrm{C}$.

Figure 1

Photographs of hornets taken with infrared cameras capable of temperature discrimination. The image in (a) was taken in summer 1998 using a Thermacom Model 290 PM camera and processed by commercial Thermonitor 95 software, both from Inframetrics USA. The images in (b)–(d) were recorded in October–November 2002 using a FLIR ThermaCam SC500 ($7.5–13\mu \mathrm{m}$) and a PC card interface Agema THV550, and were analyzed by software application ThermaCam Researcher version 2001. (a) Hornet anesthetized in its natural nest at night by ether vapor, lying on comb before recovery. (b) Similarly anesthetized hornets, at daytime in the laboratory, lying on comb during recovery stage. In (a), the color coded temperature and exhibited spot measurements of body temperature reveal that the outer shell of the hornet is as much as $3°\mathrm{C}$ cooler than the comb. In (b) one can discern six hornets in the recovery stage from ether vapor anesthesia. A hornet slightly to the right of center and two others at the lower right side are cooler than the comb. All three exhibit coldest temperatures in small isolated regions or spots. In the case of the hornet near the center, one such cold spot is located at the tip of the abdomen and an even colder spot is located at or near the compound eyes. The latter cold spot is $3°\mathrm{C}$ cooler than the comb. Above this hornet, another one can be discerned whose wings are cool though its body is warm, compared to the comb. More to the left are two more hornets with warm bodies, and even warmer heads: The head temperatures are up to $2°\mathrm{C}$ warmer than the nearby comb, and $4°\mathrm{C}$ warmer than the cold eyes of the central hornet. (c), (d) Routine activity near the entrance to a natural hornet nest which the wasps built and maintained during an entire active season (May through October 2002) in a plywood box positioned on a laboratory window sill. The entrance can be discerned as a circular hole of diameter $2.5\mathrm{c}\mathrm{m}$ in the box wall at the bottom, where the apparent temperature is somewhat higher than the exterior surface of the wall. In (c) a hornet is seen walking on the exterior of the wall just before it enters the nest, after having flown in several seconds earlier from an outing. In (d) a hornet is seen who has just exited from the nest, and is about to take off on a foraging flight. In this case, both the thorax and the abdomen are apparently somewhat warmer than the surroundings, though not warmer than the nest interior, but the wings are cooler, by about $1°\mathrm{C}$, than the plywood outer nest wall.

Figure 2

(a) Transmission electron micrograph (TEM) [17] of a section of hornet cuticle. The size scale is indicated by a vertical bar at the left which represents $10\mu \mathrm{m}$. At the top there appears a $0.2–0.3\mu \mathrm{m}$ thick epicuticular layer, and beneath it a $1\mu \mathrm{m}$ thick exocuticular layer. Proceeding toward the interior, these two layers are then followed by about 30 more roughly parallel endocuticular layers of decreasing thickness. The bottom layers, which are barely discernible, are about 10 times thinner than the exocuticular layer. (b) and (c) show scanning electron micrographs (SEM) of similar sections, at different magnifications, as denoted by the horizontal scale bars at the bottom which represent $1\mu \mathrm{m}$. (b) shows an entire cross section, comprising about 25 layers, similar to the section described in (a). (c) shows only about 10 layers of a similar section but at higher magnification. In the preparation process of the latter sample, part of the contents of individual layers was lost by material leakage, leaving mainly the skeleton of each layer. Consequently, the intralayer microstructure is more clearly visible in (c) than in (a) and (b). Note that, apart from the flat laminar structure of the nearly parallel layers, each layer seems to be additionally divided into cells which are separated by walls that are approximately perpendicular to the layer. (d) Picture of a commercial multistage thermoelectric heat pump device (Marlowe Industries Inc., Dallas, TX), whose linear dimensions, albeit variable, usually amount to a few centimeters. Note the superficial similarity in microstructure of this device and the cuticle sections displayed in (a)–(c), especially the one in (c), even though the size scale differs by a factor of ${10}^4$.