Eastern Tent Caterpillar - Tents and Temperature

Tents and Temperature

The tent of the eastern tent caterpillar is among the largest built by any tent caterpillar. The tents are constructed in the crotch of the host tree and are typically oriented so the broadest face of the structure faces the southeast, taking advantage of the morning sun. The caterpillars typically add silk to the structure at the onset of each of their daily activity periods. Silk is added directly to the surface of the tent as the caterpillars move back and forth over the structure. The silk is laid down under slight tension and it eventually contracts, causing the newly spun layer of silk to separate from the previously spun layer. The tent thus consists of discrete layers separated by gaps within which the caterpillars rest. The tent has openings to allow the caterpillars to enter and exit the structure. Openings are formed where branches jut from the structure, but are most common at the apex of the tent.

Light has a great effect on the caterpillars while they are spinning, and they always spin the majority of their silk on the most illuminated face of the tent. Indeed, if under experimental conditions, the dominant light source is directed at the tent from below, the caterpillars will build their tent upside down. Caterpillars continue to expand their tent until they enter the last phase of their larval lives. The sixth-instar caterpillar conserves its silk for cocoon construction and adds nothing to the tent. The tents are multifunctional. They facilitate basking, offer some protection from enemies, provide for secure purchase, and act as a staging site from which the caterpillars launch en masse forays to distant feeding sites. The elevated humidity inside the tent may facilitate molting.

Eastern tent caterpillars are among the earliest of caterpillars to appear in the spring. Because the early spring weather is often cold, the caterpillars rely on the heat of the sun to elevate their body temperatures to levels that allow them to digest their food. Studies show that below 15°C (59°F), the caterpillars are unable to process the food in their guts. Early instars are black and their bodies readily absorb the rays of the sun. When basking, they typically pack together tightly, reducing heat loss due to convective currents. Their long setae also serve to stem convective heat loss. The caterpillars may aggregate on the surface of the tent or within it. The tents act as miniature glass houses, trapping the heat of the morning sun and allowing the caterpillars to warm more quickly than they would if they remained outside the tent. Basking, aggregated caterpillars can achieve temperature in excess (T_body−T_ambient) of 44°C. The caterpillars can easily overheat, though, so must take evasive action when they become too hot.

Because of its layered structure, the tent is thermally heterogeneous, and the caterpillars can adjust their temperature by moving from layer to layer. They may also aggregate on the outside of the shaded side of the tent and hang from the tips of their abdomens to enhance convective heat loss and cooling.

As has been shown for some other caterpillars, eastern tent caterpillars are capable of generating a small amount of metabolic heat while they digest their meals. When recently fed caterpillars pack tightly together, the temperature in the interior of the mass may be several degrees above ambient temperature even in the absence of a radiant heat source. It is unclear whether this small heat gain has a significant effect on the rate of their growth.