Heliothermy - Many social caterpillars are heliotherms, that is, they elevate their temperature by basking in the sun.  Such basking behavior is markedly enhanced by the presence of siblings.

Caterpillars that feed at times of the year when air temperatures are low are particularly likely to benefit from aggregative basking.  The spring feeding larvae of the nymphalid butterfly Euphydryas aurinia , a non-shelter building species, bask “en masse” in the open, packing their bodies tightly together to minimize convective heat loss. Under high levels of solar radiation on cold days, gregariousness and the darkness of their cuticle enables the larvae to gain temperature excesses (T_body -T_ambient) as great as 30^o C.

The construction of shelters that trap the heat of the sun enables social species to gain even more control over their body temperature.   The extensive shelters of the tent caterpillars (Malacosoma) provide a surface large enough to enable the colony to bask “en masse” and the caterpillars oriented their nests to take full advantage of the sun.  The silk walls of the structures are dense enough to serve as barriers to convective heat loss, allowing them to function as miniature glasshouses. When the tents of M. americanum are shielded from the sun the caterpillars are unable to raise their body temperature above the cool ambient temperatures that prevail in the spring and they fail to grow.  When tents are exposed to sunlight, their layered structure creates a  thermally heterogeneous microhabitat within which the caterpillars can thermoregulate by moving from compartment to compartment.  In addition, studies show that caterpillars basking side-by-side in groups on the surface of the tent are able to achieve significantly higher body temperatures, due to boundary layer effects and convective shielding, than solitary caterpillars basking in the open on nearby branches.

Utilization of metabolic heat – In the absence of  a radiant heat source, the body temperature of aggregated social caterpillars may be several degrees centigrade above ambient temperature.  This phenomenon was first reported in 1938 by Mosebach-Pukowski in Vanessa caterpillars.  Subsequently,  Breuer and Devkota noted that the shelter of the pine processionary caterpillar Thaumetopoea pityocampa rose 2 - 3^o C when occupied by the caterpillars and cooled down when they left the structure to feed.  Recently, Ruf and Fielder conducted careful experiments that showed that when the social caterpillars of Eriogaster lanestris aggregate in their tent the internal temperature of the structure may exceed ambient temperature by as much as 6.7^o C  , though the average temperature gains of  2 - 3^o C were in line with those reported by previous investigators.  The thermal gains reported by these investigators appear attributable to the trapping of the metabolic heat generated by the caterpillars as they process food.