1. Colonies of the social caterpillar Eutachyptera psidii (Sallé) (Hymenoptera: Lasiocampidae) occurring on oak (Quercus) in upland forests of Mexico endure periods as long as 6 weeks, with little or no food or water between the time host trees shed their leaves in April and produce new leaves in June. 2. By monitoring the activity of both field and laboratory colonies with infrared activity monitors and data loggers, it was found that although colonies remain active during the period of deprivation, their foraging activity shifts from once nightly when food is available to once every second night when food-deprived. 3. Over a period of absolute food and water deprivation of 18 days, caterpillars lost an average of 36% of their initial mass but none perished. On average, the caterpillars regained their pre-starvation mass within a few days after food was provided and continued to grow thereafter. During the period of starvation, caterpillars were observed to chew on dead and dried leaves in the field and on sheets of paper in the laboratory. 4. To the authors' knowledge, there is no other documented instance of a species of caterpillar that exhibits the physiological capacity to engage in a similar level of persistent activity when forced to endure a prolonged period with neither food nor water.