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Eggs

Egg masses are typically laid on branches and trunks of trees, but may be found in any sheltered location, including rocks, foliage and vehicles. Females are flightless, so they lay eggs on a surface near where they emerged from their pupa. The eggs are covered in a coating of hairs. The covering provides protection from predators and parasites, and may be important for insulating the eggs from cold and for sealing in moisture.

The larva inside the egg becomes fully developed in about one month after being laid and then enters diapause to overwinter. The egg is in the overwintering stage lasting for eight or nine months. Development ceases in preparation for the winter. After an acclimation stage, during which the larva inside the egg reduces its water content, eggs can withstand freezing temperatures. The larva inside the egg resumes activity in the spring, and reabsorbs water. The larva will then chew through the chorion of the egg and the protective hair of the egg cluster in the spring.

Egg clusters are usually an oval about 19 millimetres (3 ⁄ 4 in) wide and 38 millimetres (1 + 1 ⁄ 2 in) long. The egg masses are buff yellow-brown, likened to a manila folder, but may bleach out over the winter months. As the female lays them, she covers them with hair-like setae from her abdomen. Egg clusters contain from 100 to 1000 eggs. Due to the appearance of the eggs, several common names have emerged; the German Schwammspinner (literally "sponge spinner") and French la spongieuse refer to the spongy texture of the egg cluster.