none The authors have studied a Swedish population of Eriogaster arbusculae and made comparisons with E. lanestris from the same area. The morphology of egg clusters, larvae, and adults as well as biological features show that E. arbusculae is a distinct species. The egg cluster is more rounded like a bulb whereas that of E. lanestris is more stretched along the twig. The eggs are often parasitized by Telenomus wullenschlegeli Mayr. The caterpillar is blueish black, the blanketspots are much smaller than those of E. lanestris . The caterpillars feed on Betula nana, Rubus chamaemorus, Vaccinium myrtillus and V. uliginosum . They are often parasitized by Apanteles calodetta Nixon. The habitat of E. arbusculae in Sweden is bogs at 700-800 m a.s.l. in the Dalarna, Haerjedalen and Jaemtland provinces. Systematics; Nomenclature; Diagnosis; General morphology; Whole animal physiology; Nutrition; Diet; Reproduction; Reproductive system; Reproductive behaviour; Life cycle and development; Life cycle; Developmental stages; Development; Metamorphosis; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Habitat utilization; Habitat; Semiaquatic habitat; Abiotic factors; Physical factors; Land and freshwater zones; Palaearctic region; Europe Eriogaster arbusculae (Lasiocampidae); General morphology; Dormancy; Pupal; Food plants; Betula; Rubus & Vaccinium; Egg; Cluster morphology & hymenopteran parasites; Genitalia; Breeding place; Oviposition sites; Oviposition; Site records; Larva, food plants & hymenopteran parasites; Pupa; Morphology, dormancy & emergence; Emergence from pupa; Temperature relationship; Hymenopteran parasites; Telenomus wullenschlegeli & Apanteles calodetta; Records; Habitat preference; Bogs; Peat bog; Habitat records; Temperature; Emergence from pupa relationship; Sweden; Biology & morphology Eriogaster lanestris (Lasiocampidae); General morphology; Dormancy; Pupal; Food plants; Betula; Rubus & Vaccinium; Egg; Cluster morphology & hymenopteran parasites; Genitalia; Breeding place; Oviposition sites; Oviposition; Site records; Larva, food plants & hymenopteran parasites; Pupa; Morphology, dormancy & emergence; Emergence from pupa; Temperature relationship; Hymenopteran parasites; Telenomus wullenschlegeli & Apanteles calodetta; Records; Temperature; Emergence from pupa relationship; Sweden; Biology & morphology Apanteles calodetta (Braconidae ); Telenomus wullenschlegeli (Scelionidae ); Lepidopteran hosts; Eriogaster arbusculae & E. lanestris; Records; Sweden; Lepidopteran host records Eriogaster arbusculae (Lasiocampidae); Diagnostic characters; P. 117 Eriogaster lanestris (Lasiocampidae); Diagnostic characters; P. 117