Mormolyce phyllodes can reach a length of 60–100 millimetres (2.4–3.9 in). These beetles possess a flat leaf-shaped, shiny black or brown body with distinctive violin-shaped translucent elytra (hence the common name). This characteristic mimicry protects them against predators, while their flat shaped body allow them to dwell in soil cracks or under the bark and leaves of trees. Head and pronotum are very elongated, with long antennae and the legs are long and slender.
Both adults and larvae are predators, feeding on insect larvae. For defense purposes, they secrete the poisonous butyric acid. The larvae live between layers of bracket fungi, genus Polyporus. Their development lasts 8–9 months, while pupation lasts 8–10 weeks. Adults fly from August to November.