What sticky substance on sundew leaves traps insects?

The sticky substance on sundew leaves that traps insects is mucilage. Sundews, members of the genus Drosera, produce it from specialized glands at the tips of their hair-like tentacles.

Mucilage is a viscous, water-rich secretion that forms glistening droplets across the leaf. The droplets can attract small insects, while their extreme stickiness prevents prey from walking or flying away. As the insect struggles, nearby tentacles often bend inward, increasing contact with the adhesive surface.

Once prey is secured, sundews release digestive enzymes, including proteases and phosphatases, that break down soft tissues and release nutrients. The plant absorbs compounds such as nitrogen and phosphorus that are scarce in the acidic, nutrient-poor soils where many sundews grow. The leaves remain photosynthetic, so captured insects supplement rather than replace ordinary plant nutrition.

Sap, resin, and nectar are common plant substances but do not name the adhesive trap secretion in sundews. Nectar-like compounds may help attract insects, but the substance responsible for holding them is mucilage. This is why sundews are classified as using a flypaper trap.