Most of a plant’s food comes from photosynthesis. The plant can make the sugars it needs to fuel it with water and carbon dioxide. But some molecules need more. Plants can pick up other elements, like nitrogen or phosphorus, through their roots. If the soil doesn’t have those elements, some plants can grab them by capturing prey, usually insects and digesting them. The sundew, Drosera spp., is one such plant that captures insects in its leaves. Its leaves are not a healthy place for an insect to be, so a fly like Toxomerus basalis, the Sundew Flower Fly, is a puzzle. A recent study by Andreas Fleischmann and colleagues has found that this fly lays its eggs on or near sundews. Not only that, but the larvae, or maggots, live their lives on sundews, crawling around them until they pupate. Why would a fly leave its children in the care of a plant that would happily eat them?
Sundews get their name from their traps. They have leaves with hairs, and the hairs hold a sticky glue that glistens, like dew, in the sun. They look pretty, but they’re deadly. A fly landing to investigate what might look like a snack gets stuck on the glue and struggles. These struggles activate the leaves. Slowly the leaf curls around, bringing more hairs into contact with the victim, making it harder for them to escape. Eventually, the whole leaf curls around the prey, and the plant releases enzymes to digest the unfortunate creature.
The leaves are very effective, so when a sundew wants to attract a pollinator, it makes sure its flowers are well away from the leaves. That way, it doesn’t load up an insect with pollen before eating it.
The efficiency of a sundew as a killer isn’t just a threat. It’s also an opportunity. Wrapping a leaf around a victim takes time. If a thief were fast enough, the sundew could do the hard work of attracting and trapping a meal, and then someone else could step in and steal the food before it was gone. If you’re enterprising enough, a sundew is less a graveyard and more a meal delivery service. This is the case for the Sundew Flower Fly.
The larvae of the Sundew Flower Fly are hungry and happy to eat most things. Fleischmann and colleagues cite work showing them eating a menu including “aphids, caterpillars, beetle larvae, planthoppers, gall midges, thrips, mealybugs, whiteflies, mites, and insect eggs”. Earlier work by Fleischmann and colleagues showed that if a sundew could catch it, then a Sundew Flower Fly larva was happy to try eating it. The team set out to see if this was a phase in the life cycle or if there was something more. Staking out sundews with cameras, they were able to track the life cycle of the Sundew Flower Fly and found that from egg to pupation, they lived on the plant.
