Carnivorous plant conned out of a meal by cunning fly larvae

Species: A hoverfly, Toxomerus basalis

Habitat: On the sticky leaves of sundew plants in Brazil

At first glance, all seemed well in the sundew’s larder. Sticky tentacles lining this carnivorous plant’s leaves had done their job most effectively, trapping small insects and condemning them to a gloopy death. Sundew leaves secrete a sweet, viscous mucilage that attracts and smothers them.

But in the forests of Brazil, a thief lurks among the carcasses. A grub less than a centimetre in size, gliding in goo and devouring the plant’s food reserves. Soon, an adult fly that looks like a bee emerges with a buzz and sets off at speed. A flower fly or hoverfly, from the family Syrphidae.

The hoverfly larvae have made a super-efficient insect death trap, their homes. And they don’t even pay rent. Hoverfly adults are vegetarian and feed on pollen and nectar, but the larvae are ravenous predators of smaller insects, typically aphids.

The discovery that they steal plants’ prey has now been reported by Andreas Fleischmann of the Botanische Staatssammlung München in Munich and colleagues. They observed larvae of the hoverfly Toxomerus basalis on six different sundew (Drosera) species, in multiple locations.

Legless criminals

More than 140 species of Toxomerus are known from Central and South America, but this is the first record of any of the larvae being thieves. In fact, this species has the distinction of being the first known thief among larvae in the entire Syrphidae family.

The T. basalis larvae look rather like maggots, flattened with no distinct head or limbs, and mouthparts designed to pierce and suck the juices out of their prey.

They vary in colour from yellow to light green, with black or red stripes, blending beautifully on the green leaves of some sundew species. In cases where the leaves are not green, the larvae hide quickly underneath or in-between the sticky leaves when disturbed, says Fleischmann.

The larvae secrete a watery fluid that lubricates their lower halves, preventing them from sticking to the leaves and getting trapped. And they don’t have legs – nothing to entangle them in the sundew’s tentacles. Their thick cuticle protects them from the digestive juices that the plant secretes to break down its prey.

Kleptobugs

An animal that feeds on stolen prey prepared by another species, without offering anything in return, is called a kleptoparasite. “It’s a surprise, but only because it’s a new feeding behaviour for syrphids,” says Francis Gilbert from the University of Nottingham, UK. “I’ve also never heard of kleptoparasitism involving stealing prey from sundews so I guess it must be a first.”

A sundew plant with its leaves full of dead insects is a sitting duck, so why aren’t there more kleptoparasites reported on it? “Most kleptoparasites have not been discovered yet”, says Fleischmann. “The story has just begun”.

Forming lasting associations with carnivorous plants is common in many members of the fly family (Diptera), but this is seen most frequently with another meat eating plant: the pitcher plant. For example, the capsid bug feeds on the insects caught by the pitcher plant, and the plant absorbs the capsids’ faeces to derive nutrition in return.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0153900