How do plant waxes cause flies to slide? Experimental tests of wax-based trapping mechanisms in three pitfall carnivorous plants.

The waxy surfaces of three carnivorous plants, Nepenthes ventrata (Nepenthaceae), Brocchinia reducta and Catopsis berteroniana (Bromeliaceae), were compared using scanning electron microscopy (SEM). Their effects on attachment and locomotion of the fly Calliphora vomitoria were studied. The waxy surface of N. ventrata is comprised of a heterogeneous layer from which only platelet-shaped crystalloids could be detached by brushing. In the two bromeliads, the crystalloids are thread-shaped and form a homogenous dense network, which was entirely removable from the epidermis. Experimental data showed that none of the flies was able to walk across any of the waxy surfaces and only a few were able to take off from those surfaces. Both the absence of sites for claw anchorage, especially in N. ventrata, and the wax itself were shown to contribute to the trapping ability of the plants. Only half of the flies quickly recovered their locomotion ability on a glass surface after 20 min of being tested on waxy plant surfaces. SEM observations revealed that the wax of C. berteroniana formed a powder of broken crystals on the tenent setae of the flies' pulvilli. In contrast, the waxes of B. reducta and N. ventrata appeared to have lost their crystal structure in contact with the tenent setae and formed an amorphous substance that adhered setae together. We hypothesize that wax interacts with adhesive fluids secreted by the fly pad and thereby prevents the tenent setae from functioning effectively.