soft but substantial membrane called the velum stretches in a curve around the middle of this platform, and helps seal the door. A second band of springy cells crosses the door just above its lower edge, and provides the flexibility for the bottom of the door to become a bendable 'lip' which can make a perfect seal with the velum. The outer cells of the whole trap excrete mucilage and under the door this is produced in greater quantities and contains sugars. The mucilage certainly contributes towards the seal, and the sugars may help to attract prey. Terrestrial species, like U. sandersonii have tiny traps (sometimes as small as 0.2 mm; 1/100") with a broad beak-like structure extending and curving down over the entrance; this forms a passageway to the trapdoor and may help prevent the trapping and ingestion of inorganic particles. Aquatic species, like U. inflata tend to have larger bladders—up to —and the mouth of the trap is usually surrounded not by a beak but by branching antennae, which serve both to guide prey animals to the trap's entrance and to fend the trap mouth away from larger bodies which might trigger the mechanism needlessly. Epiphytic species have unbranched antennae which curve in front of the mouth and probably serve the same purpose, although it has been observed that they are also capable of holding a pocket of water in front of the mouth by capillary action, and that this assists with the trapping action. Trapping mechanism The trapping mechanism of Utricularia is purely mechanical; no reaction from the plant (irritability) is required in the presence of prey, in contrast with the triggered mechanisms employed by Venus flytraps (Dionaea), waterwheels (Aldrovanda), and many sundews (Drosera). The only active mechanism involved is the constant pumping out of water through the bladder walls by active transport. As water is pumped out, the bladder's walls are sucked inwards by the negative pressure created, and any dissolved material inside the bladder becomes more concentrated. The sides of the bladder bend inwards, storing potential energy like a spring. Eventually, no more water can be extracted, and the bladder trap is 'fully set' (technically, osmotic pressure rather than physical pressure is the limiting factor). Extending outwards from the bottom of the trapdoor are several long bristle-stiff protuberances that are sometimes referred to as trigger hairs or antennae but which have no similarity to the