and seizes passing prey. In Japan, Gatzara jezoensis larvae lurk on the surface of rocks for several years while awaiting prey; during this time they often become coated with lichen, and have been recorded at densities of up to 344 per square metre. The larva is a voracious predator. Within a few minutes of seizing its prey with its jaws and injecting it with venom and enzymes, it begins to suck out the digestion products. The larva is extremely sensitive to ground vibrations, the low-frequency sounds made by an insect crawling across the ground; the larva locates the source of the vibrations by the differences in timing of the arrival of waves detected by receptors, tufts of hairs on the sides of the two hindmost thoracic segments. In trap-building species, an average-sized larva digs a pit about 2 in (5 cm) deep and 3 in (7.5 cm) wide at the edge. This behavior has also been observed in the Vermileonidae (Diptera), whose larvae dig the same sort of pit to feed on ants. Having marked out the chosen site by a circular groove, the antlion larva starts to crawl backwards, using its abdomen as a plough to shovel up the soil. By the aid of one front leg, it places consecutive heaps of loosened particles upon its head, then with a smart jerk throws each little pile clear of the scene of operations. Proceeding thus, it gradually works its way from the circumference towards the center. As it slowly moves round and round, the pit gradually gets deeper and deeper, until the slope angle reaches the critical angle of repose (that is, the steepest angle the sand can maintain, where it is on the verge of collapse from slight disturbance), and the pit is solely lined by fine grains. By digging in a spiral when constructing its pit, the antlion minimises the time needed to complete the pit. When the pit is completed, the larva settles down at the bottom, buried in the soil with only the jaws projecting above the surface, often in a wide-opened position on either side of the very tip of the cone. The steep-sloped trap that guides prey into the larva's mouth while avoiding crater avalanches is one of the simplest and most efficient traps in the animal kingdom. The fine grain lining ensures that the avalanches which carry prey are as large as possible.