 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 ) . 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 . Since the sides of the pit consist of loose sand at its angle of repose , they afford an insecure foothold to any small insects that inadvertently venture over the edge , such as ants . Slipping to the bottom , the prey is immediately seized by the lurking antlion ; if it attempts to scramble up the treacherous walls of the pit , it is speedily checked in its efforts and brought down by showers of loose sand which are thrown at it from below by the larva . By throwing up loose sand from the bottom of the pit , the larva also undermines the sides of the pit , causing them to collapse and bring the prey with them . Thus , it does not matter whether the larva actually strikes the prey with the sand showers . 
