The whistling thorn acacia protects itself with pairs of long thorns up to 3 inches long. Interspersed with these are modified thorns, called stipular spines, which are joined at the base by hollow bulbous swellings about 1 inch in diameter. These are home to four different kinds of stinging ants who pierce these swollen thorns with tiny holes. When the wind blows it turns old and abandoned spines into tiny whistling flutes, which gives the tree its name. From www.blueplanetbiomes.org/whistling_thorn.htm