Walter Johnston. Flaky Thorn Acacia. Timbavati, South Africa, 2014.
On safari in August we were told that a “gall making wasp” injects a kind of growth hormone into the thorn to make it expand (see below) and thus provide a well protected nourishing home for its eggs.
I have not been able to corroborate this. If someone else can, I'd love to learn.
Here's the best I have found:
“Myrmecophilous acacia are found in Eastern Africa and Mesoamerica …
…They develop some to most of their stipular spines into inflated, globose, ovoid, fusiform or thick cyclindrical armatures. Their spines look like galls or horns leading to species names like White swollen thorn acacia (=A. bussei), Black-galled acacia (A. malacocephala), Hairy-galled acacia (=A. mbuluensis), Bull`s Horn acacia, or Ant-galled acacia also called Whistling thorn acacia
The swollen thorns are genetically fixed. They are not randomely generated by the sting of an insect, like the galls produced by a wasp that injects her chemicals into a leaf, which then forms galls. Therefore the so-called gall-thorns are not real galls.
The fresh thorn is drilled open by an ant queen. Then it is carved out and she lays her eggs inside, starting a new colony …
The obligate mutualistic Acacia-ants (Pseudomyrex in Mesoamerica and Crematogaster in Africa) protect the plant in different ways: they fiercly attack browsing mammals, ravaging insects and epiphytic vines. They prevent any twig from neighbouring trees to touch their host – to prevent hostile ants from invading their tree. For the same reason they cut shoots of their tree that develop too far towards the canopy of neighboring trees.”
Walter Johnston. Swollen thorn of the Flaky Thorn Acacia. Timbavati, South Africa, 2014.
More on acacias here.
Photographs posted with permissin from Walter Johnston.