The term myrmecophilous means „friendly to ants“. Greek: myrmex = ant; philos = friend.
It describes plants, which offer advantages for ants. Among the hundreds of species there are some 24+ acacia s.l. species (s.l. = in the broad sense, before the split of genus acacia into 5 separate Genera). One considers the development of these specific Acacia as a prime example for coevolution.
D.H. Janzen assumed, that at first there were inflated thorns, being opened by some insect and then discovered as housing by ants. The tree then retained the ants by offering more nectar from extrafloral glands and later fixed the inflation of the thorns in the genes. That is why acacia in greenhouses also develop “galls” – even if there are no ants.
D. Seigler states, that extrafloral glands are found in many plant families. They serve as a generalized defense. They attract many insects, including ants, which then started to use bigger spines as houses, triggering coevolution.
Myrmecophilous acacia are found in Eastern Africa and Mesoamerica. A good introduction is given at http://waynesword.palomar.edu/acacia.htm
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 A. bullockii, Ant-galled acacia (=A. pseudofistula), A. myrmecophila, Spotted-galled acacia (=A. burttii), White swollen thorn acacia (=A. bussei), Black-galled acacia (A. malacocephala), Hairy-galled acacia (=A. mbuluensis), Bull`s Horn acacia (=A. cornigera) or Ant-galled acacia also called Whistling thorn acacia (=A. drepanolobium).
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 no 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 acacia offers housings (also called domatia). At the node, next to the inflated spines, the fresh leaves develop. And they feature – like most bipinnate leaves of acacia - extrafloral nectaries, providing the ants with glucosides. In Mesoamerica there can be up to 5 glands on the petiole, plus several more on the rachis, providing plenty of sweet nectar for the ants.
There are additional “glands” at the apex of the new leaflets called Beltian Bodies, named after the British Natural Scientist Thomas Belt, who discovered them in Nicaragua in 1874. These are well developed yellow or orange sacs full of protein rich substances, and they are readily harvested by the ants.
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.
In fact, when I broke off white (older) inflated thorns from A. collinsii, the ants instantly climbed my arm and bit me.
Once the ants abandon the tree, the perforated hollow thorn may whistle in the wind. Hence names like: Whistling thorn (=Acacia drepanolobium), the Coastal whistling thorn (=Acacia zansibarica) or the White whistling thorn (=Acacia seyal var. fistula).
In Eastern Africa the acacias part of the mutualism means providing nectar and housing in the inflated thorns to the ants.
In Mesoamerica the abundance of glands and the Beltian bodies offer additional incentives for the ants to colonize the acacias: Even in the dry season, the nectar from the glands is flowing and small leaves with fully developed Beltian bodies are produced to keep the ants on the plant.
In Mesoamerica, the thorns on the ant-acacias are sometimes even shorter than in the non-ant acacias, because the leaves are not protected against herbivores by thorns but by the ants.
Remains the question, why there are no Myrmecophilous Acacia in Australia.
Australian Acacia are related to Senegalias, which have no spines - but often prickles.
They lack floral nectaries and carry petiolar glands on bipinnate leaves or glands at the margin of the phyllodes. Ants patrol these plants, harvest sugar and amino acids. They also apparently play a role in protecting the acacias by eating other small insects, insect eggs or fungal spores. As the housing opportunities are lacking, the ants are only part-time visitors. There is no strict mutualism, and therfore there are no “Myrmecophilous Acacia” in Australia.
Field Guide to the Acacias of East Africa, 2006, Struik Publishers, RSA
ISBN 1 77007174 1
J.E. Ebinger, D.S. Seigler
Ant-Acacia Hybrids of Mexico and Central America
Sothwestern Naturalist 1982, Vol 37, pp 408-414
Frank M. Thomas
Unterricht Biologie 36/2005, pages 46-50 (In German)