I give numerous reasons for why I like ants, but chief among them is my fascination with just how many ways there are to be an ant. Taxonomists have formally named some 13,000 ant species and expect to discover at least another 5,000 or so. There are ants smaller than the head of a pin and ants larger than my thumb. Their societies may number several million, or just half a dozen. Some ants glisten like polished metal, others are furrier than a woolly bear. Some farm, some hunt, some scavenge. Some glide from tropical treetops, some swim in pitcher plants.
Distribution of D. attelaboides (modified from MacKay 1993).
One unusual species I was happy to encounter in Ecuador this January was Dolichoderus attelaboides. This is an ant that never outgrew its awkward adolescent physique. Dolichoderus attelaboides is more gangly than an ant should be, its limbs slightly too long. Its eyes boggle from a head supported by a false neck too thin for its size.
Dolichoderus attelaboides is a common insect in lowland South American forests. It is one of the larger species you might find wandering about in the undergrowth of a tropical rain forest. Yet like most tropical ants, little is known of its biology, and nothing is known about why it has evolved such a charmingly clumsy form.
Like most members of its subfamily (Dolichoderinae), D. attelaboides often tends hemipterous insects for honeydew. And like most members of its genus, it sports a rough, armored exoskeleton.
I will venture my own hypothesis for the strange appearance of this ant, however. I suspect the shape helps Dolichoderus attelaboides escape dismemberment by Eciton army ants.
When ants fight each other, the highest incidence of physical damage is body parts severed at their weak points, the junctions between the hardened plates. Heads detach most easily when cut at the neck, for example. If we look at the two points of unusually narrow constriction on the body of D. attelaboides– partway along the ant’s pseudo-neck and in the middle of the mesothorax- we’ll see they are adjascent to more vulnerable sutures. Any large ant attempting to bite a D. attelaboides will find their mandibles settling into grooves in the middle of armored plates rather than the membranous tissue between them. Essentially, they are false joints. The remaining vulnerability- the waist- is protected by a set of spines.
Most species of Eciton are predators of other ant species. Dolichoderus attelaboides is a sizable ant living in large colonies- a great source of protein for army ants if the colony’s defenses could be breached. Given that both ants are common in the same habitats, and that the size of both ants is similar, I think it entirely reasonable that the gawky pencil-neck form and heavy armor of D. attelaboides is a response to the army ant threat.
Of course, this is speculation. Someone will have to feed dozens of Dolichoderus workers to army ants to test whether the predators’ mandibles actually land in the false joints, and whether those joints provide any real protection from bites.
source: MacKay, W.P. 1993b. A review of the New World ants of the genus Dolichoderus. Sociobiology 22: 1-148. [(31.xii).1993.]