Archetype

Homology Weekly: Arolium

This orchid-looking thing is really the foot of an ant. The large unfolded structure in between the powerful pair of claws is the adhesive organ of the foot called arolium (pl. arolia). It is basically a soft membranous bag folded into a suction cup that allows the ant to walk on vertical or upside down smooth surfaces.

With each step the arolium is extended and retracted by a complex mechanism that involves both the indirect mechanical action from the claw muscles and hydraulic pressure from the hemolymph.¹

There is nothing particular about ant feet, they share their basic structure with that of other wasps and bees. The presence of arolium in the feet, however, is variable among the different ant groups. Arolia are well developed and functional in most ants, but having reduced arolia is not uncommon and many ants lack the structure altogether. In my experience assessing this is not easy. The structure is a small membranous balloon that will often deflate on dry specimens in collection. You may not see anything between the claws using a regular stereoscopic microscope. The only way to tell for sure is to check with a scanning electron microscope or by doing a slide mount for light microscopy. Fun but laborious task if you are trying to gather these data for phylogenetic analysis.

Curiously the presence/absence of arolia is sexually dimorphic in some species. Freeland and coworkers realized this upon observing how in colonies of Rhytidoponera ants kept in the laboratory the workers were not able to climb the plastic walls of the artificial nest while the males got out without a problem.²

One would expect to find well developed arolia in the feet of arboreal ants — those species that spend their life climbing up and down the trunks and branches of trees and bushes. This happens not to be the case, as can be seen in the image below of the foot of an Ectatomma tuberculatum worker, a medium-sized, bright orange ant common throughout the tropical forests in the Americas.

On the other end of the spectrum, some ants that spend their life crawling through the leaf-litter of the forest floor seem to have gone the velcro way. The outside surface of the arolia in the minute Probolomyrmex ants are covered by elongated projections with a pointy hook apex. This presumably provides the ant with a lot of traction on irregular surfaces, although this observation remains to be properly studied.

A colleague of mine thinks that some of these images of arolia look obscene. I have no idea what is she talking about.

Further reading

Tags: Arolia, Arolium, Ectatomma tuberculatum, Oecophylla smaragdina, Probolomyrmex guineensis, Procryptocerus, Tetraponera attenuata

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