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Big headed ants in Palmetto State Park, Texas

BugsInTheNews is a VIEWER-PARTICIPANT WEBSITE. This article by Jerry Cates and Janet FB, first published on 7 February 2011, was revised last on 7 July 2012. © Bugsinthenews Vol. 12:02(05)

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The genus Pheidole (from the Greek φειδος, pheidos, which means “sparing, thrifty”; a reference to the indiscriminate collection and transport, by these ants, of all manner of edible matter for use as food in the ant colony) is found worldwide, and is thought to comprise as many as 1,000 species of ants.

The genus was first described in 1839 by the English entomologist John Obadiah Westwood (1805-1893), who became curator and professor at Oxford University.

Most species in this genus are dimorphic, with major and minor workers distinguished by morphological characteristics: major workers, also termed soldiers, have relatively modest bodies, to which are attached unusually large heads and mandibles (hence the common name, which is applied informally to the genus as a whole, and more formally to the species Pheidole megacephala); minor workers have bodies comparable to major workers, and heads and mandibles that are not out of proportion to those bodies.

Royal castes are present, though in much fewer numbers than workers. Mature colonies contain winged imagoes of the royal castes, males and females, at certain times.

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Formicidae: Pheidole spp. ant mounds; Palmetto state park, Gonzales, Texas; Janet FB---22 Jan 2010

Formicidae: Pheidole spp. ant mounds; Palmetto state park, Gonzales, Texas; Janet FB---22 Jan 2010


Janet wrote on 22 January 2010:

Hi Jerry,

When you have time, we saw this ant colony in Palmetto State Park last spring. Can you check them out?

Janet

My first impression, after examining the photos that Janet sent me, was not very positive.

Without a close up, well-focused image of one or more of the ants that built these mounds, it seemed doubtful there was much I could tell her about them.

Of course, it seems safe to assume the ants that built these mounds are all members of a single, distinct species.

And we could point out that they, along with a huge number of other species, from a huge number of ant genera, build mounds in the soil.

End of story… But, as often happens, I was wrong.

Taking another look at the two closeup photos Janet took of the entrances to the mounds, I noticed something distinctive.

Formicidae: Pheidole spp. major worker; Palmetto state park, Gonzales, Texas; Janet FB---22 Jan 2010

Formicidae: Pheidole spp. major worker; Palmetto state park, Gonzales, Texas; Janet FB---22 Jan 2010

Formicidae: Pheidole spp. major worker; Palmetto state park, Gonzales, Texas; Janet FB---22 Jan 2010

Formicidae: Pheidole spp. major worker; Palmetto state park, Gonzales, Texas; Janet FB---22 Jan 2010

Though not one of the ants was in sharp focus, several were displayed in sufficient detail to allow a few gross anatomical features to be discerned.

And, it so happened, one particular anatomical feature was quite obvious: several worker ants had extremely large heads, quite out of proportion with the rest of their bodies.

Furthermore, the heads were clearly divided into two distinct lobes.

One genus of ants (Pheidole), in the subfamily Myrmicinae, is characterized by having dimorphic workers, such that some are fitted with enormous, bilobed heads.

These ants are omnivores, and take advantage of whatever food might be available in their immediate environment.

They appear quite ferocious, but are reputed to be among the most shy of all the ants known, tending to turn tail and hide when threatened. While termed “soldiers”, the big-headed major workers are not much good at defending the nest or other workers. Instead, their utility is more pedestrian: they use their over-large mandibles to break up seeds and to carry large objects into and out of the nest.

During seasonal periods when little food is available, minor workers may take out all the major workers by decapitating them. It is thought this practice is carried out because, during such times, the major workers cease to serve the colony’s needs.

Formicidae: Pheidole spp. ant mounds in the Palmetto state park, Gonzales, Texas; Janet FB---22 Jan 2010

Formicidae: Pheidole spp. ant mounds in the Palmetto state park, Gonzales, Texas; Janet FB---22 Jan 2010

Formicidae: Pheidole spp. ant mound entrance; Palmetto state park, Gonzales, Texas; Janet FB---22 Jan 2010

Formicidae: Pheidole spp. ant mound entrance; Palmetto state park, Gonzales, Texas; Janet FB---22 Jan 2010

These particular ants may be involved in a symbiotic relationship with root aphids that, under the soil, are feeding on tree and plant roots in the sandy soil of this portion of the Palmetto State Park.

Aphids, whether feeding above or below ground, suck the sap from their host plants, and exude a sweet liquid (honey-dew) that ants and other organisms feed on.

Many weeds are susceptible to root aphid infestations, as are a number of trees.

When ant mounds, such as the ones Janet observed in Palmetto State Park, are found scattered over a landscape, chances are the ants are feeding on something endemic in the soil. Among the likely candidates, root aphid secretions rank high on the list.

jc

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