Fire ants!

A fire ant worker in a laboratory foraging experiment climbs the wrong post and misses the sugar bait.

Amazingly I have somehow made it to my 6th month of regular posts without bringing up my favorite little study organism–the fire ant!

Let’s remedy that immediately.

My current research project involves studying variation in foraging behavior of the red imported fire ant, Solenopsis invicta.  Since I could talk about fire ants for a loooong time, this is going to be a bit of an odds and ends post where I just throw some pictures at you that I find interesting.  If you live in the south eastern United States (or California) you’re familiar with a sight something like this:

A colony of fire ants swarms in response to a disturbance to the mound.

Disturb a mound of fire ants and you are very quickly greeted by hundreds and even thousands of angry workers swarming up in defense of the colony.  In the above picture the worker ants are swarming a straw which has disturbed their nest.  (Note: never attempt to drink a fire ant colony.)  Fire ants react both to vibrations (such as footsteps) and changes in air flow (such as might be caused by breath, or a disturbance to the mound).  The workers release alarm pheromones which alert and recruit other workers to the site.  Fire ants are (in)famous for their painful stings, which have been described as a fiery, burning sensation.  In particular, their tendency to swarm and sting en masse makes an encounter with these ants potentially very unpleasant.

Fire ant sexuals and polymorphic workers.

Like other ants, the majority of a colony is made up of wingless, sterile female workers.  Sexual, or reproductive ants, called alates, are the only members of the colony who develop wings.  These wings allow them to disperse in mating swarms (called nuptial flights).  Male fire ant alates can be distinguished from females by their darker coloration and smaller heads.  (As it was once put to me, they have ‘big shoulders and tiny brains–typical males.’)

After the mating flight, fire ants drop to the ground.  Males die shortly thereafter, but the females must now attempt to join a new colony or establish their own.  These queens (or sometimes ‘gynes’) dig a small chamber and close themselves in.  The queens shed their wings, and and use the energy from their wing muscles to feed their eggs and developing brood until the first tiny workers (called minims, or nanites) mature and leave the nest to forage.

Brood development of fire ant queens and polymorphic workers. Left to right: two points in development of larvae, two points in development of pupae, and the adult form.

Fire ants, like other Hymenoptera, have complete metamorphosis (holometabolous) and undergo a resting, pupal stage as they transition from larvae to adults.

Fire ant workers are polymorphic–adult workers may mature to a broad range of sizes.  Although these sizes range across a continuum and there are no discrete castes (as exist in some other ants) workers are often grouped into broad size-based categories: majors and minors.  Ants towards the middle of the size continuum are sometimes called ‘medias.’   Worker size influences both lifespan and performance at different tasks.  For example, majors, with their large jaws, make poor nurses but excellent carvers and heavy lifters when foraging.

Polymorphic worker ants and brood (Solenopsis invicta).