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Image: ShutterstockMiniscule, yet plentiful, insects might be the antithesis of mammals.

“There’s something really strange and almost alien-like about them,” says Daniel Charbonneau, a graduate student studying entomology and insect science at the Univ. of Arizona, in an interview with R&D Magazine. “And, yet, they’re everywhere. They compose a vast majority of animal diversity and, pound for pound, vastly outweigh mammals on Earth.”

However different in appearance, social insects, such as ants, echo the complex societies of humans. Or perhaps, humans echo them. In some respects, insects appear to surpass humans, in terms of working towards a common goal. “But then, you start digging into insect societies and see that they have their fair share of problems: Half of the workers are just standing around while a few workers are doing all the work,” says Charbonneau.

In a recent study published in Bioeconomics, Charbonneau found approximately half of the 250 Temnothorax rugatulu ants studied in the laboratory were in a consistent state of inactivity. Yes, laziness isn’t purely a human phenomenon.

Charbonneau collected five colonies from the field, each consisting of 50 workers, and established artificial colonies in the laboratory. Inactive workers were immobile, and didn’t engage in any tasks. “I think the most commonly assumed explanation is that they’re reserve workers that are meant to replace workers that are killed, or begin working when colony workload increases,” says Charbonneau. However, all “studies that have attempted to activate inactive workers, either by removing workers or increasing workload, have failed to do so.”

Why are some ant inactive?
Hypotheses abound regarding inactivity in ants.

“It is also possible that inactive workers are selfishly avoiding dangerous tasks, such as foraging, and avoiding expending energy on work, while using colony resources to invest in their own reproduction,” adds Charbonneau. While workers are generally sterile, some develop ovaries and successfully lay eggs. An unpublished study, specifically using Temnothorax rugatulu ants, from a student Charbonneau mentored shows inactive ants are more likely to have oocytes, immature egg cells, than active workers.

Another theory suggests inactive ants may act as live food pantries. “Social insects have the capacity to store food in their crops (or social stomachs) and then share it with other workers or brood via trophallaxis (essentially regurgitating the food and sharing it mouth to mouth),” says Charbonneau. “When they have more food in their crops (which is located in their abdomens), they have larger, more distended abdomens. It seems that inactive workers tend to be more ‘corpulent’ than other workers.”

In the field, colonies can boast between 10 and 1,500 workers, according to Charbonneau. But most range between 100 and 300 workers. Usually, a division of labor occurs, but the degree of specialization is contingent on the species being discussed. “Species that have morphological castes (soldiers) or size polymorphism (minor, majors) have subsets of workers with morphological traits specific to a certain task,” such as large piercing mouthparts for prey, or armored plates on the head to block the colony’s entrance from invaders, says Charbonneau. “However, most social insects are monomorphic (all workers more or less look the same) and their specialization is more linked to behavior.”

How certain ants become tailor-made for certain tasks is largely unknown. A prevailing thought is each task has a stimuli associated with it, and worker ants have an array of response thresholds. “The stronger a stimulus, the more likely a worker is to respond to it and begin doing the task,” says Charbonneau. “Specialization is thought to come about from different workers having different thresholds to stimuli.”

A human analogy for the response-threshold model is the dishwasher scenario. If you have a roommate who has a low threshold for seeing dirty dishes in the sink and you have a high threshold, your roommate will consistently end up doing the dishes. “A worker with a high threshold will almost never react,” says Charbonneau.

Charbonneau will soon submit a study where different hypotheses for inactive ants are tested.

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