Are these tiny insects the world’s most bone-idle bugs?

At less than 3mm long, you may not think Dunatothrips aneurae seem like much. And – as I have shown in a new study – you’d be absolutely right. That’s because these may be the world’s laziest insects.

Dunatothrips live in the remote Australian outback where they bother nobody. They almost never leave their near-invisible miniature nests, built on Acacia trees from silk they extrude from their bottom. No known predators bother with Dunatothrips and their biggest threat is drying out in the heat if their nest is damaged. Pacifist vegetarians, they feed harmlessly on the plant surface, with no discernible effect on it.

No bigger than the hyphen on your page, they belong to the thrips, which you may know as thunderbugs owing to a myth that they come out during thunderstorms. Almost everyone gets their name wrong (one thrips is a thrips, not a thrip). Some species make a nuisance of themselves as tomato pests. But for the Dunatothrips I investigated, that sounds a bit much too much like effort.

I spent a few summers studying the social lives of these tiny insects, trying to understand their unusual habit of sometimes living alone and sometimes in groups with their sisters. I was puzzled to discover that some group members appear to do nothing. Not helping out, not breeding, nothing. Few animal societies are known where group members help no one, not even themselves.

If the silk nest was damaged, I found, usually only one or two females inside stepped up to repair it. The remaining group members didn’t do anything. The responders’ repair efforts helped everybody, so the laggards enjoyed the benefits without raising a minuscule finger.

I set out to investigate what these “lazy” thrips were doing. Were they like queen bees, specialising in producing eggs while others acted as workers? Other social insects have this arrangement, including many other Australian thrips.

But no: when I dissected helpers and non-helpers I found it was the helpers that tended to be the ones carrying eggs.

Maybe they were a reserve workforce, helping when others were lost, as in some bird societies like carrion crows. But when first responders were removed, their nestmates remained just as unhelpful as before.

I wondered whether they were biding their time, waiting for a chance to breed later, as paper wasps do. I removed all group members except for a lazy one, gifting it a nest of its very own. They declined this opportunity as well, producing few or no eggs and taking up to five times as long to repair the nest as a helpful thrips put in the same situation.

If the lazy non-helpers don’t even help themselves, doesn’t that make them an evolutionary paradox? Not really: while behaviour only evolves if it furthers individuals’ fitness, evolution tends to work on averages. Within a species, individuals are all different, and some are inevitably of poorer quality than others. They may carry mutations, inherit unfortunate gene combinations, experience poor environments, or all of the above. Perhaps they were jostled to the edge of the leaf as a kid.

If life gives you lemons

Animals in this situation will commonly make the best of a bad job. A poor quality thrips can lay only a few eggs, and can only contribute a few strands of silk to repairing a nest. She can’t build the nest she would need to raise offspring on her own. So her best option is to hang around in a group where her young can grow alongside those of others.

It’s still a mystery why nestmates of these wastrels don’t kick them out. But it may involve their being unusually chilled out in the face of any provocation, even by dangerous intruders like their cousins Akainothrips, a new species I discovered with my colleagues. The resident Dunatothrips just stand aside.

This pacifism may be related to how risky nestbuilding is. At any moment, out there on a leaf surface, you might fall, be blown out of the nest, or dry out in the outback sun. Given that you might die at any moment, it pays you to tolerate the presence of others who can carry on your nestbuilding work and help keep everyone’s babies alive.

A simple evolutionary way to achieve this is to drop all aggression towards anyone, including intruders of different species. Some spiders have done this and form cooperative nurseries involving different spider species. For our lucky waster thrips, this means they get a free pass to stay in the group.

It is also possible these bone-idle bugs may actually be helping, just in subtle ways. For example, Dunatothrips nests have rubbish dumps, so they might help by taking out the trash. In many social insects, including some thrips, workers can act as medics. Even just breathing inside the nest may raise humidity and help the group survive – cockroaches form groups at low humidity for just this reason.

So, while non-helper Dunatothrips may be among the world’s least motivated insects, they are certainly not the least interesting. The evolutionary persistence of these laggards is helping us understand how different kinds of societies evolve.

James Gilbert currently receives funding from UKRI (Biotechnology and Biological Sciences Research Council). This study was funded by a Marie Curie Fellowship (2011-2014) under the European Community's Seventh Framework Programme.