Many land animals, including humans, need to be "inhabited" by non-pathogenic microorganisms to stay healthy. Not all, however: a recent line of studies identifies, among insects, different species that seem to live very well even without bacterial flora.

Who saw them? Tobin Hammer, an ecologist at the University of Colorado Boulder, analyzed the intestinal microbes of 124 species of American leaf-eating caterpillars by sequencing a gene commonly used to identify these organisms. It was thus realized that in these animals there are no signs of resident microbes, that is, that they evolved together with their guests.

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Upstream. It is no surprise to entomologists: studies that find no evidence of stable intestinal bacteria in insects and other animals such as geese, or bats, are accumulating. At the moment, however, it is difficult for them to be published because they do not agree with current knowledge.

A big difference. For example it is thought that herbivores, like cattle, need microbes to digest the fibers of the vegetables they eat. Because the caterpillars studied by Hammer eat leaves, the researchers believed they were finding a thriving community of bacteria in their digestive tract. But the caterpillars are not cows: unlike what is observed among cattle, the few microbes found in the faeces of these insects do not come from their intestines but from the plants they eat and from the environment in which they live.

We can do without it. Microbes do not even seem necessary to keep the caterpillars healthy. When Hammer administered various levels of antibiotics to the tobacco sphinx caterpillars (Manduca sexta, a moth widespread on the American continent), to eliminate all traces of bacteria from their body, the lepidopterans reported no consequences.

You live without it. If we consider that there are around 180, 000 species of caterpillars in the world, and that the phenomenon has also been noticed in some species of ants and stick insects, we understand that the absence of microbes could be much more widespread than we think. According to some, even among vertebrates: so far we may have had only a sampling problem, and believed that what applies to the most studied species can be extended to everyone.