Behind every brilliant butterfly or moth is a hungry caterpillar. These mostly harmless insects are some for the most diverse organisms on earth, with upwards of 180,000 species, providing countless scientists plenty of opportunity for discoveries.
A recent study published in the Proceedings of the National Academy of Sciences has brought caterpillars into an entirely new sphere of influence – the microbiome. After very careful analysis and experiments, Tobin Hammer and colleagues at the University of Colorado Boulder have discovered that caterpillars do not have a resident microbiome.
Generally, scientists have assumed that all animals have microbiomes, including caterpillars. Microbiomes are the set of all microorganisms that inhabit and are associated with, for example, a human. Bacteria living inside of the human gut provide key benefits when it comes to nutrition, immunity to disease, and even mental health. Since the advent of cost-effective and widespread DNA sequencing, more evidence has accumulated for the presence of a microbiome because scientists are able to sequence the bacteria that live inside of animals easily and reliably.
Hammer and his colleagues hypothesized that caterpillars don't have microbiomes because of their peculiar lifestyles. Most caterpillars munch on leaves, constantly moving vegetative material through their very simplified and acidic intestines. Food moves through the caterpillar gut rapidly with a turnover of about two hours. Together with the rapid transit of food and the lack of suitable habitat for bacteria or other microorganisms, caterpillars have apparently lost their microbiome as result of such a hostile gut environment.
In the study, over 124 species of caterpillars from North America and Costa Rica were surveyed, identifying the microbes that were being excreted in their feces. This is similar to microbiome studies where human fecal samples are used to infer an individual’s gut microbiome.
They found that the feces contained a surprisingly low number of bacteria compared to many other organisms, including insects, fish and birds. These bacteria seemed to be as different from each other across different caterpillars as they were across different leaf surfaces from which the caterpillars ate. If caterpillars indeed had a resident microbiome, one would expect that the microorganisms living inside of their gut would be similar across caterpillars, which would suggest that they have a distinct community that probably has a biological function.
Hammer and colleagues went even further and showed that wild caterpillars don’t do much better or worse if given high doses of antibiotics to kill any and all bacteria that might be living inside of them. Their caterpillars grew to the same sizes and over the same amount of time regardless of the antibiotic treatment.
While caterpillars don’t seem to have caterpillar-specific bacteria, a recent study showed that beetles have a bacterium whose sole purpose is to produce chemicals that make their exoskeletons harden. In cicadas, bacteria produce nutritive substances that they need in order to grow. Birds have been shown to harbor communities of bacteria unique to their species and certain bacterial communities are associated with obesity and disease in various mammals.
Despite all of these examples, caterpillars may not be the only animals that don't have a resident gut microbiome. Hammer and colleagues point out that walking sticks, some species of fly, a parasitic worm, a leaf beetle, and some ants seem to lack them as well.
Diler Haji is a staff writer for The Daily Campus. He can be reached via email at firstname.lastname@example.org.