What’s in a (species) name? Using genetics to map the hidden diversity of earwigs

Written by Oliver Stuart, Image credit: Ryan Hodnett [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons

There are a lot of different insects; this cannot be overstated. Of the roughly 2 million animal species (that we know of), insects make up well over half. This mega-diversity has been a source of delight for biologists for centuries, but it has also caused no small amount of headache. The small size and great variation of insects often makes it hard to tell them apart, even when you look extremely closely.

Taxonomy is the science of classifying and categorising the living world, and uses the familiar ranking of species, genus, family, and so on. Traditional taxonomy is based on morphology, the shape and structure of organisms. This species has wings, that one doesn’t. This species has legs for digging, that one has legs for climbing. But where do we draw these lines? Within a species there can be variations in size, colour, differences between sexes, and so on. It can be hard to see where one species ends and another begins, and sometimes the lines are blurry.

In entomology, the formal study of insects, the list of unresolved taxonomic questions is long. At PEARG, we use genetic tools to answer such questions with a focus on the taxonomy of insects relevant to agriculture. One group we’ve recently trained our eyes on is earwigs: the insect order Dermaptera.

Earwigs are fast becoming a pest in Australia’s grain producing regions. Growers need management options to reduce the impacts of earwigs on their yields, but this is challenging as we just don’t know a lot about earwigs. Earwigs have slipped under the radar for a long time.

This means a lot of the information on earwig taxonomy, which would be used to identify the earwigs we find in the field, is either insufficient or unreliable. This is where we can leverage genetics to augment traditional taxonomic approaches.

We commonly use a technique called DNA barcoding to distinguish between insects that defy the eye. Barcoding uses short regions of well-known genes to distinguish between species.

To barcode a specimen, we look at the similarities and differences in the DNA of a single species. We then can see whether an unknown specimen falls within that expected level of difference. For example, let’s say we have 20 earwigs and we are very confident that they’re the same species. When we analyse their barcode genes we find that between them they only differ by about 0.5 % of the DNA in that region. Next, we bring in another individual. This new earwig looks a little like our known species, but we’re unsure if it’s the same. Maybe it’s slightly larger, but that could just be because it developed in a different environment. When we look at the barcode region of this unknown earwig, we find that it’s more than 5 % different to the DNA of the known species.

This gap between the ‘within-species difference’ and the ‘between-species difference’ (appropriately named the “barcode gap”) is a strong piece of evidence. By accumulating a lot of barcode regions for a lot of earwigs, our evidence gets stronger, and we can be more certain of how many species we’re looking at. In short: we expect members of a species to have DNA more like each other than to a member of another species.

This is a relatively simple method, that quickly gets complicated when you start looking at more individual specimens. A barcode dataset of several hundred individuals with two or three barcode regions isn’t uncommon, and teasing out the relationships among them isn’t easy. Different groups will also have varying levels of similarity and dissimilarity, so the threshold we use to define “within-species” and “between-species” can be arbitrary if we don’t investigate the group beforehand.

It’s challenging, but it’s also rewarding! Using genetic tools allows us to be at the forefront of insect taxonomic knowledge. Traditional barcoding is a comparatively old method (it turned 15 in January!),¹ but its simplicity along with the availability of cost effective standardised tools has seen it persist well past the typical use-by date of most genetic techniques. It’s likely that barcoding will continue to help us describe new, previously unknown species of earwigs that have been living unobserved in Australia for millions of years. But first, we have to go looking for them.

1. Hebert, P. D. N., Cywinska, A., Ball, S. L., and deWaard, J. R. (2003) “Biological identifications through DNA barcodes.” Proceedings of the Royal Society of London B: Biological Sciences 270(1512), 313-321.