Who has been eating our beetles?

Some beetles that do not produce defensive stink are thought to be mimics of those that do. For example, Eleodes species can shoot streams of noxious chemicals when threatened, whereas Philolithus and Stenomorpha, which have very similar size and appearance, do not. In this way, the non-stinky beetles may be protected from predators who have learned their lessons during previous meetings with Eleodes. This form of mimicry was first proposed about 150 years ago by Henry Bates, and is referred to as Batesian mimicry.

Because we humans perceive the world largely through our eyes, we tend to think of mimicry in terms of visual appearance. The examples shown in textbooks (see below) tend to be butterflies or snakes with matching colorful patterns. These are expected to be especially effective against birds or monkeys that have excellent color vision.

A close look at the snakes above will show that they are relatively easy to distinguish in photographs. Mimicry does not need to be perfect, because a mistake in the field could cost an unwary predator or hiker their life. As a human, you could memorize a rhyme that allows you to distinguish them, or you could simply avoid bright red, black, and yellow snakes.

What about the beetle mimics? Should we think about vision, or other senses? If a predator has poor vision or hunts in the dark, then what is the purpose of visual similarity?

Who Eats Local Beetles?

One of my goals for the 2026 field season was to find out who actually eats the beetles. Each of the beetles shown at the top of the page would yield 1/2 to 1 gram of water, protein, and fat, so it is worth some effort to subdue the defended ones, figure out who is undefended, or both.

Previous work by a number of authors has shown that large darkling beetles are consumed by a wide variety of predators. Depending on the location and beetle species, predators include mice, skunks, coyotes, bats, ringtails, jays, scorpions, and there are likely to be more.

Below is a summary of what I have figured out so far. As always, there is more to come, but the evidence for the ones shown below is quite solid.

Waiting in Ambush: Black Widow

When I came across some beetles wrapped up in sloppy webbing, I was not sure who had killed them. After collecting the beetles and moving the prickly pear pad below them, I saw a large spider with a red spot on its abdomen in a hole below. Then it all made sense. Black widows spin notoriously messy webs, and are perfect predators of any large beetle that comes by. When caught in the web, Eleodes‘ defensive chemicals will be sprayed ineffectually until they are used up, and the spider can then immobilize and consume the beetle at her leisure.

This spider also trapped an Embaphion contusum (also chemically defended) and a large scarab beetle. One would expect that undefended Tenebrionid beetles, such as Philolithus and Stenomorpha, would easily fall prey to such a spider. Her web is not selective, and she is able to subdue E. obscura, one of the largest and best-defended beetles.

Chomped by Bats

Pallid bats are known to eat Stenomorpha, and the resident group has dropped bits and pieces of large insects such as Jerusalem crickets and beetles. For more about the bats themselves, have a look at this post.

In August, one of the bats was particularly messy, leaving most of the front half of a small Stenomorpha, possibly S. obovata.

Now that fall is upon us, Stenomorpha rimata has become the most common Tenebrionid in the neighborhood. I was lucky enough to find some half-eaten, but readily identifiable bits of S. rimata under the roost last week.

At this point, we don’t know if the bats are focusing on undefended beetles, or whether they would eat Eleodes if given the chance. It is clear, however, that pallid bats are willing and able to eat large Tenebrionid beetles such as Stenomorpha.

Work in Progress: Beetles in Scat

During the summer and fall, one can readily find scat containing beetle remains. Predators often swallow them whole, making identification much easier.

Analysis is in the early stages, but there is lots to find. The photo above shows some of the head capsules that were in one scat. There were many Carabid beetles of a range of sizes, plenty of big and small ants, and a few good-sized Tenebrionids. Based on an abdomen that was also in the collection, at least one of the Tenebrionids was Eleodes obscura.

There is much more to find, but we can say for sure that somebody, probably a mammal, is eating a lot of beetles. Importantly, they are including Eleodes in their diets. With persistence and luck, we should be able to figure out who it is as we dig deeper into the samples.

The predators we have identified so far, spiders and bats, are not visual predators, so the outward resemblance of the mimics to Eleodes is probably not that useful. Moreover, it looks like at least some predators are happy to eat Eleodes, further reducing the possible benefit of mimicry. Nonetheless, we may find that birds such as jays and ravens are hunting the beetles, and we may yet identify some selective advantage to the visual resemblance shown by the mimics.