An interesting thing happened while I was watching the Army Ant raid near the Novomessor cockerelli nest. As I mentioned, the Novomessor cockerelli were nervous while the Army Ants passed by. There were guards around the nest entrance, and we could see agitated workers down in the entrance’s tunnel.
Then one of the Novomessor ants brought up a beetle up out of the tunnel, carrying it gently in her mandibles, parading the beetle around as though it were a favoured pet.
The beetle stayed motionless while the ant carried it. The beetle held its legs and antennae out stiffly away from its body.
The ant walked out of the nest entrance and out to the edge of the ring of guard ants, all the while carrying the spread-eagled beetle in her mandibles.
The beetle didn’t grab onto anything with its legs when the ant carried it past dirt clods and rocks, it just stayed stiffly spread out.
After roaming around outside of the nest for a few minutes, the ant turned around and took the beetle back into the nest enrance.
This beetle looks like a Tenebrionid to me – one of the Darkling Beetles, a family containing the more well-known Mealworms and Pinacate Beetles. I think the beetle is a member of the Araeoschizus genus. And I (very tentatively) think this beetle is Araeoschizus colossalis, based on the non-toothed femora, and longitudinal groove in the prothorax.
Antwiki.org lists Araeoschizus decipiens, another beetle in this genus, as occurring in Novomessor cockerelli nests. I don’t think that’s what this beetle is, though, due to the lack of toothed femora.
Hendricks and Hendricks (1999) observed Araeoschizus beetles in Pogonmyrmex Harvester Ant nests, often in the seed chambers of the nests. They saw them carry the beetles out to the middens and leave them there, and noted that sometimes the beetles struggled and did not seem to want to go. The beetles would usually turn around and walk, unmolested, back into the Harvester nests in some cases. They speculate that the worker ant may have thought the beetles were chaff or detritus, and thus taken them out of the nest.
I’ve seen Novomessor bring their larvae up out of the nest when disturbed by other ants, or if the ground is really wet…I wonder if that is how the ants prepare to abscond in times of stress. I wonder if that’s what the ant I watched was doing…bringing up something it considered to be “larva” in preparation for absconding. Except the “larva” was one of their pet beetles instead.
I don’t think it’s known exactly what the beetles do in the ant nests, and it probably varies between species of beetle and ant. The Hendricks and Hendricks (1990) paper mentions that the beetles are commonly found in seed chambers. That would suggest that the beetles might eat the seeds.
Antwiki.org mentions that one species of beetle (Araeoschizus antennatus) has been found with Neivamyrmex nigrescens, which is an Army Ant. Since Army Ants are mostly carnivorous, it seems unlikely that the beetles would be eating seeds in that situation. Perhaps in that situation the beetles eat the prey the ants bring? Or perhaps they eat something the ants produce, or the ant larvae themselves? Do these beetles differ in their behavior and diets from the other beetles in the genus?
I don’t know the answers to any of these questions. But I do know that…again…the world is a complex and spectacularly cool place.
Sources:
Antwiki.org’s Araeoschizus pages. What a marvelous resource that site is.
Hendricks, Paul, and Hendricks, Lisa M. (1999). FIELD OBSERVATIONS ON THE MYRMECOPHILOUS BEETLE ARAEOSCHIZUS AIRMETI TANNER (COLEOPTERA: TENEBRIONIDAE) AT HARVESTER ANT (HYMENOPTERA: FORMICIDAE) MOUNDS. Great Basin Naturalist 59(3), 1999. pp. 297-299.
