The desert is a rough environment for the plants and animals that live there. The unforgiving dry, rocky and stark habitat pushes life to the limit, where heat and drought shape the entire biome. Most plants are tough and spiny, while animals have to contend with a narrow range of options for shelter and food. Water sources are widely dispersed and the wildlife that lives near them includes many predators that rely on the hydration needs of their prey.
On a recent trip I took to Candelaria, in Presidio County, TX, the Chihuahuan Desert landscape appeared both amazing and intimidating. While the Rio Grande provides water, just a short distance from the river bank are steep rock cliffs, with plenty of cactus and other xeric plants clinging to the sides. The dirt road we followed along the U.S. side of the river is in the flood plain, but it seems the water only rarely expands over its usual course. When it does, naturally it is as a torrential flood that certainly disrupts the inhabitants but also replenishes the silt around the rocks in that narrow band of relatively lush growth. I had arrived after a fairly rainy period, so many plants had green foliage and were blooming.
I was part of a group of eight that included others with expertise in the biological fields, so we were on the lookout for any intriguing flora and fauna on a number of excursions, including one along the river. We had arrived in two ATVs and then got out to explore. Several folks had walked up a steep rocky path along the cliff face and I was photographing insects in the riparian vegetation when they called to me that they had found a big millipede.
After scrambling up the path, I was pleased to see one of the large rusty-colored desert millipedes that are common in the area. I did a quick photo op, then carefully started back down the loose rocks of the path. On the side of the trail was something I'd not seen when climbing up: another millipede that had apparently been stepped upon and was writhing around as yellow and black guts spewed out of its injury. I took a closer look, as something seemed very odd, but I couldn't tell just what it was. The "guts" didn't really look like the usual damage an arthropod would sustain if it were crushed between boot and stone. It took a bit more deliberation to finally figure out just what I was witnessing. Then it suddenly dawned on me; I was looking at a glowworm larva attacking a millipede.
What little I know about glowworms has come from those that I see in Austin and a bit of cursory research. Glowworms are a family of beetles, Phengodidae, in which the males have wings, large eyes and huge, feathery antennae while the females are larviform, meaning that they look pretty much like the larvae even when they are adults. I've seen a number of males which are attracted to lights, and also one larva that was crawling on a sidewalk. The larvae are brightly colored with yellow, orange and black bands. I've never seen a mature female, but I understand they are supposed to be larger and with more subdued coloration. I knew that glowworm larvae are predatory, but had never considered what they might attack.
As I watched, I realized I couldn't really tell if the glowworm was attacking the millipede or just holding onto it. The millipede was bigger and seemed very agitated but the two animals were so intertwined that it wasn't obvious who was winning this struggle. The other people were waiting to move on so I picked up the arthropod pair and put them into a container.
We drove our vehicles to another location and, when I had the time, I checked on my peculiar invertebrate guests. They were still wrapped together so I dumped them out onto my palm to get a better look. At this point, the millipede was on top of the glowworm and it looked like it might actually be the attacker. I found this puzzling because millipedes are scavengers, but, heck, I've seen some strange things. As I inspected the little beasts from different angles, I then realized that the millipede was no longer moving and the glowworm had its jaws clamped onto one of its victim's antennae. It actually looked like the glowworm was lugging its prize on its back, much like a hunter would sling a rabbit or pheasant over his shoulder.
It was time to move on again and I put the pair back into their little vial. We stopped at numerous other places and it was a couple of hours before I checked them again. The inside of the jar was a mess! When I spilled the contents out, the glowworm was busy munching through the millipede's head, hence all the slimy goo. I didn't seem to be disturbing the ravenous beetle at all, so I just replaced it once more. When we got back to the house where we were staying, I put the feasting glowworm into a larger container. By then, its head was down inside its victim, while a few remnants of millipede exoskeleton, in the form of thin rings, had been cast off.
Several hours later, the mess in the container was getting rather smelly, so I dumped it out once more to clean things up a bit. The glowworm had eaten nearly halfway into the millipede. That's about 1.5 inches of millipede going into a 2 inch beetle larva. The glowworm was significantly fatter. But it still wasn't fazed by my handling. We left early the next morning and I took the voracious little beast with me. It seemed perfectly content to keep working its way down the millipede's body as it continued to chow down. There was a litter of legs and body segment rings, along with a very rancid smelling slime that was probably a combination of rotting prey and predator feces.
Once home, I cleaned up the remains of the glowworm's banquet. The larva was now extremely rotund and rather lively. It had a strange constriction about 1/3 of the way down its body and I thought at first it might have been injured. Then I discovered that it simply had a millipede ring stuck around it. I used the tiny pair of scissors on a Swiss army knife and clipped the ring off. I could almost hear the glowworm's sigh of relief.
Putting the glowworm in a clean 2-inch deep square plastic container, I pressed down two corners of the lid and left the other two ajar to allow for a bit of air flow. The larva was restless but couldn't climb the slippery sides. Thinking the insect might now be ready to molt again or maybe even pupate, I added a bit of mulch to the container and left it on our dining room table while I attended to a few other specimens I had brought back. Checking back later, it appeared the larva had buried itself in the mulch.
Further research answered one of my pressing questions, which was about a glowworm's typical diet. It turns out that millipedes ARE their diet. Well, that little bit of information certainly fit quite neatly.
I had an entomology class to teach the next day, and was sure that my students would be delighted to see the glowworm. The next morning, there was no movement and I carefully sifted through the little bit of mulch. No glowworm. The lid was still on the container, but, obviously, my strong little captive had pushed its way out and was now finding a habitat more to its liking somewhere in our house. I checked the entire table top, along with the piles of papers and other objects that are part of the semi-permanent artifact collection there, as well as all around the floor, but to no avail. I was quite disappointed that I didn't have such an impressive prize to show everyone, although the situation was rather humorous, reminiscent of times during my childhood when a wayward snake or crayfish would escape its aquarium and I'd have to frantically search my bedroom.
That was over a month ago. Since then, I sometimes glance along the baseboards in our dining room or look up near the ceiling. If the glowworm had been female and managed to pupate successfully, it would be on the ground, emitting pheromones and using its pale greenish glow to signal to a potential mate. If it were male, it would eventually become evident as it flies around the house. Two other possibilities exist, though. It might have simply died, or it might have actually crawled out our back door, which has gaps big enough to allow roaches and spiders entry. If I never see it again, I sure hope the latter is the case. Any animal that entertaining deserves a better end than just expiring beneath our sofa.