By Leslie Mertz
If an insect ever needed a little love, it would be the immature stage of the drone fly (Eristalis tenax), which is known as a “rat-tailed maggot.” As it turns out, a biologist in London has taken this drone fly larva to heart and, in so doing, discovered never-before-seen structures that appear to keep the maggot mostly free of bacteria, despite living in stagnant, fetid waters where microorganisms flourish.
“I spend a lot of time in ponds, so I saw rat-tailed maggots — the way they move, the way they behave — and just thought, wow, they’re really amazing,” said Matthew Hayes, the lead author of a new paper on the maggots in the Journal of Insect Science.
His attention was initially drawn to the larva’s “tail,” which is actually a breathing tube, or siphon. The siphon extends like a telescope from the larva’s rear end to reach the surface of the water.
“It is an example of an unloved thing that should be loved more,” he said.
Hayes has the kind of job that allowed him to take an ultra-close look at the maggot. He’s a cell biologist, and he’s also the manager of the electromicroscopy imaging facility at the Institute of Ophthalmology at University College London in England. With scanning and transmission electron microscopes, he carefully examined the larva and saw that much of its body is covered with thin spines, or “nanopillars,” that narrow to sharp points.
Once he confirmed the spiky structures were indeed part of the maggot, he noticed a direct relationship between the presence of the spines and the absence of bacteria on the surface of the larva. He speculated that the carpet of spines simply makes it impossible for the bacteria to find enough room to adhere to the larva’s body surface.
“They’re much like anti-pigeon spikes that keep the birds away because they can’t find a nice surface to land on,” he said.
Hayes also ventured that the spines could possibly have superoleophobic properties (the ability to repel oils), which would also impede the bacteria from colonizing and forming a biofilm that could ultimately harm or kill the maggot.
The composition of the spines is as unique as the structures themselves, Hayes said. Each spine appears to consist of a stack of hollow-cored disks, the largest at the bottom and the smallest at the top.
“What I really think they look like is the baby’s toy with the stack of rings of decreasing size, but on a very small scale,” he said. “I’ve worked in many different fields and looked at lots of different things, and I’ve never seen anything that looks like it.”
If that weren’t enough, Hayes and his co-authors also noted an interesting behavior among the rat-tailed maggots.
“They come to the surface — I don’t know why — and they hang upside down underwater and crawl along on the underside of the meniscus of the water,” he said.
While there, the larva writhes.
“If you can imagine, this highly folded and spike-covered larva twists back and forth under the meniscus of the water, rolling and rubbing against itself in a complicated movement,” he said.
Hayes said the larva looks as if it’s combing itself, perhaps eliminating bacteria that may have become impaled on the spines. In fact, he has spotted some damaged bacteria near the spines.
He plans to continue looking at rat-tailed maggots to get back to his initial question about how the siphon works.
“Its siphon can be up to six inches long (about 15 cm), and it actually rotates and retracts. It’s a very intriguing mechanism,” he said.
Interestingly, the siphon doesn’t have any spines on it. He guessed that this might be related to the telescoping nature of the siphon, which may mechanically strip off any bacteria.
This work with the rat-tailed maggot is leading him to examine other insects as well, including the ability of another aquatic invertebrate — the mosquito larva — to thwart bacteria. Such antibacterial properties have applications in many different fields, including ophthalmology and other medical fields where biofilms can foul surgical instruments or implanted devices.
“If anyone asks, [the medical implications] are why we’re looking at insects, but mostly it’s because they’re just very interesting,” he said with a laugh. “I have access to this amazing microscopy equipment, and it gives me a chance to look at all sorts of things, so I have lots of projects in mind.”
For now, though, he’s thrilled about shedding light on the underappreciated rat-tailed maggot and revealing its spiny armor.
“I’ve loved insects since I was a child, when I would breed butterflies and moths,” he said. “I’m just so chuffed to have discovered something a bit new about insects!”
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Leslie Mertz, PhD, teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.