For the first time, scientists have taken 3D images of a caterpillar undergoing metamorphosis. And they are amazing.
Until now, studying metamorphosing insects, including fruit flies, blowflies and butterflies, has mainly involved dissection. Basically, scientists would take a bunch of the insects that are the same age and dissect them individually to see how their internal anatomy changes in different stages of metamorphosis. This technique is not only destructive — but it also may not give you an accurate picture of insect development, because different pupae mature at different rates.
So researchers thought: Why not use CT scans (X-ray computed tomography) to get images of the insects as they're changing in their chrysalis?
"This form of high resolution CT scanning (micro-CT), was only developed in the 80s, and has only really become widely available in the last decade or a little bit more as scanners have got cheaper, and computing power has got better," says Russell Garwood, a paleobiologist at the University of Manchester in the U.K. "So I think we're just lucky in that this is something that people have never had the chance to try before."
Now, a typical CT scan will only give you a 2D slice of a 3D object. But scientists can reconstruct 3D images if they take thousands of radiographs and put them together with computer software. One of the biggest challenges of using this technique on insects is figuring out how not to kill them with all the radiation required, Garwood tells io9.
The team initially began with half-hour scans of pupal Vanessa cardui (painted lady butterflies), says study co-author Tristan Lowe, who specializes in x-ray imaging at the University of Manchester. By fiddling with the software and with the positions of the insects in the scanner, the team eventually got the time down to 15 minutes. They were also able to reduce the number of radiographs necessary to make high-quality 3-D images.
For their study, the researchers scanned nine V. cardui specimens in their chrysalis, over a period of 16 days. They scanned different pupae on different days, further reducing the radiation exposure. All of the insects appeared to develop normally; however, those that were scanned within the first few days of the experiment couldn't break out of their chrysalis. "They tended to mature fully in chrysalis, but would be too weak to hatch out at the end," Lowe told io9. "It would seem that most of the radiation damage is done early on, though that's just speculation."
From the reconstructed 3D images, the team found that the general idea of pupal development that scientists had already come up with is correct. At the same time, though, "these scans did suggest the breathing system either developed faster than we might have expected, or that there was less remodeling and more of the breathing apparatus was carried over from the caterpillar to the adult," Garwood says.
Garwood and Lowe say that the CT scanning technique they've developed has a number of important implications. On the biological side of things, the method could help scientists understand the effects that pesticides have on insect development (particularly bees), figure out how genetic mutations affect the anatomy of model organisms, and improve forensic entomology. The fast-scanning technique could also impact industry. For example, it could be used to shorten the time it takes for a company to detect defects in automotive parts, Lowe says.
You can check out the study in the Journal of the Royal Society Interface.
All images via Tristan Lowe. Videos via Tristan Lowe and Louise Lever.