It’s Halloween today and so what better time than to bring you a couple of spooky stories from the world of physics.
First up is researchers at the University of Georgia in the US who have confirmed that six different species of bats found in North America emit a ghoulish green light when exposed to ultraviolet light.
The researchers examined 60 specimens from the Georgia Museum of Natural History and exposed the bats to UV light.
They found that the wings and hind limbs of six species – big brown bats, eastern red bats, Seminole bats, southeastern myotis, grey bats and the Brazilian free-tailed bat – gave off photoluminescence with the resulting glow being a shade of green.
While previous research found that some mammals, like pocket gophers, also emit a glow under ultraviolet light, this was the first discovery of such a phenomenon for bats located in North America.
The colour and location of the glow on the winged mammals suggest it is not down to genetics or camouflage and as it is the same between sexes it is probably not used to attract mates.
“It may not seem like this has a whole lot of consequence, but we’re trying to understand why these animals glow,” notes wildlife biologist Steven Castleberry from the University of Georgia.
Given that many bats can see the wavelengths emitted, one option is that the glow may be an inherited trait used for communication.
“The data suggests that all these species of bats got it from a common ancestor. They didn’t come about this independently,” adds Castleberry. “It may be an artifact now, since maybe glowing served a function somewhere in the evolutionary past, and it doesn’t anymore.”
Thread lightly
In other frightful news, spider webs are a classic Halloween decoration and while the real things are marvels of bioengineering, there is still more to understand about these sticky structures.
Many spider species build spiral wheel-shaped webs – orb webs – to capture prey, and some incorporate so-called “stabilimenta” into their web structure. These “extra touches” look like zig-zagging threads that span the gap between two adjacent “spokes,” or threads arranged in a circular “platform” around the web center.
The purpose of stabilimenta is unknown and proposed functions include as a deterrence for predatory wasps or birds.
Yet Gabriele Greco of the Swedish University of Agricultural Sciences and colleagues suggest such structures might instead influence the propagation of web vibrations triggered by the impact of captured prey.
Greco and colleagues observed different stabilimentum geometries that were constructed by wasp spiders, Argiope bruennichi. The researchers then performed numerical simulations to explore how stabilimenta affect prey impact vibrations.
For waves generated at angles perpendicular to the threads spiralling out from the web centre, stabilimenta caused negligible delays in wave propagation.
However, for waves generated in the same direction as the spiral threads, vibrations in webs with stabilimenta propagated to a greater number of potential detection points across the web – where a spider might sense them – than in webs without stabilimenta.
This suggests that stabilimenta may boost a spider’s ability to pinpoint the location of unsuspecting prey caught in its web.
Spooky.
