6 | NewScientist | 3 November 2018NEWS & TECHNOLOGY
Colin BarrasIN WHAT appears to be a case of
remarkably fast animal evolution,
crickets on Hawaii have begun to
purr. The discovery is the latest
twist in a decades-long battle
between crickets and a parasitic
fly that is attracted by their songs.
Male crickets normally use
their wings to “sing” to attract a
mate, but this makes Pacific field
crickets (Teleogryllus oceanicus)
on Hawaii easy targets for a
parasite. This fly (Ormia ochracea)
tracks down crickets by listening
out for their songs. It then
deposits maggots that burrow
inside their host, killing it.
In 1999, researchers noticed
that this strong incentive to stop
singing meant that crickets on the
Hawaiian island of Kauai were
evolving to stay silent. This was
the result of a mutation that
gave males unusually flat wings,
preventing them from producing
a sound. By 2003, silent males
made up about 90 per cent of the
population, making it one of the
fastest cases of evolution ever
recorded in the wild.
Similar silencing has been
detected on other Hawaiianislands. In 2014, for example,
researchers discovered that
crickets on the island of Oahu
have also evolved flat wings, but
via a different genetic mutation.
“It’s a really cool rapid convergent
evolution story,” says Robin
Tinghitella at the University of
Denver in Colorado.
But now, some crickets seem
to be regaining their voice and
singing in new ways to escape
detection by the flies. Studying
crickets on the island of Molokai,Tinghitella and her colleagues
have discovered males using what
she describes as a “cat-like purr”.
Examining these crickets in
the lab, the team found that
females are attracted by this call
(The American Naturalist, doi.org/
cwdr). However, it is probably too
low in pitch for the parasitic flies
to hear.
The team doesn’t yet know
when the Molokai crickets began
to purr. Tinghitella says that all
the males she has studied on
this island either purr or are
silent, so it is possible that purring
evolved from a silent ancestor.
Purring may be the idealsolution for the crickets of Hawaii
and their parasite problem. Mute
males find it difficult to attract a
mate, so starting to purr might
boost their chances of success.
This depends on how effective
the new sound is as a mating
song. “I definitely think it is
plausible the purring crickets
can communicate with potential
mates,” says Ann Hedrick at the
University of California, Davis.
“The experiments are
convincing.”
However, the lab studies also
revealed that the purr is a bit
messy, varying a lot in pitch, for
instance. Typically, effective
signals evolve to have little
variation for the sake of clarity.
“We’ve caught this trait so early
that natural selection doesn’t
seem to have done much to the
signal yet,” says Tinghitella.
By choosing to mate with those
that purr in a specific way, female
crickets may refine this new song
in generations to come. “The
females might whittle it down
so that it’s more like a typical
male song,” says Tinghitella.
Alternatively, the females
might learn to love the purr as
it is now. It is possible that they
become so accustomed to it that
they stop finding the original
song attractive. If this were to
lead to purring crickets no longer
breeding with non-purring ones,
it could be a step towards the
evolution of a new species. ■Fast evolution of
purring crickets
E. DALE BRODER“The laser actually sets the
properties of the liquid
light from the outside, so
they cannot be changed”THERE’S a new state of matter — and
it’s weird. It is made from light and
is somewhere between a solid and a
superfluid. It can’t be stirred, rotated
or even pushed.
“If you have some water in a
pipe and you start pushing it, it will
flow a little faster,” says Marzena
Szyman ́ ska at University College
London. “Whereas this fluid is soRigid light is a
strange new
state of matter
rigid that even pushing it will not
change its velocity.”
The new state is made from “liquid
light”. This is a fluid consisting of light
trapped in a material, in which each
photon is coupled with another
particle. These hybrid particles, called
polaritons, can flow and interact with
one another in a way that photons
alone cannot.
In the past decade or so, it has been
shown that liquid light can become a
superfluid, a fluid that flows with no
viscosity or friction. Because of the
lack of friction, superfluids cannot be
stirred or rotated. If you put one in abucket and rotate the bucket, the fluid
itself will remain stationary.
But Szyman ́ ska and her colleagues
calculated that, in certain situations,
liquid light takes things one step
further: not only is rotation impossible,
its flow also cannot be changed at all.
They call this new phase of matter and
light a rigid state.
The state arises because of how
liquid light is created. Photons arealways leaking out of the material
trap, so they must be replaced using
a laser. The team found that the laser
actually sets the properties of the
liquid light from the outside, so
they cannot be changed (Nature
Communications, doi.org/gfgbd4).
It’s not yet clear what this strange
rigid light could be used for, but it
could one day find a role in optical
communications, says David Snoke
at the University of Pittsburgh in
Pennsylvania. “Even if it’s not good for
anything, it’s interesting because it’s
different,” he says. “It really is a new
phase of matter.” Leah Crane ■The Pacific field cricket has
evolved rapidly to evade parasites