快猫短视频

IF YOU鈥橵E ever had to endure the high-pitched shriek of a flying saucer in a
B movie, then you鈥檝e already experienced the mating chorus of the periodical
cicada. Males flirt by vibrating, drumlike ridged membranes along their abdomens
with a distinct 鈥渨hooo-uhh鈥� sound. It ranks as one of nature鈥檚 most cursory
pick-up lines. But it works. In six short weeks the bugs jeer, mate, lay eggs,
lose weight and, finally, litter the ground with their carcasses. The next
generation will spend either 13 or 17 years underground before the teenage
brood鈥檚 collective hormonal alarm clock rings and it creeps en masse from its
earthy home among tree roots into the branches above.

The 鈥渨hooo-uhh, whooo-uhh鈥� of a million cicadas on a single acre of land is
deafening. In Missouri this summer, the din closed outdoor caf茅s and,
according to one reading, reached 85 decibels鈥攍ouder than a cheap vacuum
cleaner. The strange looking insects with their shiny black bodies as big as
your finger, blood-red eyes, orange legs and papery, veined wings are something
of a public nuisance. What better bug to love?

For three centuries, the periodical cicada has wowed scientists. Now, it is
stealing centre stage in biology鈥檚 top drama: the story of speciation. Ever
since Darwin, practical-minded biologists have tried to catch evolution in
action鈥攁 moth turning darker, a fish gradually shrinking. The ultimate
prize would be to observe a new species in the making. Cicada watchers believe
they have hit the jackpot.

Periodical cicadas live only in the US. Magicicada tredecim, one
cicada species, has a 13-year life cycle and is found in the midwest and south,
dipping as low as Louisiana. Its cousin, Magicicada septendecim, which
has a 17-year cycle, stretches northeast up to Massachusetts. These and other
periodical cicadas occupy more than a dozen broods, or geographically isolated
groups. Each brood emerges into the summer sun as one. In states where broods of
different ages overlap, periodical cicadas serenade the same neighbourhoods in
different years. Some years, the bugs all lie silent. And once every 221 years,
13 and 17-year broods emerge simultaneously into sheer cacophony.

The bugs look alike, with a subtle difference: 13-year M. tredecim
sport orange abdomens, while 17-year M. septendecim bear black and
orange stripes. It was a sash of stripes that, a decade ago, led Chris Simon, an
entomologist in the department of ecology and evolutionary biology at the
University of Connecticut, to a striking discovery. Her findings would start to
unravel the long-standing mystery of whether the odd-numbered life cycles of
periodical cicadas are set in stone, or whether a breakaway cast of bugs could
switch cycles to launch a new fate.

Chance matings?

Simon was in Iowa, collecting cicadas from a 13-year group dubbed Brood XIX,
when she noticed that many of the bugs had striped undersides typical of 17s. At
first, Simon assumed these darker cicadas were simply the offspring of chance
matings with nearby 17-year insects. Back at the lab, however, she and her
students studied the bugs鈥� mitochondrial DNA鈥攇enetic material in the
cell鈥檚 powerhouses, which is passed unchanged from mother to daughter. They
found that the DNA of those 13-year insects with unusual markings perfectly
matched that of 17-year cicadas. Clearly, these were 17-year bugs living the
13-year life. Rogue bands of bugs seemed to have cut four years off their life
cycle, synchronising with neighbouring native 13s. But how?

Simon had an idea. In the 1970s, University of Chicago researchers Monte
Lloyd and Jo Ann White reported that 17-year cicadas undergo a four-year
dormancy period. When these bugs bypass dormancy, they emerge in just 13 years.
Researchers have recorded several small groups of 17-year insects rising four
years early, though they were quickly gobbled by birds and other predators.
Simon suggests that a swarm of 17-year cicadas can survive an early outbreak if
they emerge at the same time as an overlapping brood of 13-year cicadas.
Predators who gorged themselves on the bug banquet would still leave enough
17-turned-13s to populate the area and adopt a faster-paced life cycle. Satiated
predators of the cicada crowd are well documented, says Richard Karban, an
entomologist at the University of California in Davis. However their role in
life-cycle switching has gone unstudied.

On a broader scale, Simon believes that in areas throughout the US where
broods of 13 and 17-year cicadas historically have overlapped, small hordes of
17s are emerging four years early, joining the adjacent party of 13-year cicadas
batting about that summer. These early risers have become, in effect, a new crop
of 13-year cicadas, with the darker belly and genotype of their 17-year
ancestors. 鈥淎t least a third of all 13-year cicadas across the eastern US are
really converted 17s,鈥� Simon suggests. The changelings appear in an arch from
Arkansas to Iowa, over to Illinois and down into Indiana. Between and below them
hover 13-year cicadas; above them, the 17-year type (see Map). The
genetic and geographic evidence combined buttress Simon鈥檚 unusual claim, says
Mark Taper, a theoretical biologist at Montana State University in Bozeman. 鈥淪he
has made the major leap鈥攕he has demonstrated that these 13-year cicadas
are just like 17s.鈥�

Why some 17-year cicadas jump the gun is unclear. Perhaps they are responding
to crowded underground living conditions, suggests Simon. Like many organisms
that lie dormant, they may be hedging their bets by maturing at different times
to boost their chances of survival. Many organisms do this, notes Karban.
Alternatively, these early risers may have a mutated 鈥渃lock鈥� gene that prompts
them to emerge ahead of schedule, says Simon. In that case, the altered
behaviour hasn鈥檛 evolved because it increases survival rates although this may
happen where 17s and 13s overlap.FIG-mg21954701.JPG

Just one thing has nagged at Simon. In theory, native and new 13-year cicadas
should mate in areas where they meet. After all, they share the same trees and
basic biology. But in Broods XIX and XXIII, where premature 17s have emerged,
the orange-bellied and black-bellied bugs have stayed separate, with little
evidence of intermediate cicadas. 鈥淚 was really surprised that the orange and
striped populations were isolated,鈥� Simon says. 鈥淲as something keeping them
补辫补谤迟?鈥�

The answer screeched from the muggy Arkansas air one morning in May last
year, as David Marshall and John Cooley slammed the doors of their pickup truck
and stood by the road. Zoology graduate students at the University of Michigan,
the two were visiting Arkansas to record periodical cicadas for a study on
mating behaviour. At the edge of stubby grass, facing a stand of trees, they
stopped and stared at each other. The cicadas didn鈥檛 sound right. Their chorus
was too shrill, too high. 鈥淟ike alien cicadas,鈥� Marshall says.

Bug music

Cooley sometimes strolls toward a group of cicadas, whistling to provoke a
female response. These females, however, did not flirt back. The researchers
soon learnt why. Armed with tape recorders and microphones, they spent the next
week sitting under the trees, recording bug music. At night, in the hotel, they
converted their taped melody to computer files, analysing the spectral
frequencies, or pitch. Sure enough, this brood鈥檚 pitch was oddly high. Another
surprise came a few miles south, where Marshall and Cooley suddenly picked up
two distinct spectral bands鈥攐ne high, one low. Here, clearly, two
different cicada songs were pulsating through the Arkansas trees.

Intrigued, the researchers started examining the bugs. 鈥淲e began to draw a
connection,鈥� Cooley says. The cicadas that were whistling low bore the classic
orange coloured abdomen of 13-year insects, but the falsettos were striped
orange and black. The researchers had stumbled across a new discovery that
perfectly fits Simon鈥檚 genetic data. There were two types of 13-year
cicadas鈥攐ne typical, the other similar to a 17-year bug鈥攁nd they
weren鈥檛 breeding together. Only now, perhaps, Marshall and Cooley knew why. The
insects were singing a different tune.

The researchers then split up, recording and mapping Brood XIX from Georgia
to Tennessee, Oklahoma to Illinois. When all was done, they had travelled 10 000
kilometres in three weeks鈥攁nd gathered enough data to draw a vivid picture
of cicada song differences. In a 100-mile swathe of geographic overlap, where
both original and newly transformed 13s live, their song frequencies differ
strongly, by as much as 500 hertz. Here, too, old-style female 13-year cicadas
flicked their wings鈥攁 sign of sexual receptivity, according to
Cooley鈥攎ore often when the researchers played back a relatively low mating
call on a tape recorder. By contrast, the time-skipping female 13s batted their
wings suggestively to significantly higher pitched calls.

Moreover, the differences in song fade the farther one travels from the
cicadas鈥� zone of geographic overlap. In areas where the conventional 13-year
cicadas live alone, or where a band of breakaway 17s has successfully
established its own territory, their songs mellow out, growing more similar.
According to Marshall and Cooley, the new 13-year cicadas fine-tune their
song over time鈥攅volving an unusually high pitch鈥攖o avoid mating with
native 13s in the same woods.

Separated in time and space from their original 17-year peers, and separated
by sex through their mating calls from their new 13-year pals, this vanguard
band of bugs has been nominated as a new cicada species, Magicicada
neotredecim. The impatient insects shed light on the events driving
evolution鈥攏ot only life-cycle switching, but also a process known as
character displacement, proposed 40 years ago by biologists Edward O. Wilson and
William Brown Jr. The idea is that two closely related species sharing a habitat
will diverge rapidly, evolving a pronounced difference in some feature鈥攁
character displacement鈥攖hat will allow them to divvy up limited resources
and reduce the chance of hybridisation. To put it a different way, two species
diverge in mating preferences and traits to avoid producing hybrid offspring
that would be less genetically fit.

While the idea of character displacement remains popular among biologists,
examples from nature have been few and far between. But this is precisely the
phenomenon driving the divergence of mating calls between the original 13-year
cicadas and the upstarts, claim Marshall and Cooley. If their hunch is borne
out, it will be sweet music to the ears of evolutionary biologists.

鈥淎lthough there are quite a few examples, not many stand up to close
examination,鈥� says Roger Butlin of the University of Leeds. These events are
hard to capture, adds Glenn-Peter Saetre of the University of Oslo in Norway,
who has recently documented a case of reproductive character displacement. His
subjects, the pied flycatcher and the collared flycatcher, resemble each
other鈥攆emales are brown and males black with small white
patches鈥攁cross much of their separate European ranges. But in parts of
central and eastern Europe, where the two species live in close association,
male pied flycatchers fade brown and male collared flycatchers appear
whiter.

Saetre wanted to know whether this was a chance geographical trend or if it
could be explained by character displacement in areas of species overlap.
Combining DNA analysis, field observations and experimental matings, he and his
colleagues showed that it is no geographical coincidence. Female flycatchers do
indeed choose males of their own species based on colouring. What鈥檚 more,
hybrids are less fertile鈥攍ess evolutionarily fit鈥攚hich gives the
pied and collared flycatchers a good evolutionary reason to differentiate.

A long wait

So, have the new breed of 13-year cicadas upped the pitch of their mating
call in an attempt to go with their own? If M. neotredecim and M.
tredecim hybrids turn out to be less fertile than pure lineage periodical
cicadas, the case for reproductive character displacement will be much stronger,
says Dolph Schluter of the University of British Columbia in Vancouver. Simon,
Marshall and Cooley hope to tackle this question in 2002, when 13-year cicada
Brood XXIII next emerges. Researchers will then trap M. tredecim and
M. neotredecim, crowd the bugs until they breed, and track the fate of
their progeny. It will be a long wait.

Meanwhile, Simon is already addressing criticisms that her ideas, although
interesting, are without grounding. 鈥淪he suggests some breakage of dormancy in
the 17-year cicada,鈥� says Jin Yoshimura, an ecologist from Shizuoka University
in Japan. 鈥淭he question is how and why?鈥� Simon hopes her current experiments
will answer this question.

Working last summer with Kim Smith of the University of Arkansas, Simon and
some students dragged a dozen trees inside tents, isolating virgin 17-year male
cicadas with virgin 13-year females and vice versa. In this cosy environment,
the bugs mated and laid eggs in the branches. Two months later, their hybrid
nymphs burrowed into the soil of plant containers below. Every year for the next
six years, Simon intends to dig up the nymphs, note their developmental progress
and analyse their DNA. She hopes to test her idea that a dominant clock gene
dictates the 17-year life cycle. In doing so, she may learn whether specific
mutations lead some xhybrids to skip the early dormancy period.

There鈥檚 more at stake here than bug theatrics. In Illinois, where M.
neotredecimhas flourished, its 17-year ancestors have slowly dwindled into
extinction. A similar pattern is emerging in other states. Simon suggests the
brazen new 13-year cicadas outcompete their slightly smaller 17-year peers for
resources underground. Eventually, this could spell doom for an old species of
cicada鈥攁nd life for a new one.

YOU don鈥檛 have to wait 13 years to hear a male cicada croon. Across the
world, cicadas with shorter life cycles emerge each summer. As garish as their
periodical cousins, these cicadas鈥攇old, green, turquoise, red and other
hues鈥攁lso attract fans.

At San Diego State University, for example, entomologist Kathy Williams
spends the summers peering into the canopies of woody plants, documenting which
varieties attract some of North America鈥檚 150 cicada species. It鈥檚 a hot
job鈥攃icadas of any variety wait until the thermometer blushes cherry red
before singing. 鈥淚鈥檝e been out in the desert when it鈥檚 blistering, at least 125
degrees Fahrenheit [52 掳C], and these guys are just singing away,鈥� Williams
says.

One reason the bugs can tolerate such heat may be because they have a secret
weapon up their wings鈥攕weat. According to research by Eric Toolson of the
University of New Mexico, cicadas are the only insect known to have sweat pores.
The tiny holes allow the bugs to perspire away excess fluids sucked from tree
branches. Keeping a cool head extends the female cicada鈥檚 egg-laying time,
possibly boosting her genetic fitness. And by basking in the summer sun, cicadas
thwart predators who crave milder temperatures, says Toolson.