i
Small insects like fruit flies don't bother with
ventilation mechanisms to breathe, they
simply rely on diffusion to supply their
tissues with oxygen ­ at least, that's what
many researchers thought. Now, a study by
Fritz Lehmann and Nicole Heymann
suggests that this conclusion was a bit
premature. Studying CO2 release in flying
fruit flies, the pair discovered that these tiny
insects do indeed ventilate, but in a rather
unconventional way: they use their "tongue"
as a pump (p.3645)!
It all started when Lehmann and Heymann
used a new gas analyser to study gas
exchange in flying fruit flies. The new tool
was sensitive enough to detect minute
respiratory changes in a tethered insect
flying in a virtual-reality flight arena. The
first time they tried it, they stumbled across
something completely unexpected: huge
cyclic fluctuations in a fly's CO2 release,
despite the fact that the flying insect's
metabolic rate was constant. This was
surprising, because a small insect's CO2
release rate is thought to match the animal's
metabolic needs, so at a constant metabolic
rate we would expect constant CO2 release.
Figuring that people had wrongly assumed
that tiny insects don't need to ventilate while
flying, Lehmann and Heymann decided to
test whether abdominal pumping, a common
ventilation mechanism in insects, could
explain the mysterious CO2 fluctuations.
Insects have flexible air sacs located in
various spots in their body, which they
compress by pumping their abdomen. To
measure abdominal pumping during flight,
the pair painted fluorescent dots on the flies
and tracked the dots' movements on video as
the insects flew. But they found no
correlation between abdomen movements
and CO2 release patterns; it seems fruit flies
don't use conventional ventilation
mechanisms to power their cyclic breathing
pattern.
The pair had to explore other explanations
for the flies' unusual breathing. `We had
noticed that fruit flies stick out their
proboscis while flying, but we had no idea
what it meant', Lehmann says. Considering
that the large air sacs in a fruit fly's head
could be compressed by the fly's large
proboscis, he wondered if flies use their
proboscis as a pump. Sure enough, when
the pair bathed a flying insect in infrared
light and tracked changes in light intensity
as the fly stuck out its proboscis, they found
a strong correlation between the fly's CO2
release pattern and its proboscis
movements. Fruit flies don't just use their
proboscis to eat; they also use it to breathe
while flying.
`This was nice, but we suspected that it
wasn't the whole story', Lehmann says. He
explains that insects mainly regulate their
gas exchange using four large spiracles
(openings) on their thorax. He constructed
an analytical model to test whether cyclic
CO2 patterns can result when these spiracles
act independently of one another. The
model revealed periods when the spiracles
were opening in phase with one another,
followed by periods when spiracle opening
patterns were not synchronised.
Astonishingly, the model's CO2 release
pattern was indistinguishable from that of a
real fly! While the model suggests that a
fruit fly's spiracles may control cyclic CO2
release patterns during flight, Lehmann is
not sure how the fly benefits from these
cyclic patterns. But he can see why a fruit
fly uses its "tongue" to ventilate its head: to
ensure that metabolically greedy organs,
like the eyes and brain, always get enough
oxygen.
10.1242/jeb.01863
Lehmann, F.-O. and Heymann, N. (2005).
Unconventional mechanisms control cyclic
respiratory gas release in flying Drosophila. J.
Exp. Biol. 208, 3645-3654.
IInnssiiddeeJJEEBB
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FRUIT FLIES STICK OUT
TONGUE TO BREATHE
TROUT BLOOD CELLS
CHURN OUT NITRIC OXIDE
Our red blood cells dutifully transport
oxygen to our tissues ­ but could they play
a more sophisticated role than we suspect?
Rather than indiscriminately supplying
oxygen, red blood cells may sense when our
tissues need an oxygen boost and release
substances to relax local blood vessels and
increase blood flow to an oxygen-deprived
area. We now know that, in our red blood
cells at least, deoxygenated haemoglobin
converts nitrite into nitric oxide. When
nitric oxide escapes from the red blood cell,
it diffuses into and relaxes the muscles
around blood vessels, increasing local blood
flow. If we can do this, perhaps fish can too,
Frank Jensen and Claudio Agnisola
reasoned (p.3665).
THE JOURNAL OF EXPERIMENTAL BIOLOGY