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There’s conjecture of how man and man’s best friend have influenced each
other’s development
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Cicadas count to 17, eclipses prove elusive
Q: Cincinnati currently is engulfed with 17-year cicadas. These handsome insects
swarm out of holes in the ground where they have been hibernating for precisely
seventeen years. My question is how do the cicadas know when seventeen years
have expired and that it is now time to emerge? (Peter, Cincinnati, Ohio)
 Freshly
transformed winged adult (right) emerges from nymphal shell (left). The whitish
adult quickly darkens as her exoskeleton hardens. [Leon Higley, University of
Nebraska-Lincoln]
A: You ask how do they ‘count to 17.' We don’t know yet but
apparently, they track seasonal changes in the flow of xylem fluid (which their
host trees use to conduct water and dissolved minerals), says
David Marshall, biologist at
University of Connecticut. This helps them note the passing years.
Cicadas must emerge from underground just before becoming
adults so, as adults, they can fly and find a mate. It takes them 17 years to
mature from nymph
to adult (somewhat like us and our 12 years to puberty). Thus, their life cycle
mainly determines when they crawl out. Also the temperature must exceed about 64EF
(18EC).
By the way, the nymphs don’t hibernate while underground. They
do experience 4 years of slow growth (not real hibernation). After that, 17-year
cicadas grow at the same rate as the 13-year cicadas. They spend their
subterranean life eating and growing. The stubby nymphs look much like adults
without wings and have six powerful legs, some modified for digging.
Both types of periodical cicadas (13- and 17-year) suck root
juices in underground burrows and grow from small-ant size to nearly adult size
(1 to 2 inches, 25 to 50 mm).
In the spring of their 13th or 17th
year, a hormone change occurs that allows them to mature. Their levels of the
so-called Juvenile Hormone (JH) drop.
"Juvenile hormone (JH) is the body’s signal for how the nymph
will molt," says
John Meyer, entomology professor at North Carolina State
University. "As long as JH titers [concentration] remain HIGH the nymph molts to
another nymph. The adult molt occurs when the JH level drops to zero."
The nymphs tunnel to the surface, leaving a small exit hole.
Sometimes they build a mud tower over the hole. A few weeks later, when the
temperature is warm enough around sunset, they crawl out of the holes and molt
into adults.
Soon, billions roar, find mates, and lay eggs. After six to
ten weeks (in midsummer), the eggs hatch, the nymphs drop to the ground, dig,
find rootlets, and start sucking. The cycle begins again.
Further Reading:
University of Michigan Museum of Zoology: Periodical Cicada page
North Carolina State University: Morphogenesis by John Meyer
Science News for Kids: Prime time for cicadas
Eclipses prove elusive
Q: How often does a solar eclipse occur? (Kyle, Sarnia,
Ontario, Canada)
 Solar
corona of the total solar eclipse of July 11, 1991. Please click
here for a larger image.
[© 2002 by Fred Espenak, www.MrEclipse.com. Used with permission.]
A: About twice a year. If the Moon orbited Earth in exactly
the same plane that Earth orbits the Sun, we’d get a solar eclipse every New
Moon and a lunar eclipse every Full Moon. But it doesn’t. The Moon’s orbit plane
is canted at a 5E angle to Earth’s orbit plane.
(The plane in which the Earth orbits the Sun is called the ecliptic because of
its relation to eclipses.)
Only about one in six New Moons produce solar eclipses — or
about two a year. Half the time the path that the Moon orbits Earth lies above
the ecliptic and half the time it lies below the ecliptic. We get a complete
solar eclipse only when the Moon’s path intersects the ecliptic. Then the
Moon can blot out the Sun (since they are the same apparent size — about as big
as a pea held at arm’s length). We get a partial solar eclipse when the Moon’s
path almost intersects the ecliptic.
So, including the partial eclipses, the average number of
solar eclipses per year is 2.26 says Guy Ottewell, author of the illuminating
Astronomical Companion.
By the way, the next partial solar eclipse occurs on October
14, 2004 and will be visible in northeastern Asia, the Pacific Ocean, Hawaii,
and Alaska.
Further Reading:
Ottewell, Guy, The Astronomical Companion, Universal
Workshop, Middleburg, VA, 2000
NASA/Goddard Space Science Center: Solar eclipse page
Mr.
Eclipse: Solar eclipses for beginners, a primer
(Answered July 16, 2004)
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Cicadas count to 17, eclipses prove elusive
Q: Cincinnati currently is engulfed with 17-year cicadas. These handsome insects
swarm out of holes in the ground where they have been hibernating for precisely
seventeen years. My question is how do the cicadas know when seventeen years
have expired and that it is now time to emerge? (Peter, Cincinnati, Ohio)
Freshly
transformed winged adult (right) emerges from nymphal shell (left). The whitish
adult quickly darkens as her exoskeleton hardens. [Leon Higley, University of
Nebraska-Lincoln]
A: You ask how do they ‘count to 17.' We don’t know yet but
apparently, they track seasonal changes in the flow of xylem fluid (which their
host trees use to conduct water and dissolved minerals), says
David Marshall, biologist at
University of Connecticut. This helps them note the passing years.
Cicadas must emerge from underground just before becoming
adults so, as adults, they can fly and find a mate. It takes them 17 years to
mature from nymph
to adult (somewhat like us and our 12 years to puberty). Thus, their life cycle
mainly determines when they crawl out. Also the temperature must exceed about 64EF
(18EC).
By the way, the nymphs don’t hibernate while underground. They
do experience 4 years of slow growth (not real hibernation). After that, 17-year
cicadas grow at the same rate as the 13-year cicadas. They spend their
subterranean life eating and growing. The stubby nymphs look much like adults
without wings and have six powerful legs, some modified for digging.
Both types of periodical cicadas (13- and 17-year) suck root
juices in underground burrows and grow from small-ant size to nearly adult size
(1 to 2 inches, 25 to 50 mm).
In the spring of their 13th or 17th
year, a hormone change occurs that allows them to mature. Their levels of the
so-called Juvenile Hormone (JH) drop.
"Juvenile hormone (JH) is the body’s signal for how the nymph
will molt," says John Meyer, entomology professor at North Carolina State
University. "As long as JH titers [concentration] remain HIGH the nymph molts to
another nymph. The adult molt occurs when the JH level drops to zero."
The nymphs tunnel to the surface, leaving a small exit hole.
Sometimes they build a mud tower over the hole. A few weeks later, when the
temperature is warm enough around sunset, they crawl out of the holes and molt
into adults.
Soon, billions roar, find mates, and lay eggs. After six to
ten weeks (in midsummer), the eggs hatch, the nymphs drop to the ground, dig,
find rootlets, and start sucking. The cycle begins again.
Further Reading:
University of Michigan Museum of Zoology: Periodical Cicada page
North Carolina State University: Morphogenesis by John Meyer
Science News for Kids: Prime time for cicadas
Eclipses prove elusive
Q: How often does a solar eclipse occur? (Kyle, Sarnia,
Ontario, Canada)
Solar
corona of the total solar eclipse of July 11, 1991. Please click
here for a larger image.
[© 2002 by Fred Espenak, www.MrEclipse.com. Used with permission.]
A: About twice a year. If the Moon orbited Earth in exactly
the same plane that Earth orbits the Sun, we’d get a solar eclipse every New
Moon and a lunar eclipse every Full Moon. But it doesn’t. The Moon’s orbit plane
is canted at a 5E angle to Earth’s orbit plane.
(The plane in which the Earth orbits the Sun is called the ecliptic because of
its relation to eclipses.)
Only about one in six New Moons produce solar eclipses — or
about two a year. Half the time the path that the Moon orbits Earth lies above
the ecliptic and half the time it lies below the ecliptic. We get a complete
solar eclipse only when the Moon’s path intersects the ecliptic. Then the
Moon can blot out the Sun (since they are the same apparent size — about as big
as a pea held at arm’s length). We get a partial solar eclipse when the Moon’s
path almost intersects the ecliptic.
So, including the partial eclipses, the average number of
solar eclipses per year is 2.26 says Guy Ottewell, author of the illuminating
Astronomical Companion.
By the way, the next partial solar eclipse occurs on October
14, 2004 and will be visible in northeastern Asia, the Pacific Ocean, Hawaii,
and Alaska.
Further Reading:
Ottewell, Guy, The Astronomical Companion, Universal
Workshop, Middleburg, VA, 2000
NASA/Goddard Space Science Center: Solar eclipse page
Mr.
Eclipse: Solar eclipses for beginners, a primer
(Answered July 16, 2004)
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