In or out?
Question: Why do some people have 鈥渋nnie鈥� belly buttons while others have
鈥涣耻迟迟颈别蝉鈥�?
Answer: I鈥檓 sorry to say this, but whether your navel is an 鈥渋nnie鈥� or an
鈥渙uttie鈥� is the result not of anything as interesting as genetics or womb
conditions but of the more prosaic practice of tying off the umbilical cord.
When you are born the cord is cut immediately after delivery and the result
is a neat little knot. As the wound heals, it takes on the vague shape of the
tissue caught between abdomen and knot. The knot falls away and leaves the
traditional puckered appearance of the belly button. Whether you end up an
鈥渋nnie鈥� or 鈥渙uttie鈥� depends on how your body heals, and the skill of whoever
tied the knot.
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Other than this, an 鈥渋nnie鈥� can be turned into an 鈥渙uttie鈥� by pregnancy or
even a hernia, which is why a larger percentage of mothers have 鈥渙utties鈥� than
the population as a whole.
Adam Wray
Perth, Western Australia
Answer: When the umbilical cord is cut after birth, it is more likely to
shrink down into an 鈥渋nnie鈥� if there is a good cut and only a small section of
cord. It will form an 鈥渙uttie鈥� if there is a longer section of cord left.
Thomas Scott
Mansfield, Nottinghamshire
Eyes up
Question: Many insects such as bees, wasps and flies have large, multifaceted
eyes. These cover a wide field of view without having to swivel as
human eyes do. However, unless these insects point their heads
downwards鈥攚hich would make flying somewhat unpredictable to say the
least鈥攕ome part of their eyes would always be looking directly at the Sun.
In view of the dangers of looking directly at the Sun, which were publicised at
the time of Europe鈥檚 total eclipse last August, why don鈥檛 these insects go
blind? Do they have some protective mechanism in their eyes, or is it that they
just don鈥檛 live long enough to suffer any significant damage?
Answer: There are important differences between our eyes and those of
insects. First, insects have compound eyes consisting of up to several thousand
optical units called ommatidia, each with a single lens. These lenses are much
smaller than ours, so less potentially damaging light reaches the sensory
cells.
Second, insects have a much wider field of view and, at the same time, fewer
receptors to cover it. This results in a much lower resolution. The diameter of
the Sun in the sky is about half a degree, which means that only between one and
three of an insect鈥檚 ommatidia are looking into the Sun at any time.
Finally, and most importantly, insects do not move their eyes to point an
area of high resolution towards an object, like we do with our fovea鈥攖he
part of the retina, packed with retinal cones, where vision is most acute. If we
look into the Sun we do it with the fovea, and if we do it for too long we
damage this region. A flying insect sees the Sun in three or fewer ommatidia,
and as those will change continually, there is not normally enough time for
damage. For the same reason, we do not go blind as long as we do not stare at
the Sun.
Almut Kelber
Department of Zoology
Lund University, Sweden
Answer: Prolonged exposure to sunlight can affect animals with compound eyes,
but the extent of the damage will depend on the species and the conditions to
which they are adapted. In the eyes of many species, mechanisms operate to adapt
the eye to the light level. These include screening pigments: tiny (between 0.3
and 1 micrometre) black or brown ommochrome granules, which may move into the
path of the light underneath each facet of a compound eye and reduce the amount
of energy that reaches the visual membranes deeper within the eye. The amount of
this screening material often varies from day to night.
Unpublished information from E. Eguchi, T. Kashiwagi and myself reveals that
light can damage the eyes of Finnish bumblebees, house crickets and two species
of butterfly. If these insects are exposed for three hours a day on three
consecutive days to the ultraviolet component of sunlight, in intensities at
least five times as strong as sunlight, two separate kinds of photoreceptor
damage occur.
The Finnish bumblebees, being avid visitors of flowers during daylight (which
lasts almost 24 hours during a northern Finnish summer), show least damage.
Crickets, being naturally crepuscular or nocturnal insects, show maximum
disruption of the visual membranes.
The two butterfly species (a papilionid and a pierid) show some damage, but
much less than the cricket, and primarily to some of the photoreceptor cell body
organelles rather than the photoreceptive membranes.
Animals such as crustaceans, which also have compound eyes and normally live
in a world where sunlight is filtered and weakened by layers of water, are even
more sensitive to sun damage. It is well documented (Crustaceana, vol
67, p 97) that lobsters can become blind if removed from water and exposed to
bright sunlight.
Victor Benno Meyer-Rochow
Department of Biology
University of Oulu, Finland
This week鈥檚 questions
Blooming fruit: Can somebody tell me what the bloom is on the skins of fruit
such as plums and grapes? How does it get there and what does it do?
Ricky Last
Miami, Florida
Playing cricket: A friend of mine assures me you can tell the temperature by
listening to the chirping of crickets. Is this true and, if so, how?
Karl Raby
San Diego, California