How Come?

Archive for the ‘animals’ Category

How come flies don’t fall off the ceiling?  asks reader J. Jones.

If we could look closely at our ceilings, we’d see the crisscrossing paths of thousands of tiny footprints, left by flies, ladybugs, and other insects (as well as by spiders).  In fact, the problem for flies and other bugs may not be holding onto the ceiling, but breaking free from it.  Turns out, a fly strolling across a ceiling is a bit like a person walking across a field of wet mud.

How do flies walk upside down, apparently effortlessly? Being tiny certainly helps.  Very-low-mass animals like wall-walking insects and spiders feel less of a pull from gravity.  So it’s easier for a fly than a pig to stick to the ceiling (even Spiderpig needed Homer’s help).

On a rough surface, an insect can use its claws, rappelling up or across like a climber on a rock wall.  But many insects and spiders also rely on special leg or foot pads, often covered with bristly hairs, when they need to climb up surfaces.  Scientists once thought that the rough, bumpy hairs allowed flies to cling to tiny nooks and crannies on even smooth-looking surfaces, including ceilings.  A substance secreted by the hairs helped, adding a bit of adhesion.

But in 2006, scientists at the Max Planck Institute in Germany discovered that the substance secreted by the hairs on a fly’s feet is a sticky glue, tailor-made for striding confidently across the ceiling, upside-down.

The glue-y stuff oozing out of a fly’s footpad hairs is a mixture of oils and sugars.  Researchers say that all insects may secrete the glue, since all 300 wall-climbing insects studied at the Institute left a trail of tiny, sticky footprints on the wall.

The adhesive is strong enough to keep each foot planted on the ceiling, fly standing still.  Walking, however, isn’t trouble-free.  Although the journey across the top of a room may look effortless from our perspective, it’s a struggle for the fly.  The researchers found that flies use at least four different techniques to get a foot unstuck and moving again.

Watching slow-motion tapes of each foot detachment, scientists found that a fly sometimes pushed a foot away from himself, popping the footpad off the surface like a freed suction cup.   Flies also twisted their footpads until they loosened from the wall, or jerked them quickly like a yanked-off band-aid.  Flies also used the handy, built-in claws on their feet to pull a footpad off the ceiling, like a person tugging off a boot.

According to the scientists, the techniques that involved peeling the pad off the ceiling or wall work best, because they require less energy.

Using four of six legs as they crawled across the ceiling also helped the flies make their gravity-defying journeys.  (On the ground, scientists say, flies often use just three legs at a time to move around:  two legs on one side and the middle leg on the other, forming a stable triangle, alternating sides with each step.)

Is it true that bumblebees shouldn’t be able to fly?  asks a reader.

No one’s sure where the myth started, but it has legs (er, wings):    Bumblebee flight is impossible.  According to the principles of aerodynamics, the story went, a big, fuzzy bumblebee, powered only by tiny wings, shouldn’t leave the ground.  A French book from the 1930s, for example, cited calculations that “proved” insects in general shouldn’t be able to fly.  But using mechanically-driven fixed aircraft wings to model the flight of bumblebees and other insects just didn’t work.

The mysteries of insect flight are still being unraveled, but today’s scientists say insects fly “in a sea of vortices,” using the swirling eddies of air created by their beating wings to stay aloft.

Bumblebees beat their wings up to 200 times a second, faster than the nerve impulses to their muscles can fire.  This works because bumblebee wing muscles don’t contract and expand with each electrical signal.  Instead, they continuously vibrate, like a repeatedly plucked guitar string.

When a bumblebee rests, its body temperature drops (or rises) to that of its surroundings. But according to entomologists, the temperature of a bumblebee’s wing muscles must be a toasty 86 degrees F. to lift the bee into the air.  So to fly to the nearest flower cafeteria, a bumblebee must dramatically raise its temperature in all but the hottest weather.

How?  Basking in the sun isn’t usually enough, so the bee will shiver her way to a higher temperature.  By rhythmically contracting and releasing her abdominal muscles (faster and faster as her temperature rises), a bee generates enough warmth to take flight.  The warmer the air temperature, the quicker the lift-off.  On a chilly, 42-degree day, a bumblebee must pump her thorax muscles for a tiring 15 minutes to reach 86 F.

Once in flight, a bumblebee’s muscle temperature remains in a range of about 86 to 111 F.  All of this activity expends enormous amounts of energy, provided by the sugars in flower nectar.  Not surprisingly, scientists have found that bumblebees are especially attracted to warm flowers, floral rooms whose color and shape keeps them heated in the sun.

According to Oxford University research published this month, bumblebee flight really is different from that of dragonflies and other insects.  Using a smoke-filled wind tunnel and cameras snapping 2,000 images a second, researchers found that bumblebee flight is surprisingly inefficient.  For example, a bumblebee’s left and right wings flap independently of each other.  And instead of using the varying air pressure at its wingtips to provide some lift, bumblebees use “brute force” to fly and hover.  Bumblebees, say scientists, are the “tanker trucks” of the flying insect world, using incredible amounts of energy to lumber (charmingly) through the sky.

In fact, bumblebees manage to fly even in places where human beings find it difficult to breathe.  On Earth’s highest peak, Mount Everest, bumblebees have been sighted at more than 18,000 feet up.  Although there’s much less air pressure for tiny wings to beat against, bumblebees power on, hot to the touch even in the frigid cold.