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Larva exist to eat, radar detects molecules, goodbye moon

Q: Why do animals have larvae? What is the importance or necessity of such a stage? Lorna, Cambridge, UK

A: Larvae exist to feed. That’s how they help their species survive.

The Monarch caterpillar (larva) differs greatly from the adult butterfly. [© Scott Camazine 2003, used with permission]

A larva is an immature animal that doesn’t look like the adult and differs markedly from the adult. We know them by many names: grub (beetles, bees, wasps), maggot (flies), caterpillar (butterflies, moths), and wriggler (mosquitoes). Not just insects either — sponges, sandworms, and snails, to name a few, hatch from an egg into a wormlike larva.

A larva’s only job is to eat and grow. They are such efficient machines that many larvae do all the eating and the adult does none. It’s a "division of labor" says Scott Camazine, biologist and coauthor of Self-Organization in Biological Systems. Mouthparts of some insects have degenerated to the extent that they can’t eat. Others may eat a little but nowhere nearly enough to reproduce. Instead, they live off body parts made and fat stored as larvae.

The mayfly is a good example. The larvae spend two years wriggling around a pond, breathing through gills, and stoking up food. As a mayfly changes into an adult, it even transforms its digestive tract into an air sac to help it stay aloft as it flits about looking for a mate. After metamorphosing, the mayfly eats nothing. The adult mouthparts are tiny and functionless. The Emperor Moth, the largest moth with a wingspan up to a foot across, is another example. Adult Emperors don’t eat.

These immature animals — larvae — are supremely adapted to eat and grow. The adult Sacred Scarab (Scarabaeus) stores dung for its larvae. After hatching, the grubs start gobbling. They consume their own weight in dung every 24 hours.

How fast a larva grows depends on the species life span, weather, and food availability. A Polyphemus Moth caterpillar increased its weight by 4,140 times in 56 days. A human baby, growing at the same rate, would weigh more than 18 tons in two months.

Larvae eat. Adults reproduce.

Further Surfing:

Scott Camazine: a series of monarch pictures depicting the metamorphosis from caterpillar to pupa and then emergence.

North Carolina State University: Insect development by John Meyer

Radar detects molecules

Q: I wonder what viewing device, in theory, would have high enough resolution to detect the movement of nitrogen and oxygen molecules? Could radar be used or would it take higher frequencies, like X-rays? Jack, Washington DC

A: Detecting movement implies a resolution small enough to capture images of the molecules. We cannot use radar to take a picture of something as small as an oxygen or nitrogen molecule. We need tremendously higher frequencies — at least X-ray radar. Our present technology doesn’t support such a thing. We do have laser radars but that’s the highest frequency and it falls short.

An oxygen molecule is 0.29 nanometers in diameter and a nitrogen molecule is only slightly bigger: 0.31 nanometers. A nanometer is a billionth of a meter, about a billionth of a yard.

We use electromagnetic waves to detect and image objects: light waves for optical microscopes and telescopes, electrons for electron microscopes and radar. The waves need to be at least as small as the object they are trying to detect. Otherwise, the object doesn’t interfere with the wave enough to disturb the wave and we get insufficient information about the object. This isn’t exactly true for radar design but "not unreasonable in practice," says Peter Wittenberg of Boeing in Saint Louis, Missouri.

So, since we want to detect oxygen and nitrogen molecules, the wavelength of the detecting wave must be less than about 0.3 nanometers, the size of the molecules.

To figure out what wave frequency that molecular size corresponds to, let’s consider how wavelength relates to wave frequency. For water waves, the smaller the wavelength is, the more frequently the wave strikes the shore. In general, for all wave types, the smaller the wavelength, the higher its frequency. The mathematics is simply

frequency = c / wavelength,

where c is the speed of light (186,000 miles per second or 300,000 km/s).

Thus, the wave frequency must be c / 0.3 nm or 1 million billion cycles per second, which is 1,000,000 gigahertz (GHz).

Most airborne radar operates in the range of 0.4 to 40 GHz. Thus, the necessary frequency is about a million times greater than the operating range of airborne radar. X-rays do vibrate at the required frequency and they are what we need.

"I’ve never heard of an X-ray radar," says Wittenberg.

A single oxygen molecule. [Wilson Ho, University of California, Irvine]

So we’re out of luck. We can, however, obtain good images of an oxygen molecule with a higher resolution device, like a scanning tunneling microscope. See Wilson Ho’s figure.

Further Surfing:

Cornell University: Researchers rotate a single oxygen molecule on command

Iowa State University: Radar frequencies

Berkeley Lab: Electromagnetic spectrum

Goodbye moon

Q: How far away is the Moon from the Earth? Katie, Erie, Pennsylvania

The Moon, a long ways away. Apollo 17 astronaut and lunar boulder. [NASA]

A: In 1969, the Apollo 11 astronauts (the first men ever) took three days to go that distance. The Moon’s average distance from Earth is about a quarter of a million miles — 30 Earth diameters. More exactly, it varies from 221,500 miles (384,400 km) to 252,700 miles (406,700 km) through the month. The Moon’s orbit about the common center of gravity between Earth and the Moon is not quite a perfect circle. Rather it is an ellipse so the Moon’s distance from Earth varies by about 7%, says Robert Massey, astronomer at the Royal Observatory Greenwich.

Furthermore, because of tidal friction, the Moon is moving away from Earth — ever so slowly — at a rate of 1.5 inches (3.8 cm) per year.

Further Surfing:

The Royal Observatory Greenwich: The Moon

Keith’s Moon Page

(Answered Mar. 26, 2004)