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In the Insect World, Size Matters: Small is Better
When faced with the question of why humans fear insects, an unknown entomologist many years ago was purported to have quipped, "Insects are large enough to see, but not large enough to see well!" This comment speaks volumes, not only about human attitudes toward insects, but also about the size of insects in general -- they are small creatures.
So how big are the biggest of these small insects? Big can be described in terms of weight or measure, and by using either, at least relative to other animals on the earth, insects are small.
The largest insect by weight is a weta, a cricket-type insect, found in New Zealand. A female weta, heavy with eggs, can weigh about 70 grams or about 14 grams more than a tennis ball. Goliath beetles are native to South America and, at least for insects, are appropriately named. These beetles weigh about 50 grams or slightly more than a golf ball.
If big is a matter of measure, then insects also come up short in the animal world. The longest insect is a walking stick that is around 14 inches in length. The largest of the Lepidoptera, the butterflies and moths, have wingspans of about 13 inches. To be sure, the fossil record shows that a dragonfly with a wingspan of 28 inches once cruised over the surface of the earth. But no more! Today, small is beautiful to insects.
So why are there no giant insects the size of those depicted in the big-bug monster movies such as "Mothra" and "Them?" One answer might be found in the basic structure of the arthropod body. Arthropods, in addition to insects, also include such creatures as lobsters, crayfish, shrimp, millipedes, centipedes, sowbugs, spiders, ticks, mites and scorpions. All of these creatures have an exoskeleton.
An exoskeleton is a hardened outside surface very much like a suit of armor. The arthropod exoskeleton has three major functions: protection, support and muscle attachment. These functions are handled by two systems in most other types of animals, including humans. In these animals, protection is the duty of the skin while the internal skeleton provides support and a place for muscle attachment.
But how does the exoskeleton limit the size of an insect? Some scientists believe it has to do with a ratio of surface area to volume. The general argument is that if an insect were to be as large as a cow that the exoskeleton would have to be so thick to support the animal that the weight of the exoskeleton would be too heavy for the creature to carry. Indeed, to support this argument, the largest arthropods by weight are lobsters, creatures that live in the sea where the water provides buoyancy.
Another biological characteristic of arthropods might also limit their size. That characteristic is how animals provide oxygen to the cells that make up their bodies.
Scientists refer to the two major approaches to oxygen distribution -- and carbon dioxide removal -- as closed or open systems.
Most of the larger animals, including we humans, have a closed system. That means that we have lungs that transmit oxygen to our blood where it is carried by hemoglobin through our arteries to all parts of our body. Our veins carry the carbon dioxide back to our lungs, all in a closed system. Closed, that is, unless we manage to cut ourselves and allow the blood to escape.
Arthropods, on the other hand, have an open system. These organisms don't have hemoglobin to carry oxygen and don't have a series of veins and arteries to transport blood. Oxygen gets to the cells in arthropods via a series of air-filled tubules called trachea that open to the outside through spiracles, the breathing holes in the exoskeleton of insects. Diffusion across a membrane moves oxygen from the air into the cells and brings carbon dioxide out. This process can only work efficiently if the organism is small in size.
Another argument for the benefits of the small size of insects is that little creatures require less food to eat and can hide from predators easier than if they were larger. In biology, having plenty of food to eat and not being eaten by someone else is always a good thing.
Entomologist H. E. Hinton once suggested that being small also allowed insects to fall great distances without getting hurt. That might not be as important as having an exoskeleton or breathing without the need for red blood, but escaping injury in a fall has got to be a good thing. Just one more reason to celebrate that being small is indeed better -- at least if you're a bug!