The first invertebrates to crawl out of the sea and populate the conti-
nents were probably crustaceans.These ancient arthropods emerged from the
ocean soon after plants began to colonize the land. The oldest known land-
adapted animals were centipedes and tiny spiderlike arachnids about the size
of a flea and found in Silurian rocks some 415 million years old. The arach-
nids are air-breathing crustaceans and include spiders, scorpions, daddy long-
legs, ticks, and mites. The early terrestrial communities consisted of small
plant-eating arthropods that served as prey for the arachnids, which were
predatory animals.
The early crustaceans were segmented creatures, ancestors of today’s
millipedes, and walked on perhaps 100 pairs of legs. At first, they remained
near shore, eventually moving farther inland along with the mosses and
lichens. Because they had the land to themselves, no competitors, and an
abundant food supply, some species evolved into the first terrestrial giants—
growing up to 6 feet long. The crustaceans became easy prey for the
descendants of giant sea scorpions called eurypterids when they eventually
came ashore.
The advent of forests, where leaves and other edible parts grew beyond
easy reach from the ground, posed new problems for the ancestors of the
insects. Climbing up tall tree trunks to feed on stems and leaves was proba-
bly less dangerous than the treacherous journey back down.The return to the
ground would have been much easier if they simply jumped or glided
through the air on primitive winglike structures.These appendages probably
originated as a means to regulate the insect’s body temperature. By natural
selection, they developed into flapping wings.They worked well for launch-
ing insects to the treetops and for escaping predators when the vertebrates
finally came to shore.
Insects are by far the largest living group of arthropods.They can eas-
ily claim the title of the world’s most prosperous as well as the most popu-
lous creatures.Insects and plants have fought with each other for more than
300 million years. The fiercest battles have been played out in the tropics,
where hordes of hungry pests attack vegetation, which in turn defend
themselves with poisons.
Ever since animals left the sea and took up a lifestyle on dry land,
insects and other arthropods have ruled the planet. Insects have three pairs
of legs and typically two pairs of wings on the thorax, or midsection. In
most cases, an insect’s body is covered with an exoskeleton made of chitin,
similar to cellulose. To achieve flight, insects had to be lightweight. As a
result, their delicate bodies did not fossilize well unless trapped in tree sap,
which became hard amber, allowing the insects to withstand the rigors of
time. In some groups, the exoskeleton was composed of calcite or calcium
phosphate, which enhanced these insects’ chances of fossilization.
SILURIAN PLANTS