Arthropods

Introduction to “Arthropods”
By nearly any measure, the most successful animals on the planet are the arthropods. They have conquered land, sea and air, and make up over three-fourths of all currently known living and fossil organisms, or over one million species in all. Since many arthropod species remain undocumented or undiscovered, especially in tropical rain forests, the true number ofliving arthropod species is probably in the tens of millions. One recent conservative estimate puts the number of arthropod species in tropical forests at 6 to 9 million species (Thomas, 1990).
Arthropods range in distribution from the deep sea to mountain peaks, in size from the king crab with its 12-foot arms pan to microscopic insects and crustaceans, and in taste from chocolate covered antsto craw fish jambalaya and lobster Newburg. Despite this unbelievable diversity, the basic body plan of arthropods is fairly constant. Arthropods have a stiff cuticle made largely of chitin and proteins, forming an exoskeleton that may or may not be further stiffened with calcium carbonate. They have segmented bodies and show various patterns of segment fusion (tagmosis) to form integrated units(heads, abdomens, and so on). The phylum takes its name from its distinctive jointed appendages, which may be modified in a number of ways to form antennae, mouthparts, and reproductive organs.

Objectives

1. Diversity and Evolution in Arthropods

2. Form and Function in Arthropods

3. Growth and Development in Arthropods

Diversity and Evolution in Arthropods
Today mostbiologists divide arthropods into four subphyla:
* Trilobita This is thought to be the oldest subphylum of arthropods. Trilobites were dwellers in ancient seas. They are now all extinct.
* Chelicerata Chelicerates include spiders, ticks, mites, scorpions, and horseshoe crabs.
* Crustacea Crustaceans include such familiar organism as crabs shrimp.
* Uniramia Unirami includes mostarthropods: centipedes, millipedes, and all insects including bees, moths, grasshoppers, flies, and beetles.

The first arthropods appeared in the sea more than 600 million years ago. Since that time, these animals have experienced several adaptive radiations. Some arthropods have remained in the water, where they have colonized all parts of the sea and most freshwater habitats. Others were amongthe very first members of animal kingdom to colonize the land. The descendants of those pioneers were on hand when the first flowering plants appeared millions of years later.
The roots of the arthropod family tree are cloaked in mystery because the ancestors of the arthropods were soft-bodied animals that left few fossils. But by studying both living and fossil invertebrates, researchers haveaccumulated many clues to arthropod evolutionary history. Insects, centipedes, and millipedes seem to have evolved from ancestors that were closely related to the ancestors of modern annelid worms. Living evidence for this line of descent can be found in the form of small wormlike animals that live today in the moist tropics only. The body form of the earliest arthropods is thought to be similarto that of the trilobites. A typical trilobite´s body had a thick, tough outer covering and was composed of many segments, each of which bore a pair of appendages. Each appendage was divided to form two branches, one a walking leg and one a featherlike gill.
Most living arthropods exhibit two evolutionary trends away from the trilobite form. First, many have far fewer body segments duringdevelopment. Second, arthropods appendages have become increasingly specialized for feeding, locomotion, and other functions.

Form and Function in Arthropods
Although living arthropods are quite different from one another, all arthropods exhibit several key features. The three most important arthropod features are a tough exoskeleton, a series of jointed appendages, and a segment body. Other...