Comparative anatomy is the scientific comparison of animal bodies. The purpose of comparative anatomy is to see their working structure, and to decide upon the phylogenetic relationships between different groups of animals. The separation of animals into phyla is done mainly by comparative anatomy: see List of animal phyla.
The main techniques used are dissection and microscopy. Dissection is an ancient method used to find out the inside structure of a living thing (usually done after it is dead). It is still used by medical students to learn about the details of the human body. Simple microscopes were first invented in the 17th century, and the compound microscope (still used today) became available in the 19th century. The purpose of microscopy is to let us see the small details of structure. Also, the careful comparison of large collections of animals (usually in museums) is often done.
The great era of comparative anatomy was from about 1800 to about 1950. It was used by those who did not believe in evolution, such as Georges Cuvier, and by those who did, such as Thomas Henry Huxley. Charles Darwin himself used comparative anatomy as the main tool in his work on barnacles. Nowadays, the main method used to find out relationships is molecular evolution, which uses DNA sequence analysis. However, for many research purposes, zoologists still dissect animals. To get a degree in biology, you need to know about the structure of animals (and plants).
Dissection
Dissection is a standard laboratory technique in biology and pathology. To dissect is to cut up a body so as to reveal its structure. The body could be that of a human, an animal, or a plant. Dissections are regularly done in autopsies to help determine the cause of death. They are done by zoologists to find out the internal structure of animal bodies. The comparative anatomy of animals, revealed by dissections, gives clues as to their evolutionary relationships.
List of animal phyla
List of animal phyla is a list of the major groups of animals usually classified as a phylum. Modern sources have been used: the list is different from that of Linnaeus or Cuvier. A list of this type may be arranged alphabetically; equally it might be arranged according to evolutionary relationships. No list will be completely satisfactory. Authorities differ in what they consider a phylum, and in the actual name of the phylum. Despite this, there is agreement on most phyla. Most modern surveys include groups above the phylum, based on evidence of common descent.
Differences of opinion about evolutionary relationships have been reduced by the use of molecular evolution and molecular clock research. These make use of protein amino acid sequences, and whole genome DNA sequence analysis. These modern techniques have led to changes and renaming of many higher categories. Classification based on traditional comparative anatomy had errors which needed to be corrected. For example, the old phylum Coelenterata, which had stood for almost two hundred years, was split down into two separate phyla, the Cnidaria and the Ctenophora.
The following list is based on evolutionary relationships:
Major groups in large type.
At least 21 phyla are exclusively aquatic, with several others in quasi-aquatic habitats on land. "None" are entirely terrestrial. This is testimony to the importance of water for life, and to the sea in particular. It is fairly certain that all phyla originated in the sea or, at any rate, in water. Most made their first showing in the Cambrian, or in the Ediacaran. Most of the soft-bodied phyla have left few fossils.
Phyla may be grouped according to evidence about their evolutionary relationships. The list above puts similar groups together.
This kind of megataxonomy is becoming more convincing as DNA sequence analysis proceeds through the phyla. Some entirely fossil groups are still placed where they are on anatomy and commonsense rather than hard molecular evidence. The trilobites are a good example. Their position in the Arthropoda is based on not much more than their bilateral symmetry and an exoskeleton. These groupings are discussed further in the references to this page.
This table has the advantage of being sortable. The terminology differs in places from the above descriptions. Also, by listing living species only for most phyla, those with huge fossil records (like Bryozoa and Brachiopods) are lower in the order despite being important aquatic forms in the Palaeozoic era.
This list is to help when you read older literature which may use out-of-date terms.
Orthoptera
Orthoptera are an order of insects. The order contains grasshoppers, katydids, and crickets. "Ortho" means "straight", so "Orthoptera" means "straight wings".
This means the front wings, called "tegmina", are stiff, straight, and not used for flying. The back wings are membranous and are folded like a fan under the front wings when the insect is not flying. Many species use their wings to make sounds, which we usually call "chirping" noises.
Orthopteran insects start life in an egg case. After three weeks – or when spring comes – the tiny nymphs come out from the egg case. After four or five molts, they have wings. This shows that they are adults and are ready to reproduce.
Their sounds are produced by using file-like edges to their wings, or wing veins. In some groups the wing edges are rubbed together, in others the hind limb is rubbed against a wing edge. Each sound is unique to the species, and its function is identification for mating purposes. It is males which make the sound, and females move towards it.
In the order Orthoptera, members chew their food, using their mandibles (jaws). Crickets are omnivores, which means they eat both plants and animals. Actually, they will eat almost anything: vegetables, cereal, and even their own mate if they are hungry enough. Katydids are mostly herbivores (plant eaters), though they will eat their own mate, too, if "they" are hungry enough. They also enjoy eating aphids and other small, slow-moving creatures. Grasshoppers almost always eat plants like grass, wheat bran, and lettuce, and they can be terrible crop pests.
It's quite hard to catch a member of Orthoptera because they jump so well. They have amazing legs - a grasshopper can jump 20 times farther than the length of its body. Their back legs are very large and long. These long, strong legs give these insects their great ability to jump.
Crickets, katydids, and grasshoppers are in the same order of Orthoptera because they are alike in many ways. However, there are several things that make them different from each other.
First, their colors are usually quite different. Since grasshoppers like to move during the daytime, their colors are similar to grass and bright flowers, making them usually green, light brown, or multicolored (lots of different colors at once). Crickets move at night, so they are dark. Katydids like to spend a lot of time on leaves, so they are often leaf-colored, and their wings can look like leaves. Their wings can have the same vein patterns as leaves, and they often have little brown spots just like the ones that might be found on a leaf.
Secondly, their behaviors are different. Grasshoppers like being active in the day; crickets, at night; katydids, in the late afternoon and evening.
Thirdly, their antennae is different. Katydids and crickets usually have long, thin antennae, while grasshoppers usually have short, thick ones. Of course, this rule does is "not" perfect – for instance, even though grasshoppers "usually" have short, thick antennae, the long-horned grasshopper has long, thin antennae like a cricket. Because of this, it is still sometimes hard to tell the members of this order apart.
Chios