As an animal doubles in size, its weight increases eight-fold, but the weight bearing capacity of its skeleton is only quadrupled and the strength of its muscles is merely doubled. Because of their great weight, large vertebrates have skeletons which are disproportionately heavy and robust compared to those of small vertebrates.
Presumably, terrestrial arthropods could reach horror-movie size simply by developing big, sturdy skeletons. But they have not done so during hundreds of millions of years on Earth.
It seems that the costs associated with large size affect arthropods more strongly than vertebrates. A heavy, cumbersome skeleton, risk of injury, and complications during molting all become more serious problems at large size. As an arthropod gets larger, the proportion of weight attributed to the skeleton will increase faster than it does for a vertebrate. At some point, the advantages of increased size will not compensate for the difficulties associated with a heavy skeleton.
When that happens, natural selection will favor the smaller individuals in a population. One important difficulty for large arthropods is the risk of injury. Without the cushioning effect of soft tissues, it is more vulnerable to abrasion and impact damage than the internal skeleton of vertebrates.
Running becomes hazardous because all of the weight of a heavy arthropod would come down on the relatively small area of the foot. Without the shock absorption provided by the hooves, paw pads, cartilage, and ligaments found in vertebrate extrem-ities, an external skeleton might be expected to fracture under the force of impact.
A simple fall might be even more damaging. Finally, molting, necessary for growth, causes other problems at large sizes. Just after molting an arthropod is essentially a soft-bodied invertebrate.
The skeleton is still soft, and does not provide good support. Worse, an arthropod cannot rely on muscles to define form the way soft-bodied animals do, because arthropod muscles are designed to exert force against a rigid skeleton, and until the skeleton hardens, many muscles are useless.
Instead, an arthropod gulps air or water in order to hold its form until the skeleton hardens. Each time an arthropod molts it must undergo this risk. Vertebrates do not molt their skeletons as part of growth so they escape these risks completely. Arthropods are the most diverse of all animals, comprising over 85 percent of all living animal species.
Estimates for the number of species in one class of arthropods, the insects, range from 1 to 10 million. The remaining 10 percent are accounted for by other invertebrate phyla, such as molluscs.
Why is there such an overwhelming number of arthropod species compared to all other kinds of animals? Why are there relatively few vertebrate species, despite their sophisticated internal skeletons and access to terrestrial environments?
Small animals can exploit habitats more fully than large ones. A single plant may be a meal to a vertebrate, but to arthropods it can be a universe.
One species might complete larval development in a flower bud, while another species spends its entire life feeding on the woody stems.
A large plant like the saguaro can support an entire community of arthropods throughout its life and after its death. Other habitats, such as the surface of water and the bodies of other animals, are used by arthropods, but are inaccessible even to the smallest vertebrates. Winged insects or ballooning spiders can travel great distances, colonizing new habitats quickly. As they invade new habitats arthropods undergo selection which favors individuals best equipped to survive in the new conditions.
Over time, the better-equipped individuals may come to differ so much from their ancestors that they become distinct species. Arthropod populations can undergo rapid change. Agricultural pests are well-known for swiftly evolving tolerance to previously devastating pesticides.
Short generations, multiple generations per year, and large populations are conducive to the prompt emergence of new forms, and under the right conditions, new species. Vertebrates are also capable of change and speciation, but because of their longer generation intervals these processes tend to require more time. Finally, arthropods have been around for a long time. Trilobites, an early and now extinct group of marine arthropods, lived million years ago.
Early terrestrial forms, like scorpions, are known from million- year-old fossils. Reptiles, the first entirely terrestrial vertebrates, did not arise until the Carboniferous period, approximately million years later.
Insects evolved flight million years before birds, dinosaurs, and mammals. There has been plenty of time for diversification and evolution of many exquisite, bizarre, and intriguing arthropod species. Remarkable parallels and contrasts can be developed when arthropods and vertebrates are compared. But there are reasons to focus on arthropods alone, without considering them in relation to other animals.
In the name of survival, arthropods have evolved forms ranging from familiar to outrageous to beautiful. They evade enemies, feed, and reproduce by methods that are sometimes ruthless, sometimes subtle, and frequently ingenious.
As is their habit, arthropods in the Sonoran Desert have diversified, giving this region a rich inventory of fascinating species. You are invited to form a closer acquaintance with native Sonoran Desert arthropods by reading about them in the following chapters. The chapters are organized on the basis of arthropod phylogeny, which reflects the evolutionary relationships between species.
As you proceed through this section, you can use the phylogenetic listing following this introduction to keep track of arthropod groups. These intriguing and largely harmless creatures are among the most visible and approachable of all Sonoran Desert animals. Keep an eye out on your next desert walk, and you may see an arthropod or two that you have previously encountered in the pages of this book.
Alcock, John. NY: W. Norton, Conniff, Richard. NY: Henry Holt, Friederici, Peter. Werner, Floyd G. Tucson: Fisher Books, In essence, phylogenies are family trees. Since most of the branching in the arthropod family tree took place before humans came into existence, the tree structure is deduced from the study of fossils, and the examination of molecular, developmental, anatomical, and behavioral characteristics of contemporary species.
Many characters important in determining relationships between species are not observable in the field. The informal notes in this phylogeny are intended to help readers develop a feel for arthropod classification by using visual characteristics alone. Animals with an exoskeleton, a segmented body, and jointed legs. The earliest arthropods probably had one pair of appendages per body segment, but there have been many divergences from the ancestral arrangement.
Segments may be fused or grouped into body regions and appendages may be exaggerated, modified, or lost. Members of this subphylum have two major body divisions, the cephalothorax the head and mid section combined and the abdomen. Some animals, like insects and crabs, have a completely different type of skeleton from ours - their skeletons are external on the outside of their bodies.
They are called invertebrates because they do not have a backbone made up of vertebrae. Some other invertebrates, like jellyfish, have no skeleton at all! Can you think of other animals? How do they move? How do you think their skeletons have changed to allow this movement? Find out more about Movement It's a simple fact, most animals move. Look at the bones that make up the human skeleton All vertebrate animals fish, amphibians, reptiles, birds and mammals have internal skeletons.
Look at some more animal skeletons on display in the Museum So, what does your skeleton do? Most organisms have a mechanism to fix themselves in the substrate. Shortening the muscles then draws the posterior portion of the body forward. Although a hydrostatic skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an efficient skeleton for terrestrial animals. Figure 2. Muscles attached to the exoskeleton of the Halloween crab Gecarcinus quadratus allow it to move.
An exoskeleton is an external skeleton that consists of a hard encasement on the surface of an organism. For example, the shells of crabs and insects are exoskeletons Figure 2. This skeleton type provides defence against predators, supports the body, and allows for movement through the contraction of attached muscles. As with vertebrates, muscles must cross a joint inside the exoskeleton. Shortening of the muscle changes the relationship of the two segments of the exoskeleton.
Arthropods such as crabs and lobsters have exoskeletons that consist of 30—50 percent chitin, a polysaccharide derivative of glucose that is a strong but flexible material. Chitin is secreted by the epidermal cells.
The exoskeleton is further strengthened by the addition of calcium carbonate in organisms such as the lobster. Because the exoskeleton is acellular, arthropods must periodically shed their exoskeletons because the exoskeleton does not grow as the organism grows.
Figure 3. The skeletons of humans and horses are examples of endoskeletons.
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