FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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ARTHROPODS OF EDGEWOOD By Paul Heiple This is the second in a series of articles on Edgewood’s arthropods by Paul Heiple. The sketches were added by the editor for clarification and were not provided by the author. —ed. I began this series in the last issue of the Explorer with two spiders that are common in the fall because it would be a year until they were seen again. Now that it is closing in on winter and few arthropods are active, I will explain the features that make an animal an arthropod and the major groups of arthropods that are found in Edgewood. I should start with a bit of information on the classification system for animals. The top ranking in the system is the kingdom of animals, Animalia. This is the group of all organisms considered animals.
To be considered an animal, the organism must be multicellular, obtain its energy from carbon compounds produced by other organisms, and have two sets of chromosomes in the nucleus derived from the fusion of haploid gametes (egg and sperm cells) that differ in size. It is not an easy definition to grasp, but for most of us, we know an animal when we see it. Arthropods are a phylum, the next category below kingdom. The classification continues down to class, order, family, genus and the specific or species. Between the major classifications there can be further divisions made by adding prefixes such as super (superorder) or sub (subphylum). These further divisions are common in the phylum Arthropoda because it is so large. In fact, Arthropoda is by far the largest phylum in the animal kingdom. If ten animal species were picked at random from a list of all animals, the most common result would be to have eight of them be arthropods. The world is dominated by arthropods in all aspects: number of species, number of individuals, and biomass. The only area in which arthropods do not dominate is in the size of individuals; they do not reach the size of most of the chordates, our phylum. The largest living arthropod is the Japanese spider crab with legs up to six feet long, but the body is only a foot or so across.
The one feature that defines an arthropod is the presence of an exoskeleton made of chitin. Chitin is the animal equivalent of cellulose, the material used by plants to make their cells stiff. Instead of glucose bound together as in cellulose, the building blocks are units of acetylglucosamine (think about the joint medicine). The chitin forms plates that are joined together by flexible joints. Arthropods are also segmented, a body plan in which parts are repeated between the head and the tail (which are different from the segments between them) with the basic plan of a pair of appendages, similar nerves, muscles and vascular systems. Each segment can then be modified and fused with other segments to make more useful parts. The fusion of parts can be so complete that the only evidence of the segmentation is found in the internal parts of the animal. The form that is closest to the simple form of a head, tail and basic segments is a centipede. Below the level of phylum are the subphyla, groupings of distinct body plans. One of these everyone has heard of is the crustaceans. The other three are less known, the trilobites which are extinct and known only to those who study fossils, the chelicerates which includes the spiders, and the uniranians which includes the insects. The question of which came first, the chitin exoskeleton or the body plans is an interesting one. All of the subphyla appeared in the Cambrian explosion, the sudden appearance of multicellular fossil organisms about 530 million years ago. This question was open thirty five years ago when I took Invertebrate Zoology and is still not answered. If the body plans came first, then the phylum Arthropoda is a superphylum and the subphyla become phyla. Edgewood contains at least six classes of arthropods, perhaps more. It would take a great deal of study to find out how many more classes were present. I have no idea how many orders, families other taxa below class are present at Edgewood. This task would take many years of work and probably always remain an incomplete inventory. That is the way arthropods are, so numerous as to be beyond counting. Arthropods are also small, sometimes cryptic and often active at only limited times of year.
In Edgewood, an arthropod you might see during the winter season is the Wingless Scorpionfly, Apterobittacus apterus. This odd insect can be found in the grassland hanging on the grass by its long front legs. It captures its pray with the long hind legs. It can be identified by the long face, typical of the order. The other feature that gives the order its common name is the bulbous male genitalia at the end of the tail that resemble the stinger of a scorpion. The species in California do not have this feature. Scorpionflies are the most primitive insect order to undergo complete metamorphosis. The larvae are caterpillar-like and feed on dead insects and other organic debris. Scorpionflies are rare in California, only four species are found in the state. The only species found in our area is the Wingless Scorpionfly. It is however common in the Bay Area. In the last Explorer, I called Argiope trifasciata the Silver Argiope. Since there is another species in the southeastern US that is known by that name, it might be better to have called it the Transverse Argiope for the lines that are transverse on its back. Such are common names; no standard is kept for species besides birds. |
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