Spiders Essay

1896 words - 8 pages

Spiders

Spiders can be found in all environments throughout the entire world, except in the air and sea. (Biology of Spiders, R.Foelix) These invertebrates of the order Aranea are one of the several groups of the Class Arachnida, with about thirty four thousand species.

They range in body size from only a few millimeters in length to almost five inches.

All are carniverous and have four pair of walking legs, one pair of pedipalps, and one pair of chelicerae. (Spiders, W.Shear) Each chelicerae consists of a base and a fang.
The fang folds up inside of a groove in the base until needed when attacking food, then moves out to bite and releases venom from a tiny opening at its end as it penetrates
the prey. (Biology Of Spiders, R.Foelix) They are also used to “chew”, getting digestive juices inside the body of the prey then squeezing out the liquid lunch. The pedipalps are mainly used to catch and rotate the prey while the chelicerae inject it with poison to tear down the tissue.

Later the bases of the pedipalps are used as chewing parts. (The Spider Book,J.Comstock) But in males, these palps are used to transfer sperm into the female. These twleve appendages are attached to a dorsal and a ventral plate, the carapace and sternum which cover the entire prosoma and provide attachment points.

The bodies of spiders consist of two parts, an anterior part called the prosoma and a posterior portion called the opisthsoma. These two portions are held together by a narrow stalk called the pedicel.

This narrow junction allows for the spider to be very limber and acts somewhat as a hinge between the prosoma and opisthosoma. So as a spider moves foward creating a web, it can continue in a straight line throwing its webbing in the direction it chooses.
This is how spiders create their zig-zag web formations. (Biology of Spiders, R.Foelix)
Covering both the prosoma and the opisthosoma is a waxy covering that enables the spider to be a very efficient water conserver. This is one of the characteristics that spiders evolved to adapt to the harsh conditions of terrestrial life. There are eight eyes located in the head region usually in two rows, varying among families. Spiders that
wait for and lunge at its prey will have a row of very large eyes well adapted at detecting the precise distance it is from its prey. Yet those spiders that make webs do not have as great a need for such advanced sight and have smaller eyes. But not all spiders have eight eyes. There are some spitting spiders that have only six, and there are some with only two or four eyes. Some cave spiders have no eyes at all and rely only on vibration. There are great differences in the ways which spiders capture prey. Some may stalk their prey, while others may lie in wait and ambush it. Other spiders may weave various types of webs used to capture passing prey, and there are some smaller commensal spiders that live in larger spiders’ webs and feed on the smaller
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