ArachnidaCamryn Baunscientific article • period 5 • November 5, 2009

picture by flickr user: Thomas Shahan

Camryn Baun • Arachnida • Period 5 1

Basic Body FunctionsExternal Transportation

Spiders walk on legs like most organisms. They have eight legs, each with seven segments. To

bend their legs, fluid gets pumped into them. The pressure allows the legs to bend at the seg-

ments. The fluid and pressure must be released to bend back.

General Structure

The general structure for a spiders body is in two segments with bilateral symmetry, the same on

both sides. The body is called the abdomen and the head is named the cephalothorax. The tiny

mouth parts on the front of the head are called pedipalps.

How They Get Nutrition

Spider stomachs can only cope with liquid food. There are two different ways that they can liquify

their food. The firs is to pump digestive enzymes into their prey and suck out the liquified in-

sides. This leaves behind an empty body shell. Another way is to grind the whole body with the

pedipalps while flooding it with the same digestive enzymes. Spiders also feed on things like nec-

tar, pollen, bananas, marmalade, milk, egg yolk, and sausage.

Reproduction

The reproduction of spiders is internal, but indirect. The males spin special webs to ejaculate on.

Then, they transfer it to their pedipalps when a female of the same species is near. The pair then

does a series of rituals or dances called courting. In the end, the male injects the sperm from his

mouth to the female’s mouth parts.

Respiration

Some spiders get oxygen in different ways. The different types of respiratory systems are book

lungs, that absorb oxygen from open air, a tracheal system, which is like humans, or both. Some

spiders have an opening on their abdomen for two sets of book lungs.

Camryn Baun • Arachnida • Period 5 2

Response to Stimuli

Most of the spider’s sensors are external. They have four pairs of eyes. These eyes put images

together like human eyes, but they can only see light and dark, basic shapes. The way they feel

things is also human-like. The hair that covers their bodies responds to force, just like human

skin. Spiders can also feel the vibrations around them in their joints. To taste and smell, spiders

have chemical sensors in their mouths.

Group BackgroundClassification

To be put in this group, organisms must have these distinct features:

• eight legs made of seven segments

•feed on insects

•inject venom into prey

•can produce silk

•body in two sections (cephalothorax and abdomen)

•appendages on head called pedipalps

Endangered or Extinct

Some extinct spiders are Spruce Fir Moss Spider, Dolloff Cave Spider, and Fen Raft Spider. The

Kauai Cave Spider is very close to extinction. It can only be found in a small population in Hawaii

in the Kauai Cave.

Other Organisms

Camryn Baun • Arachnida • Period 5 3

Other organisms found in this group include scorpions, mites, and ticks.

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by Flickr user: mnsc

by Flickr user: Alastair Rae

by Flickr user: aussiegail

Why Are They Important to Us?Yes, a lot of people are afraid of spiders, but they have a very important role in life. Spiders are a

very dominant species found all over the world and they eat a lot. This controls the huge insect

population. If we did not have spiders to eat the insects, they would eat our crops and hardly any

vegetation would survive.

Camryn Baun • Arachnida • Period 5 5

Resourcesen.wikipedia.org/wiki/spiders

sciencebulletins.amnh.org./biobulletin/biobulletin/story991

animals.howstuffworks.com arachnids/spider.htm

Camryn Baun • Arachnida • Period 5 6