Spiders have an abdomen and cephalothorax, eight legs with seven segments each, and pedipalps on their heads. They digest liquidized food that they get by pumping enzymes into prey or grinding food with their pedipalps. Males transfer sperm to females through a courting process. Spiders breathe through book lungs or tracheal systems and have various sensors like eyes and hair to navigate their environment. They are important for controlling insect populations.

1. Arachnida
Camryn Baun
scientific article • period 5 • November 5, 2009
picture by flickr user: Thomas Shahan
Camryn Baun • Arachnida • Period 5
1

2. Basic Body Functions
External 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

3. 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 Background
Classification
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

4. Other organisms found in this group include scorpions, mites, and ticks.
by Flickr user: aussiegail
by Flickr user: Alastair Rae
by Flickr user: mnsc
Camryn Baun • Arachnida • Period 5
4

5. 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

6. Resources
en.wikipedia.org/wiki/spiders
sciencebulletins.amnh.org./biobulletin/biobulletin/story991
animals.howstuffworks.com arachnids/spider.htm
Camryn Baun • Arachnida • Period 5
6