Arthropods





Arthropods 2951
Photo by: Cathy Keifer

Arthropods are invertebrate (without a backbone) animals of the phylum Arthropoda that have a segmented body, jointed legs, and a tough outer covering or exoskeleton. They include insects, crustaceans (lobsters, crabs, shrimp, crayfish), millipedes, centipedes, horseshoe crabs, arachnids (spiders, ticks, and mites) and sea spiders. Together, arthropods comprise the largest and most varied group of invertebrates on Earth.

Characteristics

The bodies of arthropods are divided into different segments, each having a specialized role. The segments have numerous paired, jointed appendages (legs, antennae, claws, and external mouth parts) that serve many varied functions. The exoskeleton acts as a protective covering to the underlying segmented body. It also provides an attachment for muscles and a barrier to water loss for animals living on land. It is made mostly of chitin (pronounced KIE-tuhn), a rigid, complex carbohydrate, and is usually covered by a hardened, waxy cuticle. The cuticle acts as a hinge between segments, allowing the body to bend and move to the right or left. Periodically, the rigid exoskeleton is shed in a process called molting. The temporarily soft animal then swells in size, and its new, larger exoskeleton hardens.

Arthropods are divided into chelicerates (pronounced kih-LIH-suhruhts), meaning "claw-horned ones," and mandibulates, meaning "jawed ones." The bodies of chelicerates are divided into two parts: a fused head and thorax, and an abdomen. They have no antennae, and most have four pairs of jointed legs. They are named for their first pair of appendages, which are modified as clawlike fangs used for feeding. The chelicerates include the arachnids, the marine horseshoe crabs, and the sea spiders.

The mandibulates have one or two pairs of appendages that function as antennae on their head, with the next pair modified as jaws for feeding. Included in this group are the crustaceans (crabs, lobsters, crayfish), the millipedes and centipedes, and the insects. The body of insects

A male black-winged damselfly. (Reproduced by permission of Field Mark Publications.)
A male black-winged damselfly. (Reproduced by permission of
Field Mark Publications
.)

is divided into three regions: a head, a thorax, and a clearly segmented abdomen. The thorax usually has three pairs of legs and two pairs of wings attached to it. Centipedes and millipedes have a head and a narrow, segmented trunk, the former having one pair of legs per segment and the latter having two. Crustaceans have many different body shapes. In most, the head and thorax are fused and separate from the abdomen. Their segmented bodies are often hidden by their hard outer shell.

Arthropods usually have more than one pair of eyes, which may include both simple and compound pairs. (A compound eye is made up of many separate units for receiving light, each with its own lens.) Breathing in land-arthropods is usually accomplished through air tubes called tracheae. Oxygen enters the air tubes from the outside through small openings in the body and is distributed to all the tissues. Arachnids, such as spiders, also breathe through book lungs, thin flaps of tissue arranged like the pages of a book. Arthropods that live in water generally breathe through gills.

Life cycle

Arthropods begin as eggs and can follow several different life cycles, depending on the group. Some insects hatch as miniature adults, while others hatch as nymphs and develop by stages into adults. Still others hatch as larvae and enter a resting stage as pupae, during which they may be enclosed in a cocoon and go through internal changes before emerging as adults. During their various developmental stages, known as metamorphosis, arthropods may shed their outer covering several times (molt).

Ecological importance

Arthropods are of ecological importance because of their sheer numbers and extreme diversity. More than 874,000 living species of arthropods have been identified, making up more than 80 percent of all named species of animals. However, it is estimated that many more thousands of arthropods exist that have not yet been named. Most of these unnamed species are small beetles and other insects, and most of these occur in old-growth tropical rain forests—areas that have not yet been well explored.

Arthropods occupy an enormous variety of Earth's habitats. Most species of crustaceans live in water (that is, are aquatic), although a few such as wood lice and land crabs occur in moist habitats on land. The spiders, mites, scorpions, and other arachnids are almost entirely land animals, as are the extremely diverse insects.

Arthropods are both harmful and helpful to humans. A few species are transmitters of bacteria or viruses that cause diseases such as malaria, yellow fever, encephalitis, and Lyme disease. Scorpions, some spiders, and bees and wasps have poison glands and can hurt or even (though rarely) kill people by injecting poison through stingers. Some arthropods are a nutritious source of food in many parts of the world, and insects play an important role in pollination (a process necessary for production in many plants).

[ See also Arachnids ; Crustaceans ; Insects ]



User Contributions:

Thomas Ashby
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May 30, 2007 @ 10:10 am
This article helped me out alot. Thank you!!!

Thomas Ashby
stacey
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Feb 11, 2008 @ 4:16 pm
thanks a lot! wouldnt have been able 2 do my lab without this information!
ana
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May 28, 2008 @ 6:18 pm
this article really helped me Thank-You!
Thank-You!
Thank-You!
JIHA
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Jun 24, 2008 @ 4:04 am
THIS ARTICLE ARE VERY GOOD 4 AL MEDICAL STUDENT...
THANKS A LOT....
AStudent
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Oct 6, 2008 @ 5:05 am
Thank you for this article, it helped me a lot with my 7th grade level science homework!
Thankyou from a student.
taylor
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Dec 10, 2008 @ 12:12 pm
Thanks for the information because this is helping me out on my project for biology thanks alot=)
tabitha
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Feb 12, 2009 @ 8:08 am
IT WAS A VERY HELPFUL ARTICLE ESPC FOR ME SINCE AM GOING TO BE DOING MY PAPER TOMORROW. THANK YOU FOR THE GOOD WORK AND I HOPE YOU WILL PUT MORE DETAILS IN FUTURE ARTICLES.
nadia
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Apr 26, 2009 @ 8:08 am
FANTASTIC! your website is very helpful. I've been looking for the information about arthropod characteristics for my school project. And finally i've got the good one. THANKS!
TuffDuff3
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May 11, 2009 @ 9:09 am
Thank you from the bottom of my heart. I love you all!
stephanie
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May 17, 2009 @ 9:21 pm
thank you! i would have failed my report project without this!
taylor wilson
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May 27, 2009 @ 6:18 pm
this website is the best. it helped me a lot wit my animal book!
THANKS!
niasha
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Aug 19, 2009 @ 11:11 am
a write up that does not try to but gives an understandable rich summary of arthropods it helped me a lot
victo
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Aug 19, 2009 @ 5:17 pm
this page is realy fantastic it is a realy good it help a lot for studing.
whith this information i will have a 7
Samaria
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Jan 3, 2010 @ 2:02 am
This has way more information than I
expected. I'm so ready for my marine bio
test now, thanks :)
Megan
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Jan 10, 2010 @ 9:09 am
This article helped me a lot with my RP (Research Practicum) paper! It was very informative and super useful, thank you very much!
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Mar 23, 2010 @ 10:22 pm
this helped immensely with the powerpoint i was completing about anthropods! thank you!!!
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May 23, 2010 @ 3:03 am
Thanks this was very helpful : for science assesment
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Oct 18, 2010 @ 10:10 am
Thank you for the article! It helped a lot with my Arthropoda project. Keep on writing!

-Rin
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Oct 29, 2010 @ 10:10 am
I'm so much benefited with staffs from this page especially i was looking ecological importance of Arthropods. Thanks to the article writer.
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Dec 5, 2010 @ 2:14 pm
THANK YOU VERY MUCH!
This helped me A LOT with my Arthropoda presentation.
Keep up the good work!
Thanks once again,
-Mo
Astriah
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Sep 17, 2011 @ 8:20 pm
Hey! This article was the most helpful on finding info about beneficial arthropods. I am in the 6th standards, and I really need more info. THANKS FOR THE INFO =) If you can update this chapter, can you write more about how arthropods help the environment too? I REALLY NEED IT!!!
christy
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Oct 25, 2011 @ 10:10 am
It was a very relevant article to read and learn more about arthropods. and the different forms in which they exist.
Mike Kercher
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Mar 5, 2012 @ 5:17 pm
I was working on a 150 yr old home the other day and a fully 9 inch long centipede skeleton fell out of from between the floor joist in the above ceiling. it has 30 pairs of legs now and looks as though it may be missing the very last segment. This happened in Jackson, Mich, and it just seems to me that this has to be extremely odd for a centipede of this size to have lived in michigan? Am I right, or is this more normal than I suppose?

I would include of photo if it was possible, but I don't see that's possible here.

thanks,
omobolanle
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May 7, 2012 @ 11:23 pm
wow this is it, this article has really helped for my assignment.
Looking forward to more artiles.
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Jun 29, 2012 @ 5:05 am
Am really really impressed for diz woundaful article,it help mi alot for my project and during my examz
doris
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Nov 19, 2012 @ 2:14 pm
THIS ARTICLE HELP ME IN SOMEHOW BUT I NEED MORE INFORMATION ON ECOLOGICAI IMPORTANCE OF ARTHROPOD.
6th grader
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Nov 12, 2013 @ 8:20 pm
it helped me allot with me homework thanks
THANK YOU FROM A YOUNG STUDENT
Matthew
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Nov 14, 2013 @ 11:11 am
thank you for helping me I have a science test to do and it really helped.

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