Phylum Arthropoda

Welcome to the World of Arthropods

Tectocoris diophthalmus

Arthropods are characterized as invertebrates with segmented bodies, jointed appendages, and an exoskeleton.  This phylum contains over 75% of all animal species! 

Their exoskeleton is one of the most prominent features of an arthropod.  It is made out of chitin, and supports the body by protecting it and keeping fluids inside.  Exoskeletons do not grow.  So, when an arthropod gets bigger, its exoskeleton must be molted (shed).   Then, it is able to grow until it produces a new one.  This whole process is called ecdysis.

Respiration in arthropods can occur in a few different ways.  They can exchange gas through gills, book lungs, or tracheae.  In tracheal respiration, gases are exchanged through small openings called spiracles.  Then, gases enter tubules that disperse to the rest of the body.

Arthropods have an open circulatory system.  This means that blood (hemolymph) moves through the body without being contained in blood vessels.  They still have hearts that, with the help of alary muscles, contract to continue blood flow.

Incomplete vs. Complete Metamorphosis

All arthropods go through some kind of change as they reach adulthood.  This change can be labeled as either incomplete or complete metamorphosis.  So, what's the difference?  In incomplete metamorphosis, there are three stages in an arthropods life cycle: egg, nymph, and adult.  A nymph looks like a miniature version of the adult.  In complete metamorphosis, there are four stages: egg, larva, pupa, and adult.  There is a drastic change from larva to adult.  The larva is very active through this cycle, eating everything it can, whereas, the pupa is usually sedentary.  The grasshopper is an example of incomplete metamorphosis.  A butterfly is a popular example of complete metamorphosis.

Types of Arthropods



Actias luna

Insects are one of the main groups of arthropods.  They are characterized by their three pairs of legs.  They have a body plan of a head, thorax, and abdomen.  Insects can be winged or not, and they usually exhibit eyes, one pair of antenna, and mandibles as jaws.  Examples of insects include ants, ladybugs, flies, and crickets.

Maratus personatus

A third group of arthropods include the chelicerates.  Their bodies can be divided into two parts--the prosoma and the opisthosoma.  These parts are like the cephalothorax and abdomen of crustaceans.  Chelicerates are predominately characterized by one pair of their appendages called chelicerae.  These are specialized mouthparts that can be claw or fang like.  In spiders, most chelicerae are poisonous.  Chelicerates do not have antenna, and are mostly terrestrial.  Examples of chelicerates include spiders, scorpions, and horseshoe crabs.

Procambarus alleni

Crustaceans are another group of arthropods.  They have a cephalothorax and an abdomen.  They have eyes, mandibles, and two pairs of antenna.  Most are aquatic animals, though a few are terrestrial.  Examples of crustaceans include lobsters, crabs, and crayfish.

Why are Arthropods So Successful?

Olenoides serratus

Arthropods are the most heavily represented animal group on the planet.  They make up over 3/4 of all species in Kingdom Animalia.  Why?  Why were these arthropods able to survive millions of years ago when the dinosaurs couldn't?  One reason is there size.  Most arthropods are not very big.  When natural disasters happen, arthropods have a better chance at surviving than other animal groups.  They can crawl into small places, and they don't, individually, need a ton of food to survive.  Another reason is that many arthropods can have thousands of offspring in one life span.  So, even if only a few arthropods survived, they would be able to keep there species going without much trouble.  Also, because they reproduce many offspring, arthropods are able to adapt faster to their environments.  They typically don't have long life spans, so when natural selection comes into play, it doesn't take too long for it to go through many generations.  Finally, even though other animal groups may have some of these advantages as well (mollusks and worms), they are usually not as advanced as arthropods.  In addition to the reasons above, arthropods are highly developed creatures.  They have body protection (exoskeletons), well developed nervous systems, and jointed appendages.  This all contributes to why arthropods are so successful!