Download Document

Amazing Arthropods Part One
Arthro = joint, Pod = foot: Jointed feet!
Diversity and Evolution
There are 4 major subphyla. A subphylum is smaller than a phylum
but bigger than a class.
1. Trilobita: The oldest subphylum. These creatures lived in ancient
seas.
 Now extinct!
 The trilobites
2. Chelicerata: Animals such as spiders, ticks, mites, scorpions, sea spiders,
and horseshoe crabs.
3. Crustacea: Animals such as crabs, shrimp, lobsters, crayfish, and
barnacles.
 Mostly aquatic
4. Uniramia: Animals such as insects, millipedes, and centipedes.
 Insects are by far the biggest class of arthropods!!!
So why are there so many arthropods?





They have been evolving for a long time.
First appeared in the sea over 600mya.
Some of the first animals to colonize land.
There have been several adaptive radiations!
Bodies can be modified like car parts.
The body of the earliest arthropods is considered to be similar to that of the
trilobites.
 Many segments which are all similar to each
other.
 Very little specialization, (customization).
These earliest arthropods had:
1. A thick exoskeleton
2. Many segments, each with a pair of legs.
3. The legs were branched.
a. One branch was a leg
b. One branch was a gill.
The evolutionary trends were:
1. A reduction in segments.
2. Increasingly specialized appendages.
Form and Function
The three most important features of an
arthropod are:
1. A tough exoskeleton.
2. A series of jointed appendages.
3. A segmented body.
Important internal features are:
1. A dorsally located brain.
2. A ventral nerve cord.
3. An open circulatory system with one heart.
So what is the exoskeleton?
Exoskeleton: A system of external supporting structures that are made
primarily of chitin, (a carbohydrate).
 Many exoskeletons are hard (crustaceans).
 Many others can be leathery and flexible (insects).
About the exoskeleton:
 They are very good for protection from predators.
 They are good at preventing terrestrial arthropods form
dehydrating.
o For this reason insects, spiders, and scorpions have
been very successful at colonizing dry areas.
 Exoskeletons cannot grow with the body.
o Therefore insects have to molt. (We will learn about
this later!)
 Movement occurs at joints between the exoskeleton.
o This is like medieval armor!
What about those appendages?
Primitive arthropods had one pair of appendages per
segment.
 Ex. Millipedes and centipedes which appear
worm-like.
More recent arthropods have fused their segments
together to reduce the number, which means there are
fewer appendages, and larger body segments.
 Ex. Insects, spiders, and crustaceans.
Over millions of years, the appendages have specialized to take
on many functions. Here are a few examples:
 Antennae: Smell
 Pincers: Defense and predation.
 Mouthparts: Feeding
 Wings: Flying/gliding
 Paddles/flippers: Swimming for aquatic insects.
 Stingers: Defense and predation.
 Ovipositors: Laying eggs.
 Legs: Walking locomotion.
 Furculums: For jumping great distances.