Download LECTURE 17 Arthropods I Phylum Arthropoda (arthros – jointed

BIO 120
Field Natural History
Spring
1
LECTURE 17
Arthropods
I
Phylum Arthropoda (arthros – jointed: podes – feet)
A. Contains 4 major lineages
1. The trilobites - now extinct (subphylum Trilobita)
2. Spiders (subphylum Chelicerata, class Arachnida)
3. Crabs, barnacles (subphylum Crustacea)
4. Millipedes, centipedes, and insects (subphylum Uniramia)
a. Much of this lecture will focus on the insects (Class Hexapoda)
B. Advent of Jointed Appendages.
1. All arthropods have jointed appendages.
2. Some are legs, but others have been modified for other uses.
3. Body plan of segmentation with jointed appendages has been very successful.
a. Nearly 2/3’s of named species are arthropods.
b. About 90% of the arthropods are insects.
c. Beetles represent one fifth of all known living organisms and a fourth of all animals.
4. Scientists estimate that there are a billion billion (a quintillion) insects alive at any one
time.
a. That’s 200 million insects for each living human!
5. Fossil records indicate that arthropods existed 630 million years ago-Trilobites.
6. As the arthropods evolved, the trend was for the fusion of segments, so that they became
fewer in number, and the appendages became more specialized.
7. The basic anatomy of insects. Head, thorax, abdomen.
a. The head is the condensation of antennae and an assortment of pairs of mouthparts.
b. Each of these pairs of mouthparts or antennae are modified appendages.
C. The exoskeleton - a beneficial adaptation for arthropods.
1. The outer shell of arthropods is called the exoskeleton.
a. It is a rigid, external skeleton made of chitin.
b. Muscles attach to the interior surface of the hard chitin shell.
2. The exoskeleton is also a line of defense against predators.
3. The exoskeleton is also impermeable to water- hence it prevents dehydration.
D. The limitation of the exoskeleton.
1. Weight of exoskeleton limits arthropod size.
a. Chitin is tough, but brittle and cannot support great weight.
b. Exoskeleton must be much thicker to bear the pull of the muscles in large insects.
c. As exoskeleton gets larger and thicker, it gets much heavier.
2. Another limitation of the exoskeleton is that it does not grow.
a. In order for an arthropod to grow, it must shed its exoskeleton.
b. This process is called molting.
c. The new exoskeleton is very soft. The arthropod uses its circulatory system and air to
puff the soft shell to full size. In this way, there is some ‘room’ to grow.
d. Because the shell is very soft, arthropods are vulnerable to predators until it hardens.
BIO 120
Field Natural History
Spring
2
E. The respiratory system –a limitation to insect size.
1. Insects do not have a circulatory system with blood cells that carry oxygen to all parts of
the body.
2. Instead, the have a system of air tubes that come into contact with the individual cells.
a. The system starts with small holes in the abdomen of the insect. These are called
spiracles.
b. The spiracles lead into small, branched, cuticle lined air ducts called tracheae.
c. The tracheae branch into smaller air ducts called tracheoles.
3. The tracheae form a network throughout the body of the insect.
a. Air passes into the tracheae via the spiracles and diffuses across the cell membrane
into the cell.
4. Because all parts of the body need to be near one of these respiratory passages so that
oxygen can diffuse into the cells, the system is less efficient than circulatory system of
vertebrates.
F. The development of wings- radiation into an aerial environment.
1. Insects are the only invertebrates that can fly.
2. Wings are extensions of the cuticle. (They are not modified appendages.)
3. Flight in insects is achieved mainly by two sets of flight muscles that don’t even attach
to the wings.
4. One set of vertical muscles attach to the dorsal and ventral sides of the thorax. These
vertical muscles force the wings up by pulling down on the top of the thorax (tergum).
5. The wings are forced back down by the relaxation of these vertical muscles.
a. A second set of longitudinal muscles contracts, causing the tergum to arch upward,
thus forcing the wings down.
b. The tergum acts like an oil can that snaps into one position or the other as it is pulled
and released.
c. It is the snapping of the semi-flexible tergum that gives insects more power per
stroke.
G. Metamorphosis.
1. Metamorphosis- Change in form after insects hatch but before they become adults.
2. Incomplete metamorphosis. Immature stages (nymphs), look similar to the adults.
a. Except they are smaller, of different proportions, and lack wings until the final adult
stage. They usually feed on the same food.
3.
Complete metamorphosis. There are two different life stages.
a. One stage is the larval stage (caterpillar), and it looks completely different than the
adult. The main job of the larvae is to eat and grow.
b. The adults have different appearance. Their main job is to mate and reproduce.
c. The main advantage of complete metamorphosis is that the adult and the immature
stage do not compete for the same food resource, so there is not as much competition
within the same species. They may also occupy different environments. Greater
chance for success.
BIO 120
Field Natural History
Spring
3
H. Miscellaneous Bug stuff.
1. Spicebush swallowtail caterpillar.
a. The large eyespots and the mouth line are meant to scare away predator – birds.
b. Mimicry - This pattern is meant to scare birds by appearing to be a snake.
2. Insect mouthparts are modified appendages.
a. Piercing and sucking mouthparts – assassin bug.
b. Horseflies- piercing and sucking mouthparts. The two blades of the mouthparts have
teeth that saw back and forth.
3. Tiger beetles- fast, colorful beetles with powerful jaws and compound eyes to detect
movement.