Download Bio 11 Zoology Lesson 8 Arthropods

Biology 11
Arthropods
Arthropods are a group of animals that share three common traits:
1. Hard exoskeleton made of chitin that is used for protection, muscle
attachment, locomotion and prevention of desiccation. Exoskeleton must be
shed as the organism grows larger. A new soft, wrinkled exoskeleton is first
excreted by the body under the old one, then the old one is shed and the
new one expands and hardens. This process is called molting.
2. Body segments – may be fused together to form body regions.
3. Jointed appendages (arthropod = jointed foot).
Arthropods have a well developed nervous system consisting of a brain and a
ventral nerve cord. Sight is achieved by a combination of compound eyes and ocelli
(light sensitive pigment spots).
Arthropods have an open-circulatory system. The heart pumps blood to the
hemocoel – a cavity in which the internal organs are bathed in the insect’s blood.
All terrestrial arthropods have internal fertilization; sperm is transferred
indirectly to the female. Once the eggs are fertilized, the female then lays the
eggs in a safe place until they hatch. Aquatic arthropods can use either internal or
external fertilization.
More than one million species have been described, but there could be millions
more that have not yet been discovered. There are five subphyla within the
Phylum Arthropoda.
Trilobites
Extinct group of organisms that disappeared from the oceans at the end of the
Permian period (250 million years ago).
Hexapods (Insects)
Insects make up the largest and most diverse group of Arthropods; there are
probably over 1 millions species of insects. The study of insects is called
entomology. Some species are solitary, but others have adapted to complex social
systems and work as a colony (bees and ants).
Insects have three body segments – the head, thorax (middle region) and the
abdomen. Often you can see segments on the abdomen of insects. Locate all the
insects on the colouring sheet and colour the head red, the thorax yellow and the
abdomen green.
Insects have three sets of legs which attach to the thorax. Colour all the insect
legs blue. You will also note that each insect has a pair of antenna; colour or shade
over the antenna in grey.
The mouthparts of insects have a pair of mandibles that chew food. Unlike the
teeth of other animals, mandibles open from side to side. Locate the mandibles
(they are only visible on two of the insects pictured) and colour the mandibles
purple. The mouth parts have been adapted in some cases depending on the insects
way of life; for example, butterflies have a long tube that is used to siphon nectar
from flowers.
Some, but not all insects have wings, which also attach to the thorax. Colour the
wings pink. An additional body part can be seen on the grasshopper, and covering
near the wings that is hard and protective called the carapace - colour the
carapace brown.
Chelicerates (Arachnids)
Arachnids are a group of arthropods that include spiders, scorpions, ticks and
mites. They have two body parts: the cephalothorax (head-middle) and the
abdomen. Colour the cephalothorax orange and the abdomen green. Arachnids
have six pairs of appendages. The first pair are the chelicerae – pointed
appendages used to grasp food and, in the case of spiders, to inject venom into
their prey. Colour the chelicerae purple. The second pair are the pedipalps; the
development of the pedipalps can vary from tactile feelers to the “claws” of a
scorpion. The remaining 4 pairs form 8 legs, instead of the 6 you find in insects.
Colour the legs blue.
Crustaceans
Crustaceans are a group of arthropods that mainly live in the water; they include
barnacles, lobsters, crabs, shrimp and crayfish. There a few terrestrial
crustaceans as well – sowbug and woodlice for example.
Crustaceans usually have two body segments, the cephalothorax (orange) and the
abdomen (green). The head region of crustaceans usually bears a pair of compound
eyes and five appendages – the first 2 pairs are antennae and the next 3 pairs
form the mouthparts used in feeding.
The number of appendages on crustaceans can vary, and many of them have large
claws, used for capturing prey. Colour the claws on the lobster brown and the large
legs attached to the thorax blue. Notice the little legs attached to the abdomen
on the lobster which the animal uses for swimming. These appendages are called
swimmerets - colour them dark blue. Crustaceans also have antennae - colour grey.
Myriapods (Centipede & Millipede)
Centipedes have long flat bodies and many legs. In fact, the word "centipede"
means "hundred legs". In reality though, the number of legs can range from less
than 20 to more than 300 – but it is always an uneven number of paired legs.
Colour the legs of the centipede blue and each of its body segments red and the
antennae grey. Note that centipedes only have one leg per body segment. The
first pair of appendages of all centipedes has been modified to form a pair of
venomous claws or forcipules. This means that all centipedes are predators.
Millipedes are round-bodied organisms that feed on detritus (dead plant matter) –
they are non-poisonous. The first segment has no legs, then next few segments
have one pair of legs each, and the remaining segments have two pairs of legs per
segment. Although the name means “thousand legs”, most of the common species
have between 36 to 400 legs, although an individual of one species in California has
been recorded as having 750 legs (on a body that measure slightly more than 2.5
cm).
Arthropod Questions:
1. What three characteristics do all arthropods have in common?
2. What type of symmetry do arthropods have?
3. What level of organization do arthropods have?
4. How many body segments does an insect have? ____ How many legs? ____
5. How many body segments does an arachnid have? ____ How many legs? ____
6. Which groups of arthropods have antennae?
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7. What are the chewing mouthparts of arthropods called?
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8. To what part of the body do the wings attach? ________________________
9. The head and thorax are fused together in some arthropods to form the
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10. What does the word "centipede" mean?
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11. What is important about the fact that all centipedes have venomous claws or
forcipules?
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12. How does millipede feeding differ from centipedes?
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