Arthropods
A) List the classes within the phylum?
The classes within the Phylum Arthropoda include Crustacea,
Arachnida, Chilopoda, Diplopoda and Insecta.
B) Describe each class in words and pictures?
Crustacea
Also known as Crustaceans this class is very diverse. Its members
display much variation in tagmata and appendages. There is up to
30,000 species of crustaceans. Most are aquatic meaning they live in
water, the majority of these are marine which means not just any
water the ocean. This class includes lobsters, crabs, crayfish, shrimp,
barnacles and several other less familiar forms including the Isopoda,
Sowbugs and Pillbugs or Roly-polys. The two most common orders of
Crustaceans are Decapoda and Isopoda.
Arachnida
This class belongs to a subphylum of the Arthropods known as
Chelicerata. This means the organisms only have two distinct body
regions, a cephlothorax and an abdomen. They six pairs of
appendages the first two pairs belong to the mouth part and the
other four pairs are the legs and they have no antennae. The first pair
of mouth parts are the chelicerae they are three-segmented and
pincer-like. For spiders the third segment is the fang. The second pair
of mouth parts are the six segmented pedipalps. They may appear leg
like for spiders or claw like for scorpions. The four pair of legs are
identical all having seven segments. Common orders of Arachnids
include Araneae (Spiders), Acari (Mites and Ticks) and Scorpiones
(Scorpions).
Chilopoda
The Centipedes, they long elongated and flattened. They are usually
found hidden and one species is even commonly found in houses and
buildings. Centipedes are predatory and feed on insects and small
animals. Larger centipedes can bite humans but it will usually hurt no
then the sting from a bee or wasp. The centipede is made up of a pair
of mandibles and two pairs of maxillae. Appendages on the first trunk
segment are two curved hollow fangs which contains venom that
quickly paralyzes its prey.
Diplopoda
Millipedes, eyes are usually present, they have one pair of antennae
usually containing seven segments, two pairs of legs on each segment
(30 or more pairs in total), one pair of mandibles and one pair of
maxillae. Millipedes are usually cylindrical which means slightly
flattened. Except for the first three segments, each segment has two
pairs of short legs. Millipedes are found in damp places like soil leaf
litter and so on, most millipedes are beneficial scavengers of
decaying plant material. Millipedes do not bite but they give off a foul
smelling odour that is strong enough to kill an insect confined with a
millipede.
Insecta
The insects may have wings, either one or two pairs, they have three
pairs of legs and one pair of antennae. Insects are the most abundant
life for now known to science there are around 1,000,000 species
that have been described and named. That is more than all the other
known animals put together.
C) Identify the characteristics used by scientists to
separate the classes?
Scientists would use characteristics such as the number of antennae
or whether the number of legs vary depending on the organism. The
will look at how many distinct body regions they have where the legs
are located how many legs. If it has wings and if it has trunk sections
on its body how many pairs of legs per segment.
D) Create a dichotomous key for the Phylum?
1a. Two pairs of antennae (one pair may be reduced, difficult to see)
Number of legs variable. Class Crustacea, go to 2.
1b. One pair of antennae or none. Go to 3.
2a. Two distinct body regions (cephlothorax and abdomen)
Five pairs of thoracic legs crayfish, lobsters, shrimp; Order
Decapoda.
2b. Three distinct body regions (head, thorax, abdomen)
Seven pairs of thoracic legs sowbugs, pillbugs, roly-polys) Order
Isopoda.
3a. No antennae; Two distinct body regions (cephlothorax and
abdomen) Four pairs of legs spiders, ticks, scorpions, etc. Class
Arachnida.
3b. One pair of antennae go to 4.
4a. Three distinct body regions (head, thorax, abdomen)
Three pairs of thoracic legs. Wings present or absent Class Insecta.
4b. Two distinct body regions (head and trunk) go to 5.
5a. One pair of legs per trunk segment centipedes Class Chilopoda.
5b. Two pairs of legs per trunk segment millipedes Class Diplopoda.