Download 14. phylum arthropoda

PHYLUM ARTHROPODA
This phylum contains four-fifths of all existing, described animal species. The number
of individuals is enormous. Many feel that arthropods are the most successful animals.
They are found everywhere. Arthropods have jointed appendages and a chitinous
exoskeleton. The earliest known arthropods had one pair of appendages per body
segment. Arthropods have an open circulatory system and an excretory system made-up
of Malphigian tubules.
Subphylum: Trilobita
This now extinct taxon is known from an extensive fossil record from the Paleozoic era.
Trilobites had a single pair of antennae and a body divided into three lobes by two
longitudinal furrows (hence the name “trilobite”). Anteriorly the head region is termed
the cephalon followed by the thorax and pygidium.
Figure
Subphylum: Chelicerata.
Members of this group lack antennae and the first post-oral appendages are modified
feeding structures or chelicerae. Posterior to the chelicerae are a pair of pedipalps. The
body is divided into a cephalothorax equipped with four pair of appendages, and an
abdomen without appendages.
Class: Merostomata.
These are the horseshoe crabs, strange, trilobite-like arthropods with a fossil record
dating back 500,000,000 years (Cambrian). Today there are only 3 genera and 5 species.
All in possession of a cephalothorax with compound eyes located laterally, an abdomen
with gills (book gills), and a long, sharp telson.
Figure
Class: Arachnida.
Include in this taxon are the spiders, ticks, scorpions and mites. The body is divided
into a cephalothorax with four pairs of walking legs and an abdomen. Find their
chelicerae. Arachnids are mostly terrestrial (some secondarily aquatic species exist)
an respire by means of book lungs. Most are predatory and have evolved prey-capturing
mechanisms such as webs, claws, stingers, fangs and, in some cases poison glands. They
feed via sucking mouthparts on the fluids of their prey.
Figure 1. Draw an arachnid 10 cm in size and label their anatomy.
Subphylum: Mandibulata.
Mandibulata possess a pair of mandibles (jaws), one or two pairs of antennae, and one
pair or two pairs of maxillae. Taxonomy varies, however, we will adopt the following
classification.
Class: Crustacea.
Included here are lobsters, crayfishes, shrimps and crabs. Most are marine with some
freshwater forms and a few terrestrial species. Crustaceans have two antennae and a
number of paired appendages on their heads. The head and thorax are combined into a
cephalothorax covered by a dorsal carapace.
Class: Branchiopoda.
Members include marine and freshwater fairy shrimp, tadpole shrimp, clam shrimp and
water fleas. They all carry leaf like, setose appendages. The water flea Daphnia is a
common freshwater crustacean that is an example of a microscopic crustacean suitable
for laboratory study.
Enlarged second antennae aid movements through water. Five pairs of thoracic
appendages, equipped with setae, remove food items from the water. These are collected
in a median ventral groove and carried anteriorly to the mouth.
Obtain a Daphnia from the culture and place a water flea in a depression slide, cover and
observe the animal first through a dissection scope and then a light microscope.
The body is laterally compressed without obvious segmentation. Notice the head,
thorax and post abdomen (the abdomen is reduced). The thoracic and abdominal
regions are encased in a chitinous, bivalved, carapace. The head is covered by an
exoskeleton. Locate the two large compound eyes, a pair of small ocelli posterior to the
the first and second antennae, and thoracic appendages used in feeding. A heart can be
seen above the anterior part of the darkened digestive system.
Figure 2. Draw a 10 cm, Daphnia and label the above-mentioned structures.
Subclass: Copepoda.
Copepods are minute aquatic crustaceans with elongated bodies and a forked tail. They
have a median eye and elongated second antennae extending perpendicular to the body.
The antennae are used in locomotion and in feeding. Rowing motion of the antennae
cause the copepod to swim in a “jerky” fashion.
Figure 3. Draw a copepod 10 cm in size and label their main structures.
Subclass: Cirripedia
This entirely marine group, known as barnacles, are sessile as adults, spending their
lives residing within calcareous plates attached to rocks, pilings and other objects. They
feed by means of feathery legs thrust out of their calcareous plates that scope-up
planktonic life forms. Barnacles and copepods share similar larval stages.
Subclass: Malacostraca.
Shrimp, crab, lobsters, pill bugs and beach hoppers are included in this taxon, which
contains three-quarters of all known species of crustaceans. Malacostracans have eight
thoracic segments and six abdominal segments followed by a telson.
Order: Decapoda.
Members of this group have five pairs of walking legs and a trunk covered by a
carapace. The anterior most pair of walking legs may be modified into claws and not
necessarily utilized for walking. The freshwater crayfish, Cambarus sp., will be the
focus of your study.
External anatomy. The body of the crayfish is divided into a rigid anterior
cephalothorax and a flexible posterior abdomen. Notice that the legs are paired and
jointed. The thoracic covering is called the carapace that is pointed anteriorly to form
the rostrum. Locate the transverse cephalic groove between the head and the thoracic
portion of the cephalothorax. Examine the exoskeleton covering the abdomen and notice
that it is divided into plates called sclerites The dorsal plate is the tergum while the
ventral plate is called the sternum or sternites. In between is the largely inconspicuous
pleuron.
Next observe the appendages. The two pairs of antennae are separated into a pair of
biramous medial pair and a much longer lateral pair. Notice the five pairs of walking
legs. The first pair is modified into chelipeds (pincers) and is not used in locomotion.
Abdominal appendages are called swimmerets or pleopods. The terminal pair of
appendages are the uropods and the single, posteriomedial element is the telson.
Determining sex in crayfish is relatively easy. Males have the first pair of swimmerets
enlarged to form rigid copulatory organs. Pleopods are uniform in females. The sperm
duct openings in the male are located at the base of the fifth walking legs, while in the
female the oviduct openings are found at the base of the third walking legs. Another
guide to sex (often unreliable) is the expanded tergum in females for holding the eggs and
young.
Figure
Internal anatomy. Carefully remove the exoskeleton from the dorsal side of the body.
Cut along the side of the cephalothorax about half way between the dorsal and the ventral
sides of the body. Now cut from the posterior edge of the cephalothorax anteriorly to
about ½ cm from the anterior edge. Then cut from left to right to expose the organs.
Underlying tissues may cling to the exoskeleton so care must be used to prevent damage
to various organs. The delicate, diamond-shaped heart is located just under the
cephalothorax. Look closely and you may see the ostia (openings in the heart) and the
anterior and posterior blood vessels exiting this organ.
In the anterior part of the cephalothorax lie the esophagus and the cardiac stomach.
Posteriorly the cardiac stomach connects to the pyloric stomach, which connects to the
intestine passing posteriorly under the heart, ending as the anal opening in the telson. On
each side of the cardiac stomach lie the digestive glands. Open the dorsal surface of the
cardiac stomach and examine the gastric mill consisting of three grinding “teeth”.
The excretory organs consist of the spherical green glands that lie ventral to each eye
and anteriolateral to the esophagus. The green glands open to the outside through an
excretory pore at the base of each antenna.
Beneath the heart lie the reproductive organs or gonads. In the gravid female the
ovaries are filled with orange masses of eggs. An oviduct leaves each ovary and exits at
the base of the third walking leg. The white testes of the male are fused to form a three
lobed sac. The conspicuous vas deferens lead to the fifth walking legs.
The brain can be found between the green glands. From the brain two nerves go
around the esophagus to connect posterior to the esophagus forming the
circumesophageal commissure. To reach the ventral nerve cord remove the tergum and
abdominal musculature. Notice that it is a paired ventral nerve cord with ganglia (a
swollen bundle of nerves) at each segment. Find the lateral nerves exiting each
ganglion.
Class: Diplopoda
Millipeds appear to have two pair of appendages per each segment, however each of the
apparent segments are actually double segments. Often called “thousand leggers”, in
reality millipeds have far fewer legs. Their head is small and carries a pair of antennae.
Class: Chilopoda
Centipeds (“hundred leggers”) have a single pair of appendages per segment. The legs
of centipeds are longer than those of millipeds and the first trunk appendage is modified
into a pair of venomous claws. The head carries a single pair of antennae.
Class: Insecta
This class is considered to be the most important arthropod group both in terms of the
number of species and their economic impact on human welfare. Three-quarters of all
known animals are in this class and two-thirds of all flowering plants are dependent upon
insects for pollination. Pollinators include bees, wasps, butterflies, moths, beetles and
flies. Mosquitoes, tsetse flies, fleas, lice and houseflies are all vectors in diseases
affecting humans, domesticated animals and cultivated crops. Some attack the wooden
frames of our homes. Overzealous use of pesticides (poisons) employed in controlling
these pests has lead to the evolution of pesticides resistant insects and the accumulation
hazardous environmental contaminates.
The grasshopper is a good representative of this class showing typical insect
characteristics and is found almost everywhere. Romalea microptera or “lubber
grasshopper” from the Southern United States is the species selected for studying the
external characteristics of an insect. Obtain a specimen and notice the hard exoskeleton
composed of chitin. Using your knowledge of the crayfish, locate the tergum, sternum
and pleura on your specimen. Notice the three major body divisions; head, thorax and
abdomen. The head carries a pair of antennae and a pair of compound eyes. Three
pairs of legs and two pairs of wings are found on the thorax. The anterior pair of wings
are modified as protective wing covers while the posterior pair are employed in flight
(flight wings). On the first abdominal segment hidden under the wings is an organ of
hearing or tympanum. Respiratory openings or spiracles can be found on the lateral
side of each segment. Reproductive structures can be seen on the terminal abdominal
segment.
Locate the mouth of the grasshopper. The labrum (upper lip) is attached to the dorsal
clypeus. Below the labrum is the dark, harden mandibles. Maxillae lie posterior to the
mandibles, and fusion of a second pair of maxillae form the labium.
Figure 1. A frontal view of the grasshopper’s face 3 cm across. Label the above
mentioned structures.
Figure 2. After dissecting out the eye of the grasshopper and cleaning it, make a
wet mount slide and examine with the light microscope. The
individual lenses are termed facets. Draw the compound eye 3 cm
across at 40X.
The honeybee, Apis mellifera, was domesticated thousands of years ago and its
biology is well understood. Using prepared slides accomplish the following:
Figure 3. Draw the third leg 5 cm in length and label the structures as seen in the
diagram.
Figure 4. Draw the honeybee stinger 5 cm across. Label as directed.
Study Questions
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2.
3.
4.
5.
6.
7.
Identify the similarities between crustaceans and insects.
On what body division are the legs and wings found in insects?
Order Orthoptera includes grasshoppers and what other
insects?
What is the function of spiracles?
Compare the exoskeleton of a grasshopper and a honeybee.
How does a grasshopper hear?
How does a grasshopper smell?
There are approximately 29 orders of insects. These taxons are based on the structure
of the wings and mouthparts, on the pattern of metamorphosis and on a number of other
features. Zoology 5 students will be required to learn the following orders of insects.
Order: Odontata (tooth)
These are the predaceous flyers known as dragonflies and damselflies. The order is
characterized by long, narrow, net-veined wings, large eyes and chewing mouthparts.
Dragonflies have stouter bodies while damselflies are slender and delicate.
Order: Isoptera (equal wing)
Termites are social insects living in colonies of winged kings and queens, and wingless
workers and soldiers. Often referred to as “white ants” (although not true ants), these
little insects destroy wooden structures by feeding on cellulose.
Order: Orthoptera (straight wing)
Grasshoppers, katydids, crickets, cockroaches, preying mantis and walking sticks are
members of this insect order. They have large heads with strong chewing mouthparts.
Many have enlarged hind legs adapted for jumping. Winged forms have leather-like
forewings protecting the more delicate membranous hind wings. Feeding habits include
carnivores and herbivores.
Order: Demaptera (skin wing)
Earwigs are elongated insects with forceps-like cerci (tail). The first pair of wings are
short and leathery and the second pair, if present, are membranous. They are scavengers
with chewing mouthparts. The term “earwig” is derived from and old superstition that
these insects enter people’s ears. This belief is without foundation.
Order: Hemiptera (half wing)
These are the “true” bugs (stinkbugs, water striders, milkweed bugs and assassin bugs),
all with piercing mouthparts and a beak arising from the front portion of the head and
extending ventroposteriorly. The membranous forewings are thickened distally and at
rest overlap the abdomen, forming a triangular pattern on the back. The hind wings are
entirely membranous.
Order Homoptera (same wing)
This taxon includes cicadas, leafhoppers, aphids and scale insects. Although related to
the Hemiptera, these herbivorous insects have entirely membranous forewings. It is
common for them to hold their wings in a tent-like position over the body. Equipped
with piercing and sucking mouthparts, that damage many cultivated plants and are
regarded as serious agricultural pests.
Order: Diptera (two wings)
True flies, horseflies, gnats, midges and mosquitoes are found in this order. The
forewings are employed in flight, and the hind wings are reduced to knob-like, balancing
organs called halteres. Mouthparts are utilized for piercing and sucking plant and animal
fluids.
Order: Lepidoptera (pretty wing)
The large wings, soft bodies and appendages of butterflies and moths are covered with
pigmented scales. Adult mouthparts are modified as a coiled proboscis used for sucking
the nectar of flowers. Many adults feed little or not at all. Lepidoptera larvae
(caterpillars) have chewing mouthparts that can prove destructive to numerous cultivated
plants.
Order: Coleoptera (sheath wing)
This order includes beetles and weevils. A famous scientist once remarked that God
must have had an inordinate fondness for beetles because 40% of all known animals are
beetles. The body is very hard, as are the modified forewings or elytra that cover the
membranous hind wings. In some species the elytra fuse making flight impossible. Food
habits include dung eating; scavenge ring, active predation and plant eating.
Order: Hymenoptera (membrane wing)
Include in this order are the ants, bees, bumble bees and wasps; many species exhibit
solitary, semi-social or social life styles. They have chewing mouthparts often modified
for sucking or lapping. Winged and wingless species exists and many have reproductive
structures modified as stingers at the end of their abdomens.