ARTHROPODS

BY JANE YAWE
 INTRODUCTION
❑Arthropods are animals belonging to the phylum
arthropoda (from greek arthron, "joint", and podos
"foot", which together mean "jointed feet"), and include
the insects, arachnids, crustaceans, and others.
❑ Arthropods are characterized by their jointed limbs and
cuticles, which are mainly made of chitin.
➢The rigid cuticle inhibits growth, so arthropods replace it
periodically by moulting.
❑The phylum arthropoda is the largest and most diverse
of all animal phyla.
❑More than three quarters of the animals on earth are
arthropods, and most of these are insects.

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 INTRODUCTION CONTD:
➢They range in size from microscopic plankton up to forms a
few meters long. They have exploited every known
environment
➢Despite their great diversity, the basic body plan of
arthropods is fairly constant. In summary, the following are
the characteristics of the phylum arthropoda:
1.Metamerism modified by specialization of body regions
for specific functions (tagmatization)
2.Chitinous exoskeleton that provides support and protection
and is modified to form sensory structures
3.Paired, jointed appendages

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 INTRODUCTION CONTD:
4. Growth accompanied by ecdysis or moulting
5. Coelom reduced to cavities surrounding
gonads and sometimes excretory organs
6. Open circulatory system in which blood is
released into tissue spaces (hemocoel) derived
from blastocoel
7. Complete digestive tract
8. Metamorphosis is often present. This reduces
competition between immature and adult
stages
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 WHY ARE ARTHROPODS SO
SUCCESSFUL?
❑ Four aspects of arthropod biology have
contributed to their success:
1. Metamerism and tagmatization
2. The exoskeleton
3. The hemocoel
4. Metamorphosis

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 METAMERISM AND TAGMATIZATION
❑Arthropod body is often composed of a series of similar
segments, each bearing a pair of appendages
➢Metamerism of arthropods is most evident externally.
Internally, however, septa do not divide the body cavity of
an arthropod, and most organ systems are not metamerically
arranged

➢Metamerism permits the specialization of regions of the

body for specific functions.

➢This regional specialization is called tagmatization. These

body regions, called tagmata (sing. Tagma), are specialized
for feeding and sensory perception, locomotion, and visceral
functions 7
 THE EXOSKELETON
❑Enclosing arthropod body is an external, jointed skeleton called an
exoskeleton or cuticle
➢ The exoskeleton is often considered the major reason for arthropod
success. It provides structural support, protection, impermeable surfaces
for the prevention of water loss, and a system of levers for muscle
attachment and movement
➢ The exoskeleton covers all body surfaces and invaginations of the
body wall, such as the anterior and posterior portions of the gut tract. It
is nonliving and is secreted by a single layer of epidermal cells
➢ The epidermal layer is sometimes called the hypodermis because, unlike
other epidermal tissues, it is covered on the outside by exoskeleton,
rather than being directly exposed to air or water
❑The exoskeleton is periodically shed in moulting process called ecdysis.
Ecdysis is controlled by nervous and endocrine systems

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 THE HEMOCOEL
❑The hemocoel is derived from an embryonic cavity called blastocoel
that forms in the blastula. Hemocoel provides an internal cavity for the
open circulatory system of arthropods. Internal organs are bathed by
body fluids in the hemocoel to provide for the exchange of nutrients,
wastes, and sometimes gases
❑The coelom was reduced in ancestral arthropods. The presence of the
rigid exoskeleton and body wall means that the coelom is no longer
used as a hydrostatic compartment. In modern arthropods, the coelom
forms small cavities around the gonads and sometimes the excretory
structures
❑In arthropods, "blood" is confined to vessels during only a portion of its
circuit through the body. The remainder of its journey takes place within
the body cavity (called the hemocoel). For this reason, arthropod blood
is called hemolymph
❑The circulation system is composed of tubular hearts and aorta running
along the dorsal side of the arthropod. The hearts pump hemolymph
into the sinuses of the hemocoel where exchange of materials take
place 9
 METAMORPHOSIS
❑Metamorphosis is a radical change in body form and
physiology as an immature stage becomes an adult

❑The evolution of arthropods has resulted in an

increased divergence of body forms, behaviours, and
habitats between immature and adult stages

❑Having different adult and immature stages means

that the stages do not compete with each other for
food or living space

➢In some arthropod, larvae also serve as the dispersal

stage
❑Metamorphosis can be complete or incomplete 10
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 CLASSIFICATION OF ARTHROPODS
❑ Arthropods are typically classified into
five subphyla:
1. Trilobitomorpha
2. Chelicerata
3. Myriapoda
4. Crustacea (biramia)
5. Hexapoda (uniramia) 12
 SUBPHYLUM TRILOBITOMORPHA
❑Members are all extinct
❑Members of this subphylum were a dominant form of life in the
oceans from the cambrian period (600 mya) to the carbonferous
period (345 mya)
❑ They crawled along the substrate feeding on annelids, mollusks, and
decaying organic matter. The trilobite body was oval, flattened. It
had three longitudinal sections and thus the subphylum name
❑All body segments articulated so that the trilobite could roll into a
ball to protect its soft ventral surface. The trilobite appendages
consisted of two lobes or rami called biramous appendages
❑ The inner lobe was a walking leg, and the outer lobe bore spikes or
teeth that may have been used in digging or swimming or as gills in
gas exchange

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 SUBPHYLUM CHELICERATA
❑This subphylum includes familiar animals, such as spiders, mites and ticks
and less familiar animals, such as horseshoe crabs and sea spiders
❑These animals have two tagmata.
❑The prosoma or cephalothorax is a sensory, feeding, and locomotor
tagma.
❑It usually bears eyes, but unlike other arthropods, never has antennae.
❑Paired appendages attach to the prosoma.
❑The first pair, called chelicerae, are often pincerlike or chelate, and are
most often used in feeding.
❑They may also be specialized as hollow fangs or for a variety of other
functions.
❑Posterior to the prosoma is the opisthosoma,
❑Contains digestive, reproductive, excretory, and respiratory organs

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❑Subphylum chelicerata, has three classes: class merostomata,
class arachnida and class pycnogonida

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SUBPHYLUM CHELICERATA:

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❑ SAND AND DEVELOP UNATTENDED

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 CLASS MEROSTOMATA
❑This class includes the extinct giant water scorpions and only four
species of living horseshoe crabs
❑ The prosoma is covered by a hard, horseshoe-shaped carapace.
❑Opisthosoma includes a long, unsegmented telson.
❑The first pair of the opisthosomal appendages covers genital pores and
is called genital opercula.
❑The remaining five pairs of appendages are book gills (the name is
derived from the resemblance of these plate like gills to the pages of a
closed book).
❑Gases are exchanged between the blood and water as blood
circulates through the book gills
❑Horseshoe crabs are dioecious.
❑The male mounts the female and grasps her with his pedipalps.
❑The female excavates shallow depressions in the sand, and as she sheds
eggs into the depressions, the male fertilizes them.
❑Fertilized eggs are covered with sand to develop unattended 19
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 CLASS ARACHNIDA
❑This class includes spiders, mites, ticks and scorpions.
❑Most of these archnids are carnivores.
❑They hold small arthropods with their chelicerae while enzymes
from the gut tract pour over the prey.
❑Partially digested food is then taken into the mouth.
❑Others inject enzymes into prey through hollow chelicerae (as in
spiders) and suck partially digested animal tissue

❑Arachnids use coxal glands and/or malpighian tubules for

excreting nitrogenous wastes
❑The body of an arachnid has a variety of sensory structures.
Most mechanoreceptors and chemoreceptors are
modifications of the exoskeleton, such as projections, pores,
and slits, together with sensory cells
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 ARACHNIDA CONT..
❑Arachnids are dioecious.
❑Paired genital openings are on the ventral side of
the second abdominal segment.
❑Sperm transfer is usually indirect.
❑The male often packages sperm in a spermatophore,
which is then transferred to the female.
❑Courtship rituals confirm that individuals are of the
same species and attract a female to the
spermatophore.
❑In some arachnids (e.g. Spiders), copulation occurs,
development is direct and the young hatch from eggs
as miniature adults.
❑Many arachnids tend their developing eggs and
young during development 22
 CLASS ARACHNIDA CONTD:
❑Many scorpions are ovoviparous;
❑Development is internal, although large, yolky eggs provide
all the nourishment for development.
❑Some scorpions, however, are viviparous, meaning that the
mother provides nutrients to nourish the embryos.
❑After birth, the young crawl into the mother’s back, where they
remain for up to a month.
❑Mites and ticks (acarines) are mostly ectoparasites on
humans and domestic animals.
❑Others are free-living in both terrestrial and aquatic habitats

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 CLASS PYCNOGONIDA
❑Members of the class pycnognida are the sea
spiders
➢ All are marine and worldwide, but are most
common in cold waters. They live on the ocean floor
and frequently feed on cnidarian polyps and
ectoprocts
➢Some sea spiders feed by sucking prey tissues
through a proboscis. Others tear at prey with their
first pair of appendages

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 SUBPHYLUM CRUSTACEA
❑Crustaceans are a very large group of arthropods.
They include various familiar animals, such as crabs,
lobsters, crayfish, shrimp, krill and barnacles
❑The majority of them are aquatic, living in either
marine or fresh water environments, but a few groups
have adapted to life on land. Crustaceans are among
the most successful animals, and are as abundant in the
oceans as insects are on land

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 Crustaceans cont ..
❑Crustaceans differ from other living arthropods in
that;
❑they have two pairs of antennae, whereas all other
arthropods have one pair or none.
❑In addition crustacean appendages are typically
biramous, meaning they are divided into two parts;
this includes the second pair of antennae, but not
the first, which is uniramous.
❑Despite their diversity of form, crustaceans are
united by the special larval form known as the
nauplius

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 CRUSTACEA
• Mandibles (biting jaws) likely evolved from a pair of
limbs that took on a chewing function
•Most crustaceans have separate sexes
•Majority develop through a nauplius stage

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 CRUSTACEA
• Decapod crustaceans include shrimps, lobsters,
crabs and crayfish
• Have ten feet
• Exoskeleton usually enforced with caco3
• Most body segments are fused into a cephalothorax
• Lobsters and crayfish have appendages that aid in
swimming
• Swimmerets and uropods

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CRUSTACEA

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 CRUSTACEA
• ORDER CIRRIPEDIA

-Barnacles are crustaceans that are sessile as adults

-Free-swimming
larvae
-Are hermaphroditic
-Some have stalks

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 SUBPHYLUM MYRIAPODA

❑ This subphylum is subdivided into four classes:

1. Diplopoda (millipedes)
2. Chilopoda (centipedes)
3. Symphyla (symphylans)
4. Pauropoda (pauropodans)
➢ They are all characterized by a body consisting of two
tagmata (head and trunk) and uniramous appendages.
➢ All modern myriapods are terrestrial.
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 CLASS DIPLOPODA
❑The class diplopoda (gr. Diploos, twofold + podus, foot)
contains the millipedes.
❑Millipedes have 11 to 100 trunk segments derived from an
embryological and evolutionary fusion of primitive metameres.
❑An obvious result of this fusion is two pairs the occurrence of
appendages on each apparent trunk segment. Each segment is
actually the fusion of two segments. Most millipedes are round in
cross section, although some are more flattened
❑Millipedes are worldwide in distribution and area nearly
always found in or under leaf litter, humus or decaying
logs to avoid desiccation since their epicuticle does not
contain much wax

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 CLASS DIPLOPODA
❑Male millipedes transfer sperm to female
millipedes with modified trunk appendages, called
gonopods, or in spermatophores.
❑Eggs are fertilized as they are laid and hatch in
several weeks.
❑Immature acquire more legs and segments with
each molt until they reach adulthood

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 CLASS CHILOPODA
❑Members of the class chilopoda (gr. Cheilos, lip + podus, foot) are
the centipedes.
❑Most centipedes are nocturnal and scurry about the surface of logs, rocks or
other forest floor debris. They lack a waxy epicuticle and therefore require
moist habitats.

➢Their bodies are flattened in cross section, and they have a single
pair of long legs on each of their 15 or more trunk segments.
➢The last pair of legs is usually modified into long sensory
appendages.

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 CLASS CHILOPODA
❑Centipede reproduction may involve courtship
displays in which the male lays down a silk web using
glands at the posterior tip of the body.
❑He places a spermatophore in the web, which the female
picks up and introduces into her genital opening.
❑Eggs are fertilized as they are laid.
❑A female may brood and guard eggs by wrapping her
body around the eggs, or they may be deposited in the
soil. Y
❑oung are similar to adults except that they have fewer legs
and segments.
❑Legs and segments are added with each moult
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 CLASS PAUROPODA AND CLASS SYMPHYLA
❑Members of the class pauropoda (gr. Pauros, small + podus, foot) are
soft bodied animals with 11 segments.
❑These animals live in forest floor litter, where they feed on fungi, humus
and other decaying organic matter.
❑Their small size and thin, moist exoskeleton allow gas exchange across the
body surface and diffusion of nutrients and wastes in the body cavity

❑Members of the class symphyla (gr. Sym, same + phyllos, leaf) are
small arthropods (2 to 10 mm in length)
❑Occupy soil and leaf mold, superficially resemble centipedes, and
❑Have 12 leg bearing trunk segments.
❑The posterior segment may have one pair of spinnerets or long, sensory
bristles.
❑Symphylans normally feed on decaying vegetation; however, some
species are pests of vegetables and flowers 39
 SUBPHYLUM HEXAPODA CLASS INSECTA

❑There are a huge amount of body shapes, but the "basic

insect" body plan is a head, thorax, abdomen, one pair of
antennae, and mouthparts in three parts (mandibles,
maxillae and a labium)

❑The head has the mouthparts and most of the sensory

organs including the eyes and antennae.
❑The thorax is subdivided into prothorax, mesothorax and
metathorax;
❑Each bears a pair of legs - never more than 3 pairs in adult
insects, and 2, 4 or no wings.
❑The thorax is simply a box of muscles linked to the legs and
wings.
❑There are eleven or fewer segments in the abdomen and they
contain the organs for digestion, excretion and reproduction
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 CLASS INSECT CONT..
❑The internal organs are bathed in haemolymph which
transports waste and nutrients and is moved around the
body by a tube shaped heart and the movement of the
insect.
❑Adult insects have six legs, but larvae, e.g., Caterpillars,
maggots may have fewer or more than six legs
(caterpillars have six true legs and up to ten prolegs).
❑Most have three simple eyes as well as compound eyes

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 WHY ARE THE INSECTS SO SUCCESSFUL?
❖ The secret of the success of the insects can be linked to the following
features:
i. Tagmosis allowing specialization in each area
ii. Jointed appendages and striated muscles giving efficient locomotion
iii. Exoskeleton which provides protection and prevents dehydration
while still being flexible and light enough to allow movement.
➢ The hardening of the cuticle is a result of sclerotization, i.e. The bonding of protein
molecules and their cross-links within and between the lammellae of the procuticle.
➢ This forms sclerotin, a resistant and insoluble protein
iv. Metamorphisim reducing competition between individuals of
different stages in the life cycle, e.g. Caterpillar and butterfly
occupy different niches
v. Flight : insects were the first animals to fly.
vi. The evolution of wings greatly helped in the dispersal and radiation
of insects.
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vii. The wings are a double cuticular membrane containing veins which
provide structural stability
 CLASS INSECTA CONTD:
❑Insects have complex sensory organs of vision,
chemoreception, hearing and touch; all enhancing
awareness of surroundings.
❑Insects detect smells mainly using their antennae.
❑The antennae are also used to touch, taste and in some cases,
to detect sound.
❑Hormones regulate many insect functions, including ecdysis and
metamorphosis.
❑Pheromones are chemicals emitted by one individual that alter
the behaviour of another member of the same species

❑Malpighian tubules attach to mid-gut and serve in

elimination of wastes.
❑Nitrogenous wastes are converted to uric acid to conserve
water 44
 CLASS INSECTA CONTD:
❑One more reason for insects’ success is their high
reproductive potential.
❑There are modifications for mating and ovipositioning (egg
laying).
❑The "claspers" seen on the final segments of many male insects,
e.g. Locusts are used to hold the female while mating.
❑Ovipositors allowing females to lay eggs in concealed sites and
in hosts e.g. Parasitic wasps; in social female bees and wasps this
has been adapted to form a sting.
➢There are also modifications to attract a mate, e.g.
Stridulatory mechanisms (sound production by rubbing
one part of the body against another), pheromone
release and antennal modifications, and
bioluminescence.
➢All these increase reproductive potential of insects 45
 CLASS INSECTA CONTD:
❖There are about 30 described
orders of insects and since insects
are so diverse, more detailed study
is reserved for a special branch of
biology called entomology (insect
science).

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THE END

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