Download Phylum Arthropoda

Arthropoda
ARTHRO – “ JOINTED” PODA “FEET”
y The arthropods are a group of animals which has
attained the greatest biological success – largest
number of species and individuals and occupy the
greatest number of habitats. At least 75% of animals
are arthropods!
All arthropods posses three key features:
y A tough chitinous exoskeleton.
y A series of jointed appendages.
y A segmented body.
Arthropoda is divided into four subphyla:
y Trilobita: an extinct group of marine arthropods
that flourished in ancient seas.
y Chelicerata: includes spiders, ticks, mites,
scorpions, and horseshoe crabs. Name of subphylum
is derived from a characteristic pair of appendages –
the cheliceras
y Crustacea – most are aquatic arthropods with gills,
mandibles, 2 pairs of antennae, and 2 compound
eyes. Eg. Crabs, lobsters, shrimp, barnacles, pill
bugs.
y Uniramia: Most arthropods are in this group –
centipedes, millipedes, and all insects. Characterized
by unbranched (uniramous) appendages
Habitat:
y Found everywhere: all parts of the sea, freshwater,
land and air
Structure:
y Very diverse, but all have a tough exoskeleton made
of chitin, jointed, appendages, and a segmented
body. Usually the body has a recognizable head,
thorax, and abdomen, but the first two of these may
be fused into a cephalothorax. Triploblast, coelomate
animals with an anus.
Ingesting Food:
y Every mode of feeding is seen – herbivores,
carnivores, parasites, filter feeders and detritus
feeders. The appendages of arthropods have evolved
in ways that enable these diverse methods of feeding.
Elimination:
y Undigested food leaves through the anus.
Internal Transport:
y A well-developed heart pumps blood through an
open circulatory system.
y Tracheal tubes in most terrestrial arthropods carry
respiratory gasses by diffusion to all tissues in the
body.
Respiration:
y Three basic types of respiratory structures are found
– gills in aquatic arthropods such as crustaceans,
book lungs are found in spiders and their relatives,
and tracheal tubes are found in most terrestrial
arthropods.
Excretion:
y Most terrestrial arthropods dispose of nitrogen-
containing wastes by using a set of Malphigian
tubules that remove wastes from the blood and then
add them to undigested food before it leaves via the
anus. Small excretory glands may be found at the
base of the legs in some terrestrial species.
Excretion Con’t:
y In aquatic arthropods, cellular wastes diffuse from
the body into the surrounding water via the gills.
Many aquatic arthropods also eliminate nitrogen –
containing wastes through a pair of green glands that
empties through a pair of openings in the head.
Locomotion:
y Arthropods have well-developed muscle systems.
The muscles are attached to the exoskeleton and
their contraction results in walking, flying and
swimming motions.
Sensing Environment:
y Arthropods have simple sense organs such as
statocysts and chemical receptors, but most also
have sophisticated sense organs such as compound
eyes, and well-developed senses of taste, touch, and
hearing. Eg. Flies can taste with their legs and
antennae, and their compound eyes have over 2,000
individual lenses.
Coordination:
y Most arthropods have well-developed nervous
systems. All have a brain that consists of a pair of
ganglia in the head, and a ventral nerve cord with a
ganglion for each original body segment.
Reproduction:
y Reproduction in most arthropods is simple. Sexes
are separate and fertilization takes place inside the
body of the female (internal fertilization).
y Development may involve either complete or
incomplete metamorphosis.
Growth and Development:
y In order to grow all arthropods must molt or shed
their exoskeletons.
y When it is time to molt the epidermis of the
arthropod digests the inner part of the exoskeleton
and absorbs much of the chitin in order to recycle
the chemicals in it.
Growth and Development Con’t
y Then the arthropod secretes a new exoskeleton
beneath the old one. This new exoskeleton is soft
and pliable; also it is larger than the animal to
provide room for growth.
y The arthropod casts off its old exoskeleton by
expanding its body and pulling itself completely out
of the exoskeleton. The arthropod must wait for the
new exoskeleton to harden – this can take from a few
hours to a few days. During this time the animal is
vulnerable to attack and must hid from predators.
Metamorphosis:
y Animals of some species experience spectacular but
predictable changes in body form and activity at some
point during their post-embryonic development. This
phenomenon is metamorphosis.
y Any aquatic tadpole changing to a terrestrial frog or a
crawling caterpillar that becomes a flying butterfly are
common example of metamorphosis.
y Aside from a very few that have no significant
transformations between the embryo and the adult,
insects species follow one of two patterns of postembryonic development.
Complete metamorphosis:
y Involves a larval and pupal stage between the egg
and the adult. All four stages are very different from
each other. Eg. Bees, beetles, butterfly.
y Egg:
Embryo developing in a protective case.
y Larva: Often worm – like and equipped to eat.
Often inhabits an entirely different habitat and eats
an entirely different diet from the adult. Eg. Aquatic
mosquito larvae feeds off of algae while adult flies
and sucks blood (female) or plant juice (male). This
serves to reduce competition for resources between
juveniles and adults of the same species.
y Pupa:
Non locomotive and non feeding stage.
Materials within the pupal case (larval body) are
recycled to create adult body parts. When animals
break out of its pupal exoskeleton it is essentially a
functioning adult.
y Adult:
Sexually active reproducing stage of the
life cycle. May have a very short life and be
nonfeeding.
Incomplete metamorphosis:
y Involves fewer distinct life stages. The animal that
hatches from the egg case is a nearly perfect
miniature of the mature adult called nymph.
Between the first nymph stage and the adult, the
animal may pass through several molts as it grows
and takes on adult feature and function such as
reproductive organs and larger wings. Because
distinct changes occur, this is a truly metamorphic
event. Eg. Grasshopper, Dragonfly