Arthropods – Insects, crustaceans…
Phylum Arthropoda
Insects are part of a bigger animal group called arthropods. The word Arthropod literally means jointed limbs. The
arthropods are the first animal group to have jointed legs. Insects have certain characteristics. They have six legs. They
have three body parts, a head, a thorax and an abdomen. Many have wings. They are by far the largest animal group.
They also have an outer or exoskeleton made of a substance called chitin.
There is another group within the arthropods called the arachnids. The arachnids are spiders, scorpions, etc. The
arachnids are similar to insects. However, they have eight legs. The largest arthropod groups are: grasshoppers,
dragonflies, beetles, bugs, aphids, butterflies, flies, bees, ants, spiders, and crustaceans [crayfish, lobsters, crabs,
shrimp…]. Crustaceans and arachnids have a combined head/thorax region called a cephalothorax.
Arthropod Systems
Muscular & Skeletal
Systems
Arthropods have an exo-skeleton, and either three [insect] or four [arachnid] pairs of
segmented legs. Muscles are connected to their outside skeleton and are generally
gray.
Digestive System
Arthropods have different kinds of mouths depending on the species. Some can chew,
some can suck objects. The digestive system is a tube that goes from the mouth to the
anus. They obtain nutrients using paired jointed appendages near their mouth
[maxillary palps] and jaws called chelicerae.
Nervous System
Arthropods often have antennae that can be used to detect odors or to touch objects.
Insects have two sets of eyes: simple eyes and compound eyes. The simple eyes have
cornea, retina, pigment cells, etc. There is a brain and nerve cells.
Circulation
Arthropods have an open circulatory system that carries food, but not oxygen
throughout its body. Since it does not carry oxygen, insect blood is green, not red like
mammal blood. The heart is a simple tube running along their backs.
Respiration
Arthropods have a respiratory system composed of openings on their abdomens [gills
in water or spiracles on land]. Terrestrial arthropods [like insects] also have tracheal
tubes that distribute gases through its body.
Reproduction
Arthropods generally reproduces sexually, although not always. Asexually, some
insects can go through a process called parthenogenesis [ovum develops without
fertilization].
Excretion
Arthropods have an excretory system by which feces can leave the body through the
anus.
Symmetry
Arthropods have bilateral symmetry.
Appearance
Arthropods are generally relatively small because of the types of systems they have.
They can be many different colors depending on the species.
Use the terms below to label the external anatomy of the ant.
abdomen - The abdomen is the segmented tail area of an ant. It
contains the heart, Malpighian tubules, reproductive organs, and
most of the digestive system (foregut, hindgut and rectum). It is
protected by an exoskeleton.
antennae - Ants have two jointed antennae. They are sensory
appendages attached to the head.
compound eye - Ants have two compound eyes. These eyes are
made up of many hexagonal lens/corneas which focus light from
each part of the insect's field of view onto a rhabdome (the
equivalent of our retina).
head - The head of an ant (or any insect) is the location of its brain,
two compound eyes, its proboscis, pharynx (the start of the digestive
system), the point of attachment of its two antennae, etc.
jointed leg - Ants, like all insects, have six jointed legs.
mandibles - Mandibles are the jaws of the ant. The
mandibles bite off food and tear it into small, easily digestible
pieces.
petiole - The small lump located between the abdomen and
the thorax Some ants have one petiole and some have two.
thorax - The thorax is the chest area of an insect (including
ants). The thorax is divided into three segments; on each
segment is a pair of legs. The thorax contains the muscles
that make the legs move.
Use the terms below to label the external anatomy of the beetles.
Abdomen - the segmented tail area of a beetle that contains the heart,
reproductive organs, and most of the digestive system
antenna - like all insects, beetles have 2 segmented antennae
compound eye - a faceted eye made up of many hexagonal lenses
elytron - (plural elytra) elytra are hardened fore wings that protect the longer
hind wings
head - the head is at the front end of the beetle's body and is the location of the
brain, the two compound eyes, the mouth parts, the pharynx (the start of the
digestive system), and the points of attachment of its two antennae.
hind wing - beetles have two hind wings, used for
flying (or swimming). These long wings can be
folded under the elytra when not in use.
legs - like all insects, beetles have 6 jointed legs
mandibles - the jaws
maxillary palps - long, segmented mouth parts
that grasp the food
thorax - the middle area of the beetle's body where the legs and wings are attached
Use the terms below to label the external anatomy of the arachnid
leg - spiders have 8 legs. Each leg is made of
abdomen - the belly, also called the opisthosoma. It contains the
seven segments and has 2 or 3 tiny claws at
guts, heart, reproductive organs, and silk glands.
the tip. If a leg is lost, it will grow back.
cephalothorax - the fused head and thorax, also called the
pedicel - the spider's waist - it connects the
prosoma. It contains the brain, jaws, eyes, stomach, and leg
cephalothorax and the abdomen.
attachments.
pedipalps - also called palps, these two
eyes - tiny eyes (also called ocelli) that can only detect light and dark sensory feelers look like very short legs
- they are located on top of the spider's cephalothorax. Most species attached to the front of the spider - they taste
of spiders have 8 eyes, but other species have 12, 6, 4, 2 or no eyes. food.
chelicera - also called chelicera, they are located below the eyes.
spinnerets - where the spider's silk is
The jaws are tipped with fangs that can inject poison.
released - they are located at the tip of the
abdomen.
Use the terms below to label the external anatomy of the hermit crab.
Abdomen - the soft, twisted body of the hermit crab (located Eye - The two compound eyes are located on
behind the cephalothorax), which contains many of the internal eyestalks at the top of the cephalothorax.
organs (including the liver, much of the digestive tract, the
Reduced 4th and 5th legs - Very short, jointed
reproductive system, etc.); it is usually hidden inside shell, which legs that are kept within the shell, and are used to
gives it much-needed protection.
hold onto the inside of the shell.
Antennae - The long sensory organs (feelers) located towards Shell - A hard shelter (made by a gastropod like a
the front of the crab (each of the long pair of feelers is called an snail or a whelk) that is temporarly used by the
antenna; each of the short pair is called an antennule).
hermit crab. When the crab grows bigger than the
Carapace - The hard, protective outer shell of the crab. The shell, it must find a new shell.
carapace is made of chitin.
Walking legs - Two pairs of long, jointed legs used
Cheliped - One of two big claws used for defense and food
for locomotion (walking); they are attached to the
handling; one cheliped is much bigger than the other.
thorax.