Arthropods Made Easy
The Phylum Arthropoda
Phylum Arthopoda Review
1. What are some examples of arthropods?
Ants, flies, cockroaches, shrimps, crabs, spiders and scorpions are examples of arthropods.
Phylum Arthropoda - Image Diversity:
arthropods
2. What are the classes into which the phylum Arthropoda is divided?
What are the three main ones and some of their representative species?
The three main classes of arthropods are: insects (cockroachs,
ants, flies, bees, beetles, butterflies), crustaceans (crabs, lobsters,
shrimps, barnacles) and arachnids (scorpions, spiders, mites). Other
classes are onychophorans (velvet worms), diplopods (millipedes) and
chilopods (centipedes).
3. What are the main morphological features of arthropods?
Arthropods present three distinguishing features: they are
metameric beings (segmented body), they have an exoskeleton made of
chitin and they present articulated limbs.
Phylum Arthropoda - Image Diversity:
arthropod structure
4. Concerning germ layers and the presence of coelom how are arthropods characterized?
Arthropods are triploblastic (they have three germ layers) and coelomate beings.
5. Considering the presence of segmentation (metameres) in their body to
which other already studied phylum are arthropods proximal?
Considering their metameric feature arthropods are proximal to
annelids that also have segmented bodies. In the embryonic development
of some arthropods there are fusions of metameres forming structures
like, for example, the cephalothorax of arachnids.
Phylum Arthropoda - Image Diversity:
athropod cephalothorax
6. What is the external rigid carapace of arthropods called? Of which
substance is it made? Which type of organic molecule is that substance?
The external carapace of arthropods is called exoskeleton. The
arthropod exoskeleton is made of chitin, a nitrogen-containing
polysaccharide.
Phylum Arthropoda - Image Diversity:
exoskeleton
7. How do arthropods grow?
Due to the presence of exoskeleton the growth of an arthropod is
periodical. During the growth period the animal loses the exoskeleton,
grows and develops a new exoskeleton. This process is named ecdysis, or
molting.
Phylum Arthropoda - Image Diversity:
ecdysis
8. How does the presence of exoskeleton explain the general small size of arthropods?
Since they have exoskeleton and periodic ecdysis, the growth of
arthropods is limited to avoid the animal becoming vulnerable to
environmental harm. There are however some arthropod species with
relatively large-sized individuals, like “giant” cockroaches, crabs and
spiders.
9. How can the features of the arthropod exoskeleton explain the terrestrial adaptation of some species of the phylum?
In the arthropod exoskeleton there is a layer of wax which is
impermeable. This feature was fundamental for primitive arthropods from
the sea to survive on dry land without losing excessive water to the
environment.
10. What is the type of digestive system present in beings of the phylum
Arthropoda? Are these animals protostomes or deuterostomes?
The digestive tube of arthropods is complete, containing mouth
and anus. Arthropods are protostome animals, i.e., in their embryonic
development the blastopore originates the mouth.
11. How is the extracorporeal digestion associated to predation in arachnids?
Arachnids can inoculate poison to paralyze or kill their preys
using structures called chelicerae. The prey is partially digested
outside the body of the arachnid by digestive enzymes inoculated
together with the venom or injected posteriorly. After this
extracorporeal digestion the food is ingested and gains the digestive
tube of the predator where the extracellular digestion continues.
12. Which organs or respiratory adaptations do aquatic and terrestrial arthropods respectively present?
In crustaceans, typical aquatic beings, there are richly
vascularized gills that make contact with water and permit gas exchange.
In terrestrial insects the respiration is tracheal and gases flow
inside small tubes that connect the animal external surface and ramify
to tissues and cells without the participation of blood. In arachnids,
besides the tracheal respiration, book lungs (thin folds resembling
leaves in a book) may also exist.
Phylum Arthropoda - Image Diversity:
crustacean respiratory system
insect respiratory system
arachnid respiratory system
13. In arthropods why isn't gas exchange done through cutaneous diffusion?
In arthropods the impermeability of the exoskeleton makes the
passage of gases difficult. In addition the new methods of respiration
present in arthropods were preserved by evolution because they were more
efficient for those animals.
14. What is the type of circulatory system present in arthropods? Do these animals have heart and respiratory pigments?
In arthropods the respiratory system is open (lacunar). Blood,
also known as hemolymph, is pumped by a heart and falls into cavities
(lacunas) irrigating and draining tissues.
All arthropods have a heart. Crustaceans and arachnids have
respiratory pigments. Most insects do not have respiratory pigments
since their blood does not carry gases (in them gases reach tissues and
cells through tracheal structures). However, some few insects do have
respiratory pigments, hemoglobin (contribution from Jacob Campbell).
15. What are respiratory pigments? What is the respiratory pigment
present in some arthropods? Which is the analogous molecule in humans?
Respiratory pigments are molecules able to carry oxygen and other respiratory gases present in circulatory fluids.
In crustaceans and in arachnids hemocyanin is the respiratory pigment. In humans the analogous pigment is hemoglobin.
16. How is the respiratory system of insects (with its independence
between circulation and respiration) related to the motor agility of
some species of this arthropod class?
Even having low speed and low pressure circulatory system, since
it is a lacunar (open) circulatory system, insects perform extremely
fast and exhaustive movements with their muscle fibers, like wing
beating. This is possible because in these animals the respiration is
independent from the open circulation. Gas exchange is done with great
speed and efficiency by the tracheal system that puts cells in direct
contact with air. Muscles can then work fast and hard.
17. How are the excretory systems of the three main arthropod classes constituted?
In crustaceans a pair of excretory organs called green glands
exists. The green glands collect residuals from the blood and other
parts of the body. They are connected by ducts to excretory pores
located under the base of the antennae and these pores release the
excretions outside.
In insects small structures called malpighian tubules gather
wastes from the blood and throw them into excretory ducts that open in
the intestine. In these animals excretions are eliminated together with
feces.
In arachnids, besides malpighian tubules, there are coxal glands
located in the cephalothorax near the limbs that also participate in
excretion.
Phylum Arthropoda - Image Diversity:
arthropod excretory system
18. What are the noteworthy features of the nervous system of arthropods?
In arthropods the nervous system has more sophisticated sensory
receptors with well-advanced cephalization. In the anterior region of
the body there is a fusion of ganglia forming a brain connected to two
ventral ganglial chains having motor and sensory nerves.
The boosted development of the sensory system of arthropods
provides more adaptive possibilities for these animals to explore many
different environments.
Phylum Arthropoda - Image Diversity:
arthropod nervous system
19. What are compound eyes?
Arthropods have compound eyes made of several visual units called
ommatidia. Each ommatidium transmits visual information through the
optic nerve to the brain, which interprets the image. Because they are
round and numerous, these ommatidia, whose external surfaces point in
different directions creating independent images, cause arthropod eyes
have a large visual field, larger than the visual field of vertebrates.
Some insects have one or more simple eye besides their pair of compound
eyes.
Phylum Arthropoda - Image Diversity:
compound eyes
20. How is arthropod reproduction characterized?
Reproduction in beings of the phylum Arthropoda is sexual, with
larval stage in some insects and crustaceans (arachnids present only
direct development).
21. What are the types of fecundation that occur in arthropods? What is the predominant type?
In arthropods there are species having external fecundation and
other species having internal fecundation. Internal fecundation is
predominant.
22. How is fecundation done in insects (external or internal)? Is there copulation between insects?
Fecundation in insects is internal, with copulation.
23. How are the main classes of arthropods classified according to the presence of larval stage in their embryonic development?
In crustaceans there are species with direct and others with
indirect development. In insects there are species without larval stage
(ametabolic insects), others undergoing indirect development beginning
with an egg stage followed by a nymph stage (hemimetabolic insects) and
others with indirect development beginning with the larval stage
(holometabolic insects).
The transformation of a larva into an adult individual is called
metamorphosis. Hemimetabolic insects undergo incomplete metamorphosis
while holometabolic insects undergo complete metamorphosis.
Phylum Arthropoda - Image Diversity:
metamorphosis
24. What are nymph and imago?
Nymphs are larvae of hemimetabolic insects (like grasshoppers).
They are very similar to the adult insect although smaller. In
holometabolic insects (like butterflies) the larva makes a cocoon
(chrysalis, pupa) where it lives until emerging into the adult form.
Imago is the name given to the adult form of insects with indirect
development.
25. Is the stage when an insect larva is within a cocoon a stage of total biological inactivity?
The period when the larva is within its cocoon is a time of
intense biological activity since the larva is being transformed into an
adult animal.
26. How are the three main arthropod classes characterized according to the presence of wings?
Crustaceans and arachnids do not have wings. Most insects have wings.
27. Most insects have wings. Which is the other animal phylum that contains creatures with analogous organs?
Besides the phylum Arthropoda another animal phylum with flying
creatures is the chordate phylum, birds and chiropterans mammals (bats)
have wings. In the past some reptiles that possibly originated the aves
had wings too. There are also amphibians and fishes that jump high
exploring the aerial environment.
28. How are the three main arthropod classes characterized according to the presence of antennae?
Crustaceans have two pairs of antennae; insects have one pair; arachnids do not have antennae.
29. How are the three main arthropod classes characterized according to the body division?
In crustaceans and arachnids the head is fused with the thorax
forming the cephalothorax. Their body thus is divided into cephalothorax
and abdomen.
In insects there are head, thorax and abdomen.
30. How are the three main arthropod classes characterized according to the number of limbs?
Most crustaceans have five pairs of limbs. Insects have three pairs and arachnids present four pairs of limbs.
31. Which arthropod class is the most diversified animal group of the planet? How can this evolutionary success be explained?
The insects are the animal group with most diversity of species.
Almost 750000 insect species are known, about 55% of the total already
cataloged species of living beings (compare with mammals, with no more
than 4000 known species). It is calculated however that the number of
unknown species of insects may be over 2 million. The insect population
on the planet is estimated to be more than 10 quintillion
(1000000000000000000) individuals.
The great evolutionary success of insects is due to factors such
as: small size and alimentary diversity, making possible the exploration
of numerous different ecological niches; wings that provided more
geographic spread; the tracheal respiration that gave them motor
agility; high reproductive rates with production of great numbers of
descendants.
32. What are some examples of beings of the phylum Arthropoda that present a high level of behavioral sophistication?
Insects like some species of bees, wasps, ants and termites form
societies that include hierarchy and job division among members. Spiders
build sophisticated external structures, webs, mainly to serve as a
trap for capturing prey. Another example is the communication mechanism
in some bees known as the bee dance by which an individual signal to
others information about the spatial position of flower fields and other
nectar sources.
33. Arthropod identity card. How are arthropods characterized according
to examples of representing beings, basic morphology, type of symmetry,
germ layers and coelom, digestive system, respiratory system,
circulatory system, excretory system, nervous system and types of
reproduction?
Examples of representing beings: cockroaches, flies, crabs,
lobsters, shrimps, spiders, scorpions, mites. Basic morphology:
segmented body (metameric), articulated limbs, chitinous exoskeleton,
periodic ecdysis. Type of symmetry: bilateral. Germ layers and coelom:
triploblastics, coelomates. Digestive system: complete. Respiratory
system: tracheal in insects, branchial in crustaceans, tracheal and book
lungs in arachnids. Circulatory system: open, hemocyanin in crustaceans
and arachnids. Excretory system: malpighian tubules in insects, green
glands in crustaceans, malpighian tubules and coxal glands in arachnids.
Nervous system: ganglial. Types of reproduction: sexual, with or
without larval stage in insects and crustaceans, metamorphosis in some
insects, no larval stage in arachnids.
