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8.7. Arthropods

Phylum Arthropods

The arthropods (from the Greek, árthron, "joint" and, poús, "foot") are the most numerous and varied phylum in the animal kingdom. They are invertebrate animals, which have an exoskeleton (external skeleton) of chitin and their legs are articulated, that is, they are divided into pieces that move.
There are more than a million different species of arthropods, most of them insects, and they are the most abundant group of living beings and the one that has had the greatest evolutionary success, since they have adapted to all possible environments and living conditions, being able to be aquaticterrestrial and parasitic.
Arthropods have bilateral symmetry.
The chitin exoskeleton gives them protection against attack by predators and against desiccation. Throughout its life, it needs to be changed several times in order to grow. This exoskeleton change is called moulting.
The body of arthropods is usually divided into three parts (although two of them can be merged):
  • Head. It contains the antennae, the eyes, and the mouth, which varies depending on the type of feed.
  • Chest.
  • Abdomen.
The respiration of arthropods can be:
  • Gill respiration: aquatic arthropods.
  • Tracheal respiration: the terrestrial arthropods.
Arthropods have appendages formed by articulated parts with different shapes, depending on their function: antennae, legs, mouthparts.
Its digestive system is complete, with morphological adaptations to the type of food.
The nervous system of arthropods is relatively complex. They have a clear cephalization and several senses quite developed, such as the view.
Arthropods are oviparous, and their reproduction is sexual and internal fertilization. They are born from eggs, but they have to go through several stages, sometimes with changes in shape and lifestyle, until they reach adults. These changes are called metamorphosis.
According to their number of legs, the main groups of arthropods are:
Las principales clases de artrópodos son: insectos, arácnidos, crustáceos y miriápodos

Una foto publicada por Mosa Marimbas (@mosamarimbas) el


Insect Class

Insects are the most abundant arthropods. Its name refers to the marks in the form of an incision that they present and that divides the body into three sections or parts:
  • Head.
    • Most have a pair of compound eyes.
    • A pair of mobile antennas with tactile and olfactory function.
    • Mouth parts. There are different types of oral appliances depending on the type of food.
  • Chest.
    • Three pairs of articulated legs.
    • One or two pairs of wings, although in some adult insects there are no wings.
  • Abdomen.
The nervous system of insects is quite developed, with sense organs and complex behavior, as seen in the case of social insects .
There is often sexual dimorphism, with males and females being different in both shape and size.
Because they have a rigid quinine exoskeleton, insects need metamorphosis in order to grow, and they undergo several moltsMetamorphosis can be:
  • Incomplete metamorphosis. The nymph that hatches from the egg is similar to the adult, although smaller and without wings.
  • Complete Metamorphosis. The larvae that hatch from the egg go through a pupal or chrysalis stage until they become an adult.
Although sexual reproduction is characteristic, asexual reproduction, from unfertilized ovules, by parthenogenesis is possible in some groups. Thus, for example, the drones of bees are born.
In some groups, as occurs with butterflies, they present a pupa phase in their metamorphosis in which the organism does not have any type of vital activity but which will give rise to an adult individual that is very different from the larva that originated it.
Examples of insects are flies, mosquitoes, beetles, butterflies, grasshoppers, wasps, ants, etc.
Tettigonia veridissima
Tettigonia veridissima (saltamontes verde)
Interactive activity: Complete the text about insects.
Interactive activity: Dissection of a beetle.

Class Arachnida

Arachnids are terrestrial arthropods  whose body is divided into:

  • Cephalothorax (a single piece that includes the head and the thorax). Here are the appendices:
    • Four pairs of legs that help them move.
    • A pair of chelicerae, mouth appendages ending in nails.
    • A pair of pedipalps, sensory appendages with tactile and defensive function.
  • Globular abdomen.

They differ from other arthropods in that they have neither antennae nor wings.

Their diet is usually carnivorous, they often inject poison with which they capture their prey.

Anatomía interna de un arácnido

Spider_internal_anatomy-en.svg : * Spider_internal_anatomy.png : John Henry Comstocktrabajo derivado: Pbroks13 (Ryan Wilson) ( hablar )Trabajo derivado: Xvázquez ( charla ) [CC BY 3.0], via Wikimedia Commons

Some of the most common arachnids are:

  • Spiders In the back of the abdomen they have structures, the rows, with which they make the spider web that they use to capture their prey.
  • Scorpions. They have very small chelicerae and pedipalps transformed into pincers. They have a segmented abdomen, with its end connected to a poisonous gland.
  • Mites. They have welded cephalothorax and abdomen. Ticks belong to this group.

Interactive activity: Parts of an arachnid.

Video: Scorpion molting (time lapse).

Class Crustaceans

The name of crustaceans comes from the Latin crusta, 'crust', due to the shell (usually a calcareous exoskeleton) that covers and protects them.
The body of crustaceans is divided into:
  • Cephalothorax (united head and thorax).
    • Two pairs of antennae.
    • Five pairs of legs.
    • A pair of moving eyes.
    • A pair of jaws.
  • Abdomen segmented and with appendages.
    • They may have non-jointed appendages for swimming, breathing, etc.
They are animals, generally aquatic, and although most are marine, they can also live in fresh and brackish water. They breathe through gills.
They are characterized by having two pairs of antennae and five pairs of legs, although the first one is usually transformed into a pair of pincers.
They are usually carnivorous, but there are also others that feed by filtering the water.
In their development, crustaceans undergo metamorphosis, with several larval stages before reaching adulthood.
Crustaceans include several groups of animals, such as lobsterscrabs, prawns, barnacles, and soil - dwelling sowbugs. 
Interactive activity: Complete the text about crustaceans.

Curiosity: the largest crab in the world

Do you want to see what the largest crab in the world looks like?

It is the Japanese spider crab (Macrocheira kaempferi).

Class Myriapods

The name myriapods comes from the Greek myria, "innumerable" and podos, "foot", thus indicating that this class of arthropods has many legs.

The millipedes are terrestrial, and your breathing is tracheal.

All myriapods are oviparous.

The body of myriapods is elongated and is made up of:

  • Head, containing:
    • Mouth, with chewing jaws and surrounded by taste buds.
    • pair of antennae.
    • Eyes.
    • First pair of legs modified into large claws, the forcipulas , associated with a poisonous nail.
  • Trunk divided into equal rings that bear a pair of legs (like centipedes) or two pairs of jointed legs (like millipedes).

Some myriapods curl up on themselves if they are in a dangerous situation.

The two most important groups of myriapods are:

  • Chilopods: Centipedes have a pair of legs for each ring, and their body is flattened. They are carnivorous, and the first pair of legs is adapted for capturing prey. For example, the scolopendra.

Escolopendra (Scolopendra morsitans)

B. Navez / CC BY-SA

  • Diplopods:  Millipedes have two pairs of legs in each ring and a cylindrical body. Most are herbivores or detritivores, few predators.


Interactive activity: Characteristics of myriapods.