Unit 8: Invertebrates                                                    1   2   3   4   5   6

5. Arthropods

a) Habitat

     Arthropods represent close to 90% of all animal species (more than one million).

     They are the largest and most varied group of animals.

     Most part of them is terrestrial and some are aquatic, of sea and fresh water.

b) Morphological characteristics

- They have bilateral symmetry.

-   They have a segmented body covered by a rigid exoskeleton

 (an external skeleton of chitin)

-   Their body is divided into three parts: head, thorax and abdomen.

 In some groups head and thorax are joined and form the cephalothorax.

-   They have jointed legs, that can be used for walking, swimming, hopping, digging,

    or catching prey.

-   They have appendages (mouthparts, antennae, legs, wings…)

c) Vital functions:

     Nutrition:

The digestive system has a mouth, anus and glands. They can be herbivorous or carnivorous. The mouthparts of arthropods are adapted to different styles of feeding. In general, arthropods have mouthparts for cutting, chewing, piercing, sucking and licking.

In addition, they can have other appendages near the mouth with different characteristics in every group. For example, spiders have chelicerae that inject digestive juices directly into prey, and insects have pedipalps.

Aquatic species breathe through gills, and terrestrial species breathe through tracheae. Tracheae form an exclusive respiratory system. It consists on a system of tiny tubules that branch into the body, bringing oxygen directly to cells.

Arthropods have an open circulatory system.

Interaction

They have a well-developed brain and sense organs: eyes (compound and simple), ears, pedipapls and antennae. Antennae are sensory organs which can feel smells, flavours, heat or touch. They are attached on the head between the eyes and their size and shape can vary.

Exoskeleton gives protection from the outside environment and strong points for muscle attachment on the inside. The hard exoskeleton does not grow or stretch. As the animal grows, it needs to shed its old exoskeleton a number of times (moulting). After the old exoskeleton has been moulted, the soft covering underneath hardens and becomes the new exoskeleton.

Reproduction:

Most have male and female sexes.

They are oviparous.

Fertilization is internal.

Larva hatches and undergoes metamorphosis to become adult.

d) Classification:

The scientific criteria used for classifying them are based on the number of legs, antennae and body parts that they have.

- Arachnids (E. g. Spiders, scorpions, ticks and mites).

- Crustaceans (E.g. Lobsters, crabs, prawns, etc)

- Myriapods (E.g. Centipedes, milipedes, etc).

- Insects (E. g. Butterflies, ants, bees, grasshoppers, beetles, etc).

5.1. Complete the following table about Arthropods.

Then, listen and relate each sentence with the correspondant group.

5.2. Identify every type of mouthparts with its correspondent picture.

        Then, listen and indicate which types of mouthparts are described.

5.3. Listen and identify which characteristic of Arthropods is described:

5.4. Listen and complete the text: