Field guide training: Arthro­pods – Part 1

Arthropods probably belong to the most extraordinary and mysterious animals on the planet. Even though many arthropod species are known, biologists assume that a high number of species has not yet been discovered. Due to their important function for the ecosystem, a detailed knowledge about their behaviour and characteristics is a must for every field guide and everyone interested in nature.

The group of the arthropods which includes insects, spiders and shellfish is by far the most diverse group of all animals. Unique forms, colours and behaviours have developed over time within the category of arthropods and no other group of animals has managed to conquer almost every area of the planet including regions with hostile environment.

Despite the great diversity, there are some characteristics which all arthropods have in common:

-They all have an outer skeleton made of chitin which is renewed through ecdysis
-Single body segments are not flexible, only the joints
-The body segments are coalesced and can be separated in different body parts depending on the species:

1. Head, thorax and abdomen are separate
2. Head, thorax and abdomen are coalesced (Tagmata)
3. Head and thorax are coalesced (Cephalothorax), the abdomen is separate

• Their extremities are in pairs and bound to the body segments. The extremities fulfill different functions (running, swimming, eating, sexual organs, etc.)
• Cephalization (formation of the head) is very distinct. The head is often equipped with compound eyes
• There are special organs for breathing (trachea, gill)
• The haemocoel is the part of the body where arthropods store their circulatory system
• They have an open circulatory system with only a few vessels

Arthropods can be divided into four main groups, separated by the number of legs, antennas and body parts.

1. Insects (Insecta) with 6 legs, 2 antennas, 3 body parts (head, thorax, abdomen)
2. Spiders (Arachnida) with 8 legs, no antennas, 2 body parts (cephalothrax, abdomen)
3. Shellfish (Crustacea) with up to 8 legs, 4 antennas, and different numbers of body parts
4. Centipede (Myriapoda) with up to 18 legs, 2 antennas, head and a segmented body

Being able to characterize especially spiders and insects is essential when staying in the wild and training to become a field guide. Not only because they are ubiquitary, meaning they can be found almost everywhere, and play a vital role for all ecosystems, but also because some of them are poisonous and can be dangerous for animals and humans.