Five Characteristics of an Arthropod

The phylum Arthropoda is the largest and most diverse group of living creatures, containing more species than all the other phyla combined. The exact number of species in this group is unknown, but it is estimated to be in the tens of millions. Insects, spiders, crustaceans and scorpions are all part of this group. As diverse as the species in Arthropoda are, they do all share some common traits in terms of body shape, support structure, growth, internal organs and reproduction.

  1. Body shape

    • Arthropods, as a general rule, are bilaterally symmetrical meaning that one side of the animal is identical to the other side. Body segmentation of some type is present in all arthropods but may be more pronounced in some species. Segments can be fused to create tagmata, which are larger, more defined body sections such as heads, thoraxes and abdomens. Segments generally also possess at least one pair of legs, which can vary from species to species, but which are usually jointed.

    Support Structure

    • Unlike mammals, fish, birds and reptiles who have internal skeletons to support their body structures, all arthropods have exoskeletons. This is an external form of skeleton that holds the soft internal tissues inside the animal. This exoskeleton is made up mostly of chitin along with lipids, proteins and calcium carbonate. The exact structure of an exoskeleton varies. Some are soft and thin, while others are dense and hard, forming a more protective shell such as in crabs.

    Growth

    • In order to grow, arthropods have to cast off their exoskeletons, which are fixed as a particular size and shape. In some species this can happen numerous times with a new exoskeleton forming when the old one is cast off. In others, the same process may happen only once and is more of a metamorphosis such as a caterpillar becoming a butterfly.

    Internal Organs

    • Internally, arthropods all have digestive, circulatory, respiratory and reproductive organs of some form or another. The space between these organs can be empty or filled loosely with blood, tissue or sinuses. In many species breathing is accomplished through small holes in the surface of the exoskeleton that lead to tubes called tracheae.

    Reproduction

    • Generally, arthropods are dioecious, meaning the species has both male and female members. Reproduction therefore usually takes place via internal fertilization although some species do exhibit external fertilization. Arthropods start life as an egg and will likely go through some metamorphosis during their lifetimes.