Arthropods are a group of animals forming the phylum Euarthropoda. The group includes insects, crustaceans, myriapods, and arachnids. They are all invertebrate animals.
Segmented body, paired jointed appendages, and the presence of an exoskeleton are some of the features characterizing the phylum. Most of these animals are bilaterally symmetrical. They exhibit the greatest diversity among all animal groups. Arthropods account for more than 80% of all extant species on Earth. The species are distributed in a variety of ecosystems ranging from marine and freshwater to land and air.
Some of the main characteristics of arthropods have been mentioned below:
7. Reproduction In Arthropods
Except for a few species, most arthropods have separate sexes. Each sex has paired sex organs that open onto the ventral surface of the trunk via ducts. Sperms in these creatures are usually protected within sealed packets called spermatophores that ensure that the sperms are not diluted by water in aquatic environments and not desiccated in terrestrial environments. In some species, spermatophores are released on the ground while in others they are deposited during a nuptial dance between the species. However, in most species, the males deposit them directly into the female genital opening during copulation. Some species have specialized and unique methods of reproduction. In some species, parthenogenesis can be observed while others give birth to living young instead of eggs.
6. Nervous System In Arthropods
Arthropods have a very basic nervous system consisting of a brain located dorsally and a ganglionated longitudinal nerve cord on the ventral side. Nerves extend laterally into each segment from the nerve cord.
5. Open Circulatory System In Arthropods
Arthropods have an open circulatory system. Their heart is placed dorsally. The arthropods have either an extensive or limited arterial system that delivers blood into hemocoels or tissue spaces. The heart and the arteries are linked via paired openings or ostia where blood movement is controlled by valves. Crustaceans and arachnids have blue blood as it contains the oxygen-carrying pigment hemocyanin. In insects, such pigments are lacking since oxygen is delivered to the tissues directly from the tracheal system. Hemoglobin is, however, present in the blood of a few small crustacean species and insect larvae.
4. Respiratory System In Arthropods
Terrestrial arthropods possess tracheae and book lungs as respiratory organs. The former is a network of tiny tubes that allow gases to find their way to the interior of the body. The latter are internal pockets lined by chitin with blood-filled plates. Air brought in by the tracheae circulates over these plates allowing the exchange of gases. Aquatic arthropods, on the other hand, have gills for respiration. Although the gills which are outgrowths of the skin are covered by an exoskeleton, it is thin in this area permitting gaseous exchange.
3. Diverse Feeding Habits
Arthropods have diverse feeding habits. There are herbivores, carnivores, parasites, detritus feeds, etc., in this group. Usually, appendages that aid in feeding are present around the mouth. Each species has appendages specialized per the dietary habit of the species. For example, fiddler crabs have small claws that allow them to scoop up surface sand and place it within their mouth where tiny hairs are used to sift the sand to keep the organic parts and expel the mineral material. On the other hand, aphids have appendages that are used to pierce vegetation to suck out plant juices.
Unlike most other invertebrates, arthropods lack locomotory cilia at all stages of life. They move with the co-ordinated action of their exoskeleton, appendages, and muscles. Their exoskeleton is segmented to allow for movement. The appendages of these animals are specially adapted to allow different types of locomotion like walking, swimming, burrowing, running, etc. Some insects also possess wings that help them fly.
Arthropods possess a hard, nonliving, exoskeleton that not only provides protection and support of the creature but also helps in locomotion. The exoskeleton is made of two layers, an inner thick chitin-protein layer (procuticle) and an outer protein layer (epicuticle). To allow the arthropod to grow, the exoskeleton is periodically shed in a process called molting or ecdysis, and new exoskeleton is generated.