The phylum Arthropoda contains animals with segmented appendages on their body segments. Arthropods occupy every habitat, and are in many respects the most successful animal group on Earth. There are conservatively over 1 million species of living arthropods. Biologist E.O. Wilson estimates there are 10 million species, 9 million of which are arthropods. Certain groups of arthropds have extremely complete fossil records.
Arthropod features that have contributed to their success, includes:
1. A hard exoskeleton, a strong but flexible outer covering composed primarily of the carbohydrate chitin. This functions in protection, attachment for muscles, locomotion, and prevention of desiccation.
2. Presence of jointed appendages. Trilobites, which flourished during Cambrian Period and were important animals in marine ecosystems for the remainder of the Paleozoic Era, had a pair of appendages on each body segment. Modern arthropod appendages are specialized for walking, swimming, reproduction, etc. These modifications account for much of the diversity and success of arthropods.
3. A complex nervous system with a brain connected to a ventral solid nerve cord. The head bears various sensory organs. Compound eyes have many complete visual units, each of which collects light independently. The lens of each visual unit focuses the image on light sensitive membranes of a small number of photoreceptors within that unit. In simple eyes (like our own), a single lens brings the image to focus into many receptors, each of which receives only a portion of the image.
4. A unique respiratory system that employs a variety of respiratory organs. Marine arthropods utilize gills composed of a vascularised, thin-walled tissue specialised for gas exchange. Terrestrial forms have book lungs (e.g., spiders) or tracheae (e.g., insects). Book lungs are invaginations to serve in gas exchange between air and blood. Tracheae are air tubes that serve as ways to deliver oxygen directly to cells.
5. A complex, yet adaptable, life cycle. Metamorphosis is a drastic change in form and physiology that occurs as an immature stage becomes an adult. Metamorphosis contributes to the success of arthropods because the larval stage eats food and lives in environments different from the adult; reducing competition between immature and adults of a species. Reduction in competition thus allows more members of the species to exist at one time.
The arthropod body consists of three major collections or zones of body segments:
Classification of Arthropods:
Due to their great diversity of appendages, lifestyles, and other features, arthropods are usually separated into several subphylums.
i. Subphylum Chelicerata:
The subphylum Chelicerata includes spiders, scorpions, ticks, mites, horseshoe crabs, etc. The first pair of appendages is chelicerae, second pair is pedipalps, and the next four pairs are walking legs. Chelicerae are appendages that function as feeding organs. Pedipalps are feeding or sensory in function; although in scorpions, they are large pincers. All appendages attach to a cephalothorax, a fusion of the head and thoracic regions. The head lacks antennae, mandibles, or maxillae appendages.
a. Class Merostomata:
The class Merostomata contains the extinct “sea scorpions” (or eurypterids) and the extant (living) horseshoe crabs. Eurypterids are extinct, but were important elements of faunas 200-500 million years ago during the Paleozoic Era. Some were huge, reaching a length of over 10 feet.
Some eurypterids may have been amphibious, emerging onto land for at least part of their life. Horseshoe crabs are an ancient group consisting today of only 5 species. Members of this class have a large shield that covers the cephalothorax. The compound eyes are reduced.
The second pair of appendages, the pedipalps, resembles walking legs. They have a long, spike-like appendage called a telson that projects from the rear of their bodies. Respiration is via book gills (precursors to book lungs?).
The horseshoe crab genus Limulus is a familiar sight along the east coast of North America. The anterior shield is a horseshoe-shaped carapace with two compound eyes. The long, unsegmented telson projects to the rear. They possess book gills that resemble the pages in a book. Limulus is considered a living fossil due to its great similarity to fossil forms from the Paleozoic Era.
b. Class Arachnida:
The class Arachnida includes over 60,000 described species (and most likely a very large number of as yet undescribed ones) of spiders (around 35,000 species), mites and ticks (25,000 species), scorpions (1200 species), and other forms. Nearly all arachnids are terrestrial.
Arachnids have a cephalothorax covered with a carapace-like shield. The abdomen may be segmented or unsegmented. Appendages on the abdomen are absent or modified, for example forming the spinnerets of spiders. Respiration is via tracheae or book lungs. Scorpions are arachnids.
They are the oldest terrestrial arthropods known from fossils. All scorpions are nocturnal and spend most of the day hidden under a log or rock. Their pedipalps are large pincerlike appendages, and their abdomen ends in a stinger containing venom.
Spiders have a narrow waist separating the cephalothorax from the abdomen. Spiders have numerous simple eyes rather than compound eyes. The chelicerae are modified as fangs with ducts from poison glands.
The abdomen has silk glands used to spin a web to trap prey. Invaginations of the body wall form lamellae (pages) of the book lungs; air flows across the lamellae in the opposite direction from blood flow to exchange gases more efficiently.
ii. Subphylum Crustacea:
The Subphylum Crustacea, contains 30,000 mostly marine species. A few species live in freshwater. Lobsters, crabs, crayfish, shrimp, copepods, barnacles, and several other groups of organisms belong to this subphylum.
All crustaceans possess two pairs of antennae, a pair of mandibles, a pair of compound eyes (usually on stalks), and two pair of maxillae on their heads, followed by a pair of appendages on each body segment. Crustacean bodies usually have a head, thorax, and abdomen. Crustaceans utilize gills for gas exchange.
Most crustaceans are free-living, but some are sessile and a few are even parasitic. Some crustaceans filter tiny plankton or bacteria from the water, while others are active predators. A few crustaceans scavenge nutrients from detritus.
Many species, including lobsters, crayfish, barnacles, and crabs are economically important (yum, yum). Krill, and a few other species, form the base of extremely important marine food chains. Still others are crucial in recycling nutrients trapped in the bodies of dead organisms. The subphylum contains several taxonomic classes.
The class Malacostraca is the largest taxonomic class of Crustaceans, having over 20,000 primarily marine species. Some malacostracans are freshwaters, while others occupy diverse terrestrial habitats. Typical malacostracans include sowbugs, krill, and a very large order, the Decapoda that contains many kinds of shrimp, crabs, and crayfish.
Malacostracans typically possess a body with eight thoracic and six abdominal body segments, each bearing a pair of appendages. Class Malacostraca contains a number of economically significant species, such as edible lobsters, shrimp, crayfish and crabs. Many malacostracans contribute to plankton and as such are at the base of an immensely important marine food chain.
iii. Subphylum Uniramia:
This subphylum contains arthropods that have unbranched appendages. The uniramian body has two or three tagmata, and an abdomen that has many segments. Appendages in the head region include paired antennae and mandibles, and also two pairs of maxillae. Gas exchange is by means of tracheae and spiracles. This subphylum includes millipedes, centipedes, and insects.
a. Class Chilopoda:
This taxonomic class includes 20 families and more than 2500 species of centipedes, all terrestrial. Most centipedes are small, but a few can attain a length of up to 10 inches (25 cm). Centipedes have bodies which are made up of a chain of many (up to 177) flattened segments.
With the exception of the segment behind the head and the last body segment, each segment has a single pair of appendages (legs). The appendages of the first body segment have been modified to form large, poisonous fangs that are used to capture prey. The bite of a large centipede, however, can be painful to an adult and dangerous to a small child.
b. Class Diplopoda:
Millipedes comprise this class containing some 8000 species. Bodies of members of this class are made up of numerous segments. Millipedes lack poisonous fangs and do not bite. Prerdators are discouraged by the millipede’s rolling into a defensive ball. Production of poisonous or foul-smelling substances also serves to dissuade any predators. Most millipedes are apt to burrowing herbivores or scavengers.
c. Class Insecta:
Insects are the largest group, with probably over one million identified and named species. Insects live in almost all terrestrial and freshwater habitats, with a few species living in the oceans.
Many insects have some thoracic appendages modified for flight. Insects are important as pollinators for flowering plants, as well as for the damage they do annually to crops, and the diseases they transmit (malaria, some forms of encephalitis, Dengue Fever, the West Nile virus, etc.).
Insects display a wide huge variation in body styles, although there seems to be a size limit on the insect-style of body organisation.
Common features shared by most living insects include:
I. Body composed of three tagmata
V. One pair of relatively large compound eyes
VI. Usually three ocelli located on the head
VII. One pair of antennae on the head
VIII. Mouthparts consisting of a labrum, a pair of mandibles, a pair of maxillae, a labium, and a tonguelike hypopharynx
IX. Two pairs of wings derived from outgrowths of the body wall
X. Three pairs of walking legs
Insects have a complete, complex digestive system. They exchange gases through a tracheal system, with external openings called spiracles dividing into finely branched tubules that carry gases directly to metabolising tissues. Aquatic forms may exchange gases through the body wall or may have various kinds of gills.
Excretion of nitrogenous waste takes place via Malpighian tubules. The nervous system of insects is complex, including a number of ganglia and a ventral, double nerve cord. Sense organs are complex and acute. In addition to ocelli and compound eyes, some insects are quite sensitive to sounds, and their chemoreceptive abilities are excellent.
Growth patterns are quite variable. Some insects hatch from eggs as miniature adults, which in turn shed their exoskeleton. Most insect species have newly hatched young that are completely different in appearance from adults. These larval forms usually live in different habitats, eat different foods, and look completely different from their adult stages.
When larval growth is completed, the larva stops feeding and builds a case or cocoon around itself. In this non-feeding condition (pupa or chrysalis) the larva undergoes a complete transformation or “metamorphosis” of its body form, eventually emerging as a fully- formed adult.
Insects are very valuable to us. While insects eat our food, feed on our blood and skin, contaminate our dwellings, and transmit diseases, we could not exist if they were not here.
Insects are a vital part of our ecosystem, functioning in:
I. Pollination of many flowering plants.
II. Decomposition of organic materials.
III. Recycling of carbon, nitrogen, and other essential nutrients.
IV. Control of populations of harmful invertebrate species (including other insects).
V. Direct production of certain foods like honey.
VI. Manufacture of useful products such as silk and shellac.