Phylum Arthropoda

Phylum Arthropoda (arthro = joint; poda = foot) is the most numerous phylum of all living organisms, both in number of species and in number of individuals. One, very conservative, estimate is that there are well over one million species of insects alone. In terms of number of individuals, there are more ants than anything else, and in terms of numbers of species, there are more kinds of beetles than anything else: 40 to 50% of all insect species are beetles. There are more species of insects than all other plants and animals together.

An arthropod has a segmented body covered by an exoskeleton made from chitin and other chemicals. This exoskeleton serves as protection and provides places for muscle attachment. Arthropods must molt because their exoskeletons don’t grow with them. Arthropods have open circulatory systems consisting of a dorsal heart which collects blood from the body cavity and pumps it back into the body cavity again. In insects, the anterior portion of the heart (which is located in the abdomen) is extended into a tube that is called an aorta which directs the blood forward as it goes out into the body cavity. Arthropods have a well-developed, mesodermal, ventral, solid nerve cord and well-developed sense organs. The body feature from which the phylum takes its name is the jointed appendages, which include antennae and mouthparts as well as walking legs.

It is thought that the early arthropod ancestors (descended from organisms that looked like marine worms or, later, Peripatus) looked sort of like a centipede: they had a number of body segments, each with a pair of jointed appendages. From there, some of these segments became fused to form a head and some of the appendages became modified to form mouthparts or antennae. Early on, there was an evolutionary split which led to the various modern subphyla and classes. Currently, three living subphyla are recognized, with trilobites representing an extinct fourth subphylum.

Earthworm-like Ancestor:


Clamworm-like Ancestor

Peripatus-like Ancestor

Various Arthropods
Generic Crustacean:

Generic Arachnid:

Generic Insect:

• Subphylum Trilobita (tri = three) are all now extinct. Their bodies were divided sideways into three sections (the “lobes”) as well as a head and body regions.
• Subphylum Chelicerata (cheli = a claw, hoof) has the first pair of appendages modified as pincer-like mouthparts called chelicerae (these are the fangs in spiders). Their bodies are divided into a cephalothorax (cephalo = head) and an abdomen.
  • Class Xiphosura (xipho = a sword; ura = tail) are called horseshoe crabs because of the horseshoe-shaped carapace covering the cephalothorax. Their abdomen is thin, long, and pointed, hence the class name. They are marine, typically found in intertidal areas.
  • Class Arachnida (arachni = spider) includes scorpions, mites and ticks, daddy-long-legs, and spiders. They have simple eyes on top of their cephalothorax. The first pair of appendages is modified as chelicerae, small pincer-like mouthparts, which are further modified as fangs in spiders. The second pair of appendages is modified as pedipalps (pedi = foot; palpi = a feeler), which in spiders, look like small legs, but in scorpions are modified as large pincers (to capture food and for defense). Arachnids have four pairs of walking legs. Occasionally there are other appendages, like the spinnerets in spiders. Arachnids breathe via book lungs. Arachnid orders include:
    • Order Scorpionida, the scorpions, which have a poison sting at the tip of their abdomen, just above their anus,
      
    • Order Phalangida (phalang = finger, toe), the daddy-long-legs and harvestmen, which have long, slender legs, are not spiders, and do not spin webs, and
      
    • Order Araneida or Araneae (aranea, aranei = spider), the spiders, which have the chelicerae modified as fangs with poison glands (used to paralyze prey), have spinnerets on the abdomen from which they produce silk for their webs, and are beneficial predators
  
  
• Subphylum Crustacea (crusta = crust, rind) includes crayfish and lobsters, crabs, pillbugs, and several other groups. They have gills, thus terrestrial pillbugs need to maintain a 100% humidity environment around their gills to be able to “breathe.” Crustaceans have the head and thorax combined into one body region, the cephalothorax, as well as an abdomen. They have two pairs of antennae, mandible-type mouthparts (of different evolutionary origin than mandibles in insects), and other mouthparts which include two pairs of maxillae and three pairs of maxillipeds, all of which are formed from modified appendages.
  • Class Malacostraca, Order Decapoda (deca = 10) includes crabs, crayfish, and lobsters. They have five pairs of walking legs including a large pincer, the cheliped (ped = foot).
  
  
• Subphylum Atelocerata (formerly Mandibulata) includes insects, centipedes (centi = 100), and millipedes (milli = 1000). Centipedes and millipedes don’t really have 100 or 1000 legs, but they do have lots. Members of this subphylum have one pair of antennae, mandibles (mandibul = jaw), and two other pairs of mouthparts (either two pairs of maxillae or one pair of maxillae plus a labium), which are modified appendages. Note that their mandibles are modified appendages and are NOT homologous to our mouth or teeth.
  • Class Chilopoda (chilo = lip) is the centipedes. Their bodies are divided into head and trunk regions. The first pair of legs on the trunk is modified as poison jaws to capture and kill prey and assist mouthparts (also used for defense). On the trunk region, centipedes have one pair of appendages per segment. Centipedes are predatory/carnivores, and do have a poisonous “bite” to subdue their prey.
    
  • Class Diplopoda (diplo = double, two) is the millipedes. They also have head and trunk regions. Many of their trunk “segments” are actually two segments fused together into one apparent segment. Thus millipedes appear to have two pairs of legs per apparent segment. As their name would suggest, millipedes generally have more legs than centipedes, and the segmental pairing of legs is pretty obvious. Millipedes are scavengers, so it’s highly unlikely that one would bite if handled. Some millipedes are able to secrete small amounts of toxic chemicals like cyanide to protect themselves if threatened. While this is not enough to be a threat to humans, it has been reported that if an appropriate millipede is enclosed in a small jar and shaken, it will emit enough cyanide to kill itself and any insects placed in the jar with it (I do not recommend killing animals just to watch them die).
    
  • Class Hexapoda or Insecta (hexa = 6) is the insects. Insects have three body regions: head, three-segmented thorax (prothorax, mesothorax, and metathorax), and abdomen. Most also have both compound and simple eyes; mouthparts (covered by a labrum in front, and highly modified in some orders) which include mandibles, maxillae, and a labium; three pairs of walking legs, one on each of the three thoracic segments. Many insects have wings, which are NOT modified appendages, merely flaps of the exoskeleton of the meso- and metathoracic segments. There are about 28 to 30 orders of insects, depending on whose classification scheme you’re using.
    Insects exhibit one of two types of metamorphosis. Those with gradual metamorphosis change from egg to nymph to adult. In these insects (grasshoppers, roaches, true bugs), the nymphs look like miniature adults without wings, usually living in the same environment and eating the same food. Insects with complete metamorphosis go from egg to larva to pupa to adult (larva = ghost, specter; pupa = doll). Larvae of these insects look very different from the adults, usually live in a totally different environment and eat different food. The pupa is a “resting” stage where much transformation takes place. Probably the example of complete metamorphosis with which most people are familiar is that of a caterpillar (larva) changing to a chrysalis (pupa) then to a butterfly (adult).
    
    Many of the insect orders are easily distinguishable:

• Insects with gradual metamorphosis:
  • Order Odonata (odonto = a tooth, in ref. to projections on the mandibles) includes dragonflies and damselflies. These have aquatic immatures, and adults with long, narrow wings.
    
  • Order Orthoptera (ortho = straight; ptera = wing, feather) includes grasshoppers, katydids, and crickets. These have jumping back (metathoracic) legs. The front wings are modified as leathery tegmina, and the hind wings are membranous. Their mouthparts are “regular” chewing mouthparts. Orthopterans have “XO” sex determination: an individual with two X chromosomes (XX) is a female, and an individual with only one X chromosome (often referred to as XO where the “O” means “nothing there” — there is no Y chromosome) are males.
    
  • Order Dictyoptera (dictyo = a net) includes roaches, mantids, and walkingsticks. These are similar to orthopterans, but without jumping legs. They also have leathery tegmina. The forelegs of mantids are adapted for capturing prey. Many mantids and walkingsticks are well camouflaged, resembling sticks, leaves, or flowers.
    
  • Order Isoptera (iso = equal) is the termites. Their front and back wings (if present) are nearly the same size and shape. Termites have a caste system which includes both males and females, and they are colonial. The reproductive caste (kings and queens) initially have wings to swarm out and mate, then lose the wings as they settle down to found a colony. The workers can be distinguished from ants because they are whitish, more soft-bodied, and have no “waist.”
    
  • Order Hemiptera (hemi = half) is the true bugs. Their front wings are leathery on the basal half and membranous on the distal half, hence are called hemelytra. At rest, the overlapping wings form an “X” over the insect’s back. Bugs have long, pointed, piercing sucking mouthparts.
    
  • Order Homoptera (homo = same, like, alike) includes cicadas, aphids, leafhoppers, and scale insects. Many features of the Homoptera are similar to Hemiptera (mouthparts., etc.), but the front wings are membranous and held roof-like over the body at rest.
  
  
• Insects with complete metamorphosis:
  • Order Coleoptera (coleo = a sheath) is the beetles (note spelling!!!). Their front wings are hardened and shell-like, thus are called elytra (elytr = a sheath, cover). Beetles fly with their membranous hind wings, folding them under the elytra when at rest. Beetle larvae are often called grubs. Order Coleoptera has the most species of any insect order. Some species of beetle only reach 1 to 2 mm in size, while others are 4 to 6 inches with grubs so large they look like bratwurst with heads.
    
  • Order Lepidoptera (lepido = scale) includes butterflies, skippers, and moths. The larvae are called caterpillars. Butterflies and moths are characterized by colorful scales on the wings of the adults. The adults have siphoning mouthparts if present (some adult moths lack functional mouths and do not eat). SOME moths spin cocoons, and we get silk from one (Bombyx mori). The lowly silkworm is of great economic importance, and wars have been fought and empires founded on silk, not to mention Louis Pasteur’s career. Pupae in butterflies are called chrysalises. Many moths pupate underground, but those that do spin a cocoon subsequently molt to a pupa inside the cocoon, then molt to adult before coming out. Butterflies and moths use the “ZW” system of sex determination: females are heterogametic or ZW and males are homogametic or ZZ. Thus, the sex of the offspring is determined by the type of egg (Z or W) which the mother produces, not by the father’s sperm which are all Z.
    
  • Order Diptera (di = two) includes flies and mosquitoes. Their hind wings are modified as knob-like balance structures called halteres (singular = halter, halter = a weight held to give impetus in leaping), so they look like they have only one pair (= 2) wings. Most flies have sponging mouthparts (like houseflies that walk around “tasting” things as they go), although some have sharp, cutting mandibles. These bite a host, then sponge up the blood. The piercing-sucking mouthparts of mosquitoes are notorious, but interestingly, only female mosquitoes bite people: they need the protein from a blood meal for their developing eggs.
    
  • Order Siphonaptera (siphon = tube; a- = not, without) is the fleas. Adult fleas have sucking mouthparts and suck blood from their host (cat, dog, etc.). They are wingless and parasitic, with flattened bodies, the better to crawl between their host’s hair. Adults are also able to jump (which aids them in moving to a new host). Eggs laid by the females fall off into carpeting, the pet’s bedding, etc., where they hatch. The larvae live in rugs and the pet’s bedding and feed on organic material there. Eventually, they pupate in silk cocoons, often in the crevice between the rug and a table leg. In the Middle Ages, Bubonic Plague (Black Death) was spread by the bite of rat fleas infected with the bacterium.
    
  • Order Hymenoptera (hymeno = a membrane) includes bees, ants, and wasps. Unlike termites, worker ants have a “waist,” and they are darker color than termites. Many hymenopteran species are colonial with castes. Generally, workers are sterile females, and there are very few males in a colony. Most hymenopterans have a haplodiploid system of sex determination: males are 1n and females are 2n. Sex is determined by whether the egg was fertilized or not. Workers in many colonial species are sterile females and have stingers to defend the colony. A stinger is a modified ovipositor (ovi = egg; posit = placed), an “egg-layer,” thus males can’t sting. Many hymenopterans are of economic importance: honeybees make honey, wasps and hornets are beneficial predators, eating undesirable garden “pests,” and many species of bees, ants, and wasps are involved in pollinating many of the plants upon which we depend for food.

Insect Orders Matching Game

Try to match an insect picture with the name of the order in which that insect belongs. Click on two squares, and wait for feedback before clicking two more. When all the squares are uncovered, clicking anywhere on the picture will reload the game.