Phylum Arthropoda

2-D says, “Finally! You got here. Here’s where all my relatives and I fit in. There are more of us than of you, so that makes us really important. We insects live pretty-much everywhere on earth except out, deep in the ocean, but our cousins the Crustacea have that covered.”

Background Information:

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.

Cicada Molting
Molting Cicada
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.

Arthropod Evolution:

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
Worm Segments

Clamworm-like Ancestor

Peripatus-like Ancestor

Various Arthropods
Generic Arachnid:
Arachnid Appendages
Generic Crustacean: Crustacean Appendages
Generic Insect:
Insect Appendages

Arthropod Taxonomy and Characteristics:

Number of
Number of legs
& attachment
Number of body parts
& attachment
Subphylum Trilobitomorpha
Class Trilobita
These were most common during the Cambrian and Ordovician periods of geological history, and can be found preserved in a number of rock formations in the Cincinnati area. They are now extinct.
Subphylum Chelicerata
Class Merostomata
     horseshoe crabs
Class Arachnida
0 4 prs, att. to cephalothorax, (chelicerae & pedipalps are m.p.) cephalothorax & abdomen spider
Class Pycnogonida
     sea spiders
Subphylum Crustacea
Class Malacostraca
2 pair 5 prs incl. cheliped att. to cephalothorax, and swimmerets, m.p., incl. mandibles cephalothorax & abdomen
Note: there are quite a number of other classes of Crustacea not listed here.
Subphylum Atelocerata
Class Diplopoda
1 pair many, 2 pr per apparent segment bec of fused segm., m.p. incl. mandibles head and “trunk” segments, every two segments fused into one apparent segment
Class Chilopoda
1 pair many, 1 pr per segment, m.p. incl. mandibles & poison claw on next segm. head and “trunk” segments centipede
Class Pauropoda
Class Symphyla
Class Hexapoda (Insecta)
1 pair 3 pr, 1 pr. per thoracic segment, m.p. incl mandibles, some abdominal head, three-segmented thorax, segmented abdomen
(wings are not appendages)

Some further notes on various subgroups within Arthropoda:

Gradual Metamorphosis
Gradual Metamorphosis
Complete Metamorphosis
Complete Metamorphosis

Characteristics of the Orders of Class Hexapoda

This list only includes some of the more common insect orders. There are other, less common ones that have been left off the list.

Type of Front Wing Type of Back Wing Other Notes Photo
Subclass Entognatha
(endo, ento = within, inner; gnatho = the jaw), mouthparts within the head, primarily wingless, simple metamorphosis, no longer considered to be insects
none none (coll = glue; embola = a bolt or wedge) — collophore on bottom of 1st abdominal segm., for water uptake + furcula = jumping organ on ventral abdomen Collembola
Subclass Insecta
(ecto = outside, out, outer) ectognathous — mouthparts stick out from head
     (a- = not, without; ptero = wing, feather) primarily wingless, simple metamorphosis
none none (thysan = fringe) — somewhat flattened body, three taillike structures on posterior end, body often covered with scales  
     winged (a few are secondarily wingless)
          (exo = out, outside) — gradual metamorphosis, wing pads develop externally,
          young are called nymphs (naiads if aquatic) —
          Ephemeroptera, Odonata, and Plecoptera, which have aquatic naiads,
          are said to be hemimetabolous (hemi = half).
membranous membranous, smaller than front wings (ephemer = for a day, temporary) — aquatic naiads, winged subimago, then adult; very short-lived as adults  
long & narrow
long & narrow
(odonto = tooth) — have “teeth” on mandibles, aquatic naiads; chewing mp; long & slender damselfly
     leaf insect
leathery tegmina
(or absent)
(or absent)
(phasmato = apparition, phantom) — chewing mp; look like sticks or leaves, & well-camouflaged walkingstick
     camel cricket
leathery tegmina
(or absent)
(or absent)
(ortho = straight) — jumping back legs; chewing mouthparts grasshopper
leathery tegmina
(or absent)
(or absent)
(manti, mantid, mantis = a soothsayer, a kind of grasshopper) — chewing mp; front legs adapted for catching prey Chinese mantis
leathery tegmina
(or absent)
(or absent)
(blatta = cockroach) — chewing mp; legs adapted for running hissing roach
(or absent)
membranous; same size as front (or absent) (iso = equal) — light-colored; no “waist”; chewing mp; small size; social with castes, winged reproductives;  
shortened = brachypterous
(or absent);
leathery, called tegmina or elytra
membranous; folded under front wings (or absent) (derm = skin) — forceps-like cerci at end of abdomen earwigs
membranous membranous, “bottom” area folded under at rest (pleco = twine, twist, braid, twisted, folded) — aquatic naiads  
Suborder Mallophaga
     chewing lice;
Suborder Anoplura
     sucking lice
none none (phthir = lice) — ectoparasites (mostly on birds or mammals)  
     true bug
half-leathery, half-membranous hemelytra;
“X” when folded
membranous (hemi = half) — piercing-sucking mp; green stink bug
     scale insect
(or absent)
(or absent)
(homo = same, like, alike) — piercing-sucking mp periodical cicada
similar to each other (unlike true bugs), held rooflike or tentlike over body when at rest
          (endo = within, inner) — complete metamorphosis,
          wing pads develop internally until pupal stage, young called larvae
membranous membranous (neuro = nerve, sinew, cord) — named for wing veins; dobsonfly larvae are aquatic; many prey on other insects
(Photo © D.B. Fankhauser)
adult antlion
hard, shell-like elytra membranous (coleo = a sheath) — chewing mouthparts; largest order with ~40% of all insects Milkweed Leaf Beetle
membranous membranous (meco = long, length) — tip of male’s abdomen curls up, resembling shape of scorpion’s (photo is female); long, snout-like head female scorpionfly
none none (siphon = tube, pipe) — pupa in cocoon; blood-sucking; jumping; small & flat cat flea
membranous modified as halteres (di = two) — adults with sponging, cutting-sponging, or piercing-sucking mp robber fly
bright color due to scales bright color due to scales (lepido = a scale) — siphoning mp in adults, chewing in larvae (caterpillar) alfalfa butterfly
(or absent)
smaller than front
(or absent)
(hymeno = a membrane) — have a “waist”; chewing mp; many can sting; many social in colonies; often black & yellow bodies honey bee

Some further notes on various subgroups within Arthropoda:

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. OK... Javascript weirdness... it wouldn’t show the second picture clicked before “turning them back over.” So, I got that fixed – sort-of. It will now display the first two pictures clicked, then when you click on a third one, it will go back and check the first two. I know, that makes it easy to lose track of whether you’re on a “first” or “second” click, but eventually you should be able to get to the end of it. When you click anywhere on the squares after you match the last pair, the pictures will reload in a different order so you can try again.

#01 #02 #03 #04 #05
#06 #07 #08 #09 #10
#11 #12 #13 #14 #15
#16 #17 #18 #19 #20
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