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.”
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
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
Arthropod Taxonomy and Characteristics:
|Number of legs
|Number of body parts
|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.
||4 prs, att. to cephalothorax, (chelicerae & pedipalps are m.p.)
||cephalothorax & abdomen
||5 prs incl. cheliped att. to cephalothorax, and swimmerets, m.p.,
||cephalothorax & abdomen
|Note: there are quite a number of other classes of Crustacea not listed here.
||many, 2 pr per apparent segment bec of fused segm., m.p. incl.
||head and “trunk” segments, every two segments fused into one
||many, 1 pr per segment, m.p. incl. mandibles & poison claw on next segm.
||head and “trunk” segments
Class Hexapoda (Insecta)
||3 pr, 1 pr. per thoracic segment, m.p. incl mandibles, some
||head, three-segmented thorax, segmented abdomen
(wings are not appendages)
Some further notes on various subgroups within Arthropoda:
- 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
- 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
- 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
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
|Type of Front Wing
||Type of Back Wing
ento = within, inner; gnatho = the jaw), mouthparts within the head, primarily wingless, simple metamorphosis, no longer considered to be insects
||(coll = glue; embola
= a bolt or wedge) — collophore on bottom of 1st
abdominal segm., for water uptake + furcula = jumping organ
on ventral abdomen
|(ecto = outside,
out, outer) ectognathous — mouthparts stick out from head
| (a- = not,
without; ptero = wing, feather) primarily wingless, simple
||(thysan = fringe) — somewhat
flattened body, three taillike structures on posterior end, body often
covered with scales
| winged (a few are secondarily wingless)
= out, outside) — gradual metamorphosis, wing pads develop externally,
young are called nymphs (naiads if aquatic) —
Ephemeroptera, Odonata, and Plecoptera, which have aquatic
are said to be hemimetabolous
(hemi = half).
||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
|(phasmato = apparition,
phantom) — chewing mp; look like sticks or leaves, &
|(ortho = straight) —
jumping back legs; chewing mouthparts
mantis = a soothsayer, a kind of grasshopper) — chewing mp; front
legs adapted for catching prey
|(blatta = cockroach) —
chewing mp; legs adapted for running
|membranous; same size as front (or absent)
||(iso = equal) — light-colored;
no “waist”; chewing mp; small size;
social with castes, winged reproductives;
|shortened = brachypterous
leathery, called tegmina or elytra
|membranous; folded under front wings (or absent)
||(derm = skin) — forceps-like
cerci at end of abdomen
||membranous, “bottom” area folded under at rest
||(pleco = twine, twist,
braid, twisted, folded) — aquatic naiads
||(phthir = lice) —
ectoparasites (mostly on birds or mammals)
|half-leathery, half-membranous hemelytra;
“X” when folded
||(hemi = half) —
|(homo = same,
like, alike) — piercing-sucking mp
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
||(neuro = nerve, sinew, cord)
— named for wing veins; dobsonfly larvae are aquatic; many prey
on other insects
(Photo © D.B.
|hard, shell-like elytra
||(coleo = a sheath) — chewing
mouthparts; largest order with ~40% of all insects
||(meco = long, length) — tip
of male’s abdomen curls up, resembling shape of scorpion’s (photo is female);
long, snout-like head
||(siphon = tube, pipe) —
pupa in cocoon; blood-sucking; jumping; small & flat
||modified as halteres
||(di = two) — adults with
sponging, cutting-sponging, or piercing-sucking mp
|bright color due to scales
||bright color due to scales
||(lepido = a scale) — siphoning mp in adults, chewing in larvae (caterpillar)
smaller than front
|(hymeno = a membrane) — have a
“waist”; chewing mp; many can sting; many social in colonies; often black
& yellow bodies
Some further notes on various subgroups within Arthropoda:
- Exopterygota: 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) is a group that
shows you that not even all entomologists totally agree on the “best”
classification scheme to use.
In the table, above, roaches, mantids,
and walkingsticks are each listed as being in their own orders
(Blattaria, Mantodea, and Phasmida, respectively), but you will also
find that those three orders are frequently combined in one order,
Order Dictyoptera (you may even run across some really-old entomology
book that puts all of these into Order Orthoptera).
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.
Talk about confusing taxonomy! This is it!
these days use “Hemiptera” and the following order, “Homoptera,”
but over the years, those names and “Heteroptera” (hetero =
other, different) have “flip-flopped” back and forth. I’ve seen
“Order Heteroptera” containing “Suborders Hemiptera and Homoptera,”
as well as “Order Hemiptera” containing “Suborders Heteroptera and
Homoptera,” while still other people don’t even use “Hemiptera” at
all, preferring to use just “Heteroptera” instead.
- 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 here,
the front wings are membranous and held roof-like over the body at
- Endopterygota: 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
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.
2-D says, “Here I am! Here I am! Hurray, you
finally found me. I’m so glad you’re here! If you have time,
check out the pictures and information from when Ms. Carter’s
General Biology Lab class raised
for an international research project.”
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
- 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.
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.
Copyright © 1997 by J. Stein Carter. All rights reserved.
This page has been accessed times since 19 Mar 2001.