Phylum Arthropoda
Arthropods include an incredibly diverse group of taxa such as insects,
crustaceans, spiders, scorpions, and centipedes. There
are far more species of arthropods than species in all other phyla combined, and
the number of undescribed species in the
largest assemblage of arthropods, the insects, probably numbers in the tens of
millions. Members of the phylum have been
responsible for the most devastating plagues and famines mankind has known. Yet
other species of arthropods are essential for
our existence, directly or indirectly providing us with food, clothing,
medicines, and protection from harmful organisms.
A number of important characteristics are shared by most members of this phylum.
Arthropods are bilaterally symmetrical
protostomes with strongly segmented bodies. Segmentation affects both external
and internal structure. Some segments are
fused to form specialized body regions called tagmata; these include the head,
thorax and abdomen, and the process and
condition of fusion is called tagmosis. The body is covered with an exoskeleton
made up primarily of the protein chitin; lipids,
other proteins, and calcium carbonate also play a role. Primitively, each body
segment bears a pair of segmented (jointed)
appendages; in all living arthropods, many of these appendages are dramatically
modified or even lost. Arthropods generally
grow by molting their exoskeletons in a process called ecdysis. Movement of
appendages is controlled primarily by a complex
muscular system, divided into smooth and striated components as in chordates.
Cilia are not present. Most arthropods have a
pair of compound eyes and one to several simple ("median") eyes or ocelli;
either or both kinds of eyes may be reduced or
absent in some groups. Arthropods are eucoelomate with the coelom formed by
schizocoely, but the volume of the coelom is
much reduced and usually restricted to portions of the reproductive and
excretory systems. Most of the body cavity is an open
"hemocoel,"
or space filled loosely with tissue, sinuses, and blood. The circulatory system
is open and consists of a heart,
arteries, and the open spaces of the hemocoel. The gut is complete. Respiration
takes place through the body surface, and/or
by means of gills, tracheae, or book lungs. The nervous system is annelid-like,
with a brain (=cerebral ganglion) and a nerve
ring surrounding the pharynx that connects the brain with a pair of ventral
nerve cords. These cords contain numerous ganglia.
Most arthropds are dioecious and have paired reproductive organs (ovaries,
testes). Fertilization is internal in most but not all
groups. Most lay eggs, and development often proceeds with some form of
metamorphosis.
These are the scorpions, spiders, mites, horseshoe crabs, and "sea spiders."
Their bodies have two major subdivisions, an
abdomen and a cephalothorax. Chelicerates have six pairs of appendages, which
are uniramous (unbranched). These include a
pair of chelicerae, a pair of pedipalps, and four pairs of walking legs.
Chelicerates lack mandibles and antennae. Respiration is
by means of book gills, book lungs, or tracheae.
(horseshoe crabs, eurypterids)
The Merostomata includes two rather different groups of marine organisms, the
eurypterids and the horseshoe crabs.
Eurypterids are now extinct; they lived 200 to 500 million years ago. Some were
huge, reaching a length of 3 m. Their
morphology suggests that they fed on a variety of kinds of foods. Some may have
been amphibious, emerging onto land for at
least part of their life cycle. The horseshoe crabs are an ancient group, but
only 5 species exist today. They feed on small
invertebrates. Horseshoe crabs are often used as laboratory animals by
physiologists.
Members of this class have a large shield that covers the cephalothorax. The
compound eyes are reduced. The second pair of
appendages, the pedipalps, resemble 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.
(sea spiders)
Sea spiders appear to be a sort of marine "spider," but in fact their
relationships are enigmatic. They may represent a very early
branching of the chelicerate lineage. There are approximately 1000 described
species of pycnogonids, all of which are marine.
Pycnogonids can be found from the intertidal regions to depths of around 7000 m.
Most are small, but a few deep-sea forms
reach up to 70 cm diameter across the legs. They feed by sucking juices from
soft-bodied invertebrates through a long
proboscis.
Pycnogonids vaguely resemble spiders, with small bodies and relatively long,
hinged legs. Unique characteristics include an
unusual proboscis, which varies in size and shape among species, but amounts to
a chamber with an opening at the distal end
(the true mouth lies between the proboscis chamber and the esophagus). The body
itself is not divisible into neatly- organized
tagmata or regions as it is in most other arthropods. An anterior region bears,
besides the proboscis, three or four pairs of
appendages, including the first pair of walking legs. Special appendages called
ovigers when present make up the four pair;
these play a role in the brooding of young and are used in cleaning. Following
the first segment is a series of segments making
up a "trunk," each segment bearing a pair of walking legs. A terminal segment
includes a tubercle that projects dorsally and an
anus. Some species have more than four pairs of walking legs. Pycnogonids are
also unique in bearing multiple gonopores,
found on the second segment of some or all of the walking legs.
This large Class of arthropods includes over 60,000 described species (and most
likely a very large number of so-far
undescribed ones). Spiders make up the majority of these (over 50,000 described
species); with mites and ticks next largest
(around 48,200 species). The Arachnida also includes a diverse array of smaller
groups, including scorpions (1200 species),
whip scorpions (100 species), palpigrades (60 species), pseudoscorpions (2000
species), solpugids (900 species), and
harvestmen (5000 species). Nearly all species are terrestrial.
Arachnids have a pair of tagmata called a prosoma and opisthosoma. The prosoma
is partially or completely covered with a
carapace-like shield. The opisthosoma may be segmented or unsegmented. The
appendages on the opithosoma are absent or
modified, being used as spinnerets (spiders) or pectines (probably sensory in
function, found in scorpions). Respiration is via
tracheae or book lungs; it is cutaneous in many small arachnids.
The Arachnid Class contains the following orders:
Eurypterida (extinct)
Scorpiones -- scorpions
Pseudoscorpiones (Chelonethida)
Opiliones (Phalangida) -- daddy long legs
Uropygi -- whip "scorpions", vinegaroon
Amblypygi -- tailless whip scorpions
Schizomida
Palpigradi
Solifugae (Solpugida) -- sun "spiders", wind "scorpions"
Ricinulei
Araneae -- spiders
The Acari (unranked, but within Class Arachnida) includes the following orders:
Parasitiformes -- ticks and parasitiform mites
Acariformes -- acariform mites
Approximately 30,000 species make up this Subphylum. Most are aquatic; of these,
the majority are marine but some are
found in fresh water. Members of the Subphylum include lobsters, crabs,
crayfish, shrimp, copepods, barnacles, and several
other groups of organisms. All have 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 are
made up of head, thorax, and abdomen, although the segments composing these
tagmata differ among different Classes). The
appendages are primitively branched (biramous), and although this condition is
modified in many species, adults always have at
least some biramous appendages. Crustaceans respire via gills. Like other
arthropods, all have a hard but flexible exoskeleton.
Most crustaceans are free-living, but some are sessile and a few are even
parasitic. Most use their maxillae and mandibles to
take in food. The walking legs, including specialized chelipeds, may be used to
help capture prey. Some crustaceans filter tiny
plankton or even bacteria from the water; others are active predators; while
still others scavenge nutrients from detritus.
Most crustaceans are dioecious. The actual mechanisms by which fertilization is
achieved vary greatly. Some crustaceans hatch
young that are like miniature adults; others go through a larval stage called a
nauplius.
Many species, including lobsters, crayfish, barnacles, and crabs are important
to human economies, some very much so.
Others, such as krill, are at the base of extremely important marine food
chains. Still others are crucial in recycling nutrients
trapped in the bodies of dead organisms.
These primitive crustaceans, which were discovered in submerged caves only in
1980, have a long trunk made up of many
segments, each bearing a pair of biramous swimming appendages. Not much is known
of their biology.
These tiny crustaceans make a living by feeding on benthic marine detritus. Only
nine species are known. The second pair of
maxillae of cephalocarids closely resembles the appendages of the thorax. In
this and other characteristics cephalocarids
resemble what we imagine primitive crustaceans might have been like.
Cephalocarids also lack abdominal appendages. They
have small compound eyes that are buried in the exoskeleton, rather than being
raised on stalks as in most other crustaceans.
As in branchiopods and malacostracans, cephalocarids feed by generating currents
with their thoracic appendages. These
currents bring in food particles, which are trapped and and passed anteriorally
along a ventral groove leading to the mouthparts.
(fairy shrimp, water fleas, etc.)
The four orders that make up this class include brine shrimp, tadpole shrimp,
water fleas, and clam shrimp. Most live in fresh
water or in small, salty lakes or ponds. Most are small (under 2 cm). The number
of segments and appendages possessed by
an individual varies considerably among groups, but in most forms the number of
segments is large and tagmosis is minimal.
About 800 species are known; most feed by filtering small organisms and organic
particles from the water. Currents that move
water for feeding are generated by thoracic appendages, and food particles are
passed anteriorally along a ventral groove
leading to the mouthparts.
With over 20,000 species, Malacostraca is by far the largest Class of
Crustaceans. Its members, which are primarily marine
but also occupy some freshwater and terrestrial habitats, are extremely diverse.
They include isopods (sowbugs), amphipods,
euphausiids (krill), beachhoppers, mantis shrimp, and a very large order, the
Decapoda, that contains many kinds of shrimp,
crabs, and crayfish. Malacostracans are characterized by a trunk with eight
thoracic and six abdominal somites, each bearing a
pair of biramous appendages.
As in branchiopods and cephalocaridans, primitive malacostracans feed by
filtering food from water; currents that move water
for feeding are generated by thoracic appendages. Food particles are passed
anteriorally along a ventral groove leading to the
mouthparts. Other malacostracans are predatory, capturing prey by a variety of
means and tearing it into edible pieces with
their appendages. Yet others scavenge organic material by scraping it from the
substrate, from living fish in the case of cleaner
shrimp, or even from fallen leaves and other plant material in the case of
terrestrial isopods. A few species are even parasitic,
living on other crustaceans or on fishes.
The Class Malacostraca contains a number of species of considerable economic
significance. These include edible lobsters,
shrimp, crayfish and crabs. Equally or more significant are the many
malacostracans which, as larvae or adults, contribute to
plankton and as such are at the base of an immensely important marine food
chain.
Uniramians are arthropods whose appendages are unbranched. Most appendages are
made up of several articulating pieces.
The uniramian body has two or three tagmata, and the abdomen contains numerous
segments. The head appendages include
paired antennae and mandibles, and also two pairs of maxillae (the second pair
may be fused or sometimes absent). "Breathing"
is by means of tracheae and spiracles. The sexes are separate, but most other
aspects of reproduction are extremely varied.
This enormous group include millipedes, centipedes, and insects, plus two small,
primitive Classes, Pauropoda and Symphyla.
Centipedes are uniramian arthropods whose bodies are made up of a chain of many
(up to 177) flattened segments, each
except the one behind the head and last two bearing 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.
Centipedes are predatory, feeding on soil invertebrates such as earthworms and
terrestrial insects. All centipedes are terrestrial,
but they require moist microhabitats. Fertilization is internal, with
spermatophore transferred in ways similar to many arachnids.
Centipedes lay eggs, which in some species are carefully brooded by the female.
When they hatch, the young resemble
miniature adults.
Centipedes are a diverse group, including some 20 families and over 2500
species. Most are small, but a few attain up to 10
inches in length.
Like centipedes, millipedes have bodies that are made up of numerous segments.
The first four thoracic segments each bear a
single pair of legs, but the following abdominal segments all have two pairs.
Millipedes lack poisonous fangs and do not bite;
rather, to discourage predators they roll into a defensive ball and many emit
poisonous or foul-smelling substances.
Most of the approximately 8000 species of millipedes are herbivorous or
scavengers, living primarily on decaying plant and
animal matter in moist microhabitats. They are adept and powerful burrowers.
Like centipedes, female millipedes lay eggs in
nests, which are often carefully guarded. Newly hatched millipedes usually have
only 3 pairs of legs, adding legs and body
segments with each molt as they grow.
With around one million named species and perhaps several times that number
unnamed, insects account for a great majority of
the species of animals on earth. They are a tremendously successful group.
Insects can be found in almost all terrestrial and
freshwater habitats, from the driest deserts to freshwater ponds, from the
canopy of a tropical rainforest (where their diversity is
unbelievably great) to the arctic wastes. A few species are even marine. Their
feeding habits are similarly varied; almost any
substance that has nutritive value is eaten by some group of insects.
Insects also show huge variety in shape and form. Almost the only condition
their group does not attain is very large body size.
A number of features, however, are shared by most kinds of living insects. In
addition to the general characteristics of
uniramians, these include a body composed of three tagmata, a head, thorax, and
abodmen; a pair of relatively large compound
eyes and usually three ocelli located on the head; a pair of antennae, also on
the head; mouthparts consisting of a labrum, a pair
of mandibles, a pair of maxillae, a labium, and a tonguelike hypopharynx; two
pairs of wings, derived from outgrowths of the
body wall (unlike any vertebrate wings); and three pairs of walking legs.
Insects have a complete and complex digestive tract. Their mouthparts are
especially variable, often complexly related to their
feeding habits. Insects "breathe" through a tracheal system, with external
openings called spiracles and increasingly finely
branched tubules that carry gases right to the metabolizing tissues. Aquatic
forms may exchange gases through the body wall or
they may have various kinds of gills. Excretion of nitrogenous waste takes place
through Malpighian tubules. The nervous
system of insects is complex, including a number of ganglia and a ventral,
double nerve cord. The ganglia are largely
independent in their functioning; for example, an isolated thorax is capable of
walking. Yet ganglia also use sensory output. A
grasshopper with one wing removed can correct for this loss and maintain flight,
using sensory input from its brain. 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 astounding.
Insects are dioecious and fertilization is internal in most. The ways in which
mating is accomplished, however, are incredibly
variable; study of this variability by evolutionary biologists has greatly
advanced our understanding of the evolution of behavior,
social evolution, and traits such as number, size of young and patterns of
investment in them. Reproduction by insects often
involves a male locating a receptive female through chemicals (pheromones)
released by the female. In most species, females
store the sperm in a special receptacle in their abdomens; even species that lay
huge numbers of eggs (in honeybees, for
example, the number may be over one million), females mate only once and rely on
sperm stored during that mating for the rest
of their lives.
The manner in which growth is accomplished is an especially important
characteristic of insects. In some, hatching eggs produce
miniature adults, which to grow must shed their exoskeleton in a process called
ecdyisis. In almost 90% of insect species,
however, newly hatched young are completely different in appearance from adults.
These larval forms usually live in different
habitats, eat different foods, and assume a body form completely different from
that of their parents. The larva feeds and grows,
molting its skin periodically. At some point larval growth is completed, the
larva stops feeding and builds a case or cocoon
around itself. In this nonfeeding condition it is called a pupa or chrysalis.
While so encased, the larva undergoes a complete
transformation or "metamorphosis" of its body form, and a fully-formed adult
emerges. Insects that experience this sort of
complete change are called "holometabolous." Other species undergo a more
gradual process, in which the newly hatched
young are more similar to the adult but are small in size, lack wings, are
sexually immature, and may differ in other, relatively
minor ways as well. The young in these insects are called nymphs, and the
lifestyle is referred to as "hemimetabolous."
Insects are incalculably valuable to man. Usually, we think of them in a
negative context. Insects eat our food, feed on our
blood and skin, contaminate our dwellings, and transmit horrible diseases. But
without them, we could not exist. They are a
fundamental part of our ecosystem. A brief and incomplete list of their positive
roles would include the pollination of many,
perhaps most higher plants; the decomposition of organic materials, facilitating
the recycling of carbon, nitrogen, and other
essential nutrients; the control of populations of harmful invertebrate species
(including other insects); the direct production of
certain foods (honey, for example); and the manufacture of useful products such
as silk and shellac.
There is no general agreement on the details of how different groups of insects
are related. The following is modified list is the current listing of the
insects used in Zoology Class at South Dakota State University.
Entognatha
Diplura
Collembola (springtails)
Protura
Insecta
Microcoryphia
Thysanura (silverfish)
Paleoptera
Ephemeroptera (mayflies)
Odonata (dragonflies)
Neoptera
"Orthopteroids"
Grylloblattaria
Phasmida (stick and leaf insects)
Orthoptera (grasshoppers, crickets, katydids)
Mantodea (mantises)
Blattaria (cockroaches)
Isoptera (termites)
Dermaptera (earwigs)
Embiidina
Plecoptera (stone flies)
"Hemipteroids"
Zoraptera
Psocoptera (book and bark lice)
Phthiraptera
Heteroptera (true bugs)
Homoptera (cicadas, aphids, scale insects)
Thysanoptera
(thrips)
Holometabola
Neuroptera (lacewings, ant lions, dobsonflies, etc.)
Coleoptera (beetles)
Strepsiptera
Mecoptera (scorpion flies)
Siphonaptera (fleas)
Diptera (flies)
Trichoptera (caddisflies)
Lepidoptera (moths and butterflies)
Hymenoptera (ants, bees, wasps
SCORPION ANATOMY
SPIDER ANATOMY
WATER FLEA
LOBSTER ANATOMY
BASIC INSECT ANATOMY
BUTTERFLY ANATOMY
BEETLE ANATOMY
WASP ANATOMY
WALKING STICK ANATOMY
GRASSHOPPER ANATOMY
