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.


Subphylum Chelicerata


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.


Class Merostomata


(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.



Class Pycnogonida


(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



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.


Class Arachnida


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



Solifugae (Solpugida) -- sun "spiders", wind "scorpions"


Araneae -- spiders

The Acari (unranked, but within Class Arachnida) includes the following orders:

Parasitiformes -- ticks and parasitiform mites

Acariformes -- acariform mites


Subphylum Crustacea


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.


Class Remipedia


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.


Class Cephalocarida


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.


Class Branchiopoda


(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.


Class Malacostraca


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.


Subphylum Uniramia


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.


Class Chilopoda


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.


Class Diplopoda


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.


Class Insecta


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.





          Collembola (springtails)




          Thysanura (silverfish)


               Ephemeroptera (mayflies)

               Odonata (dragonflies)




                    Phasmida (stick and leaf insects)

                    Orthoptera (grasshoppers, crickets, katydids)

                    Mantodea (mantises)

                    Blattaria (cockroaches)

                    Isoptera (termites)

                    Dermaptera (earwigs)


                    Plecoptera (stone flies)



                    Psocoptera (book and bark lice)


                    Heteroptera (true bugs)

                    Homoptera (cicadas, aphids, scale insects)

                    Thysanoptera (thrips)


                    Neuroptera (lacewings, ant lions, dobsonflies, etc.)

                    Coleoptera (beetles)


                    Mecoptera (scorpion flies)

                    Siphonaptera (fleas)

                    Diptera (flies)

                    Trichoptera (caddisflies)

                    Lepidoptera (moths and butterflies)

                    Hymenoptera (ants, bees, wasps