A Close Look at Arthropods

Learn everything about arthropods!

Arthropods Dissection Pre-AP Biology March 30, 2014- Ryan Xu

Today's Objective

We will observe and dissect arthropods and learn about the external and internal anatomy of arthropods. We will focus on the organs and structures inside the arthropods. We will also learn about the integumentary system of arthropods.


An arthropod is an invertebrates animal having an exoskeleton (external skeleton), a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda.The habitat of arthropods include lakes, bay heads, seasonal ponds, flatwoods and local pastures.

Interesting Facts

FACT: Living arthropods are divided into four subgroups.

Chelicerates (Subphylum Chelicerata) are a group of arthropods that include spiders, mites, horseshoe crabs, and scorpions. The first evolved about 445 million years ago during the Late Ordovician Period. Chelicerate bodies are divided into two regions, the cephalothorax and the opisthosoma.

Crustaceans (Subphylum Crustacea) include about 50,000 species of lobsters, crabs, shrimp, barnacles, brine shrimp and others. Most crustaceans live in marine or freshwater environments but there are also some terrestrial species.

Hexapods (Subphylum Hexapoda) are a group of arthropods that includes the insects. The body of a hexapod is divided into three sections, a head, thorax, and abdomen.

Myriapods (Subphylum Myriapoda) include about 13,000 species of millipedes and centipedes. Myriapods are terrestrial arthropods that are most abundant in forest habitats. They are notable for their long bodies and for having many pairs of legs.

FACT: Ancestral arthropods were the first land animals.

The oldest known animal to have lived on land is the arthropod Pneumodesmus newmani. Fossil evidence of this species consists of a single specimen that was discovered in 2004 in Scotland. Pneumodesmus newmani is classified as a millipede and is thought to have lived 428 million years ago, during the Late Silurian Period.

FACT: Arthropods have segmented bodies.

The body of an arthropod is divided into a series of semi-repetitive segments. Each segment bears a pair of appendages such as antennae, legs, wings, or mouthparts. Segmentation enables the arthropod body to develop specializations in each region of the body for specific functions such as feeding, sensory perception, locomotion, and visceral functions. The specialization of the various segments is known as tagmatization and each area of speciality is called a tagmata.

FACT: Arthropods have an exoskeleton.

The arthropod body is encased in a hard structure called an exoskeleton. The exoskeleton acts as an external skeleton and provides the animal with structural support and protection. The exoskeleton consists of cuticle, a multi-layered substance secreted by the epidermis. Cuticle consists of chitin, proteins and lipids. In many aquatic crustaceans, the exoskeleton is mineralized with calcium carbonate that is acquired from the surrounding water. This produces a tougher, more rigid structure. Since the exoskeleton is rigid, it must be jointed to allow the arthropod to move and acts as a framework to which the animal's muscles are attached.

FACT: Arthropods are a highly successful group of animals—they account for over three quarters of all currently known living and fossil organisms.

Of all animal groups, arthropods are the most diverse and their diversity is due in no small part to the diversity of one subgroup, the insects. Scientists approximate that there may be as many as 30 million species of insects alive today. To date, over one million have been identified. The success of arthropods has been attributed to their versatile exoskeleton, the process of metamorphosis, and their metameric body structure.

FACT: During their life cycle, arthropods undergo a transformation called metamorphosis.

Metamorphosis is a biological process that involves a radical transformation of body form and physiology. The immature form of an organism, called a larva, undergoes metamorphosis to become an adult. Since the larva and adult forms differ greatly in the way they live and what they eat, metamorphosis enables organisms to reduce competition for resources that otherwise would occur between the immature and adult forms.

FACT: Arthropods employ a variety of reproductive methods.

All terrestrial arthropods undergo internal fertilization and sperm is usually transferred to the female indirectly. Aquatic arthropods vary in their reproductive methods, with some species using internal fertilization and others external fertilization. Most arthropods lay eggs. Scorpions are the exception to this rule, they incubate their eggs internally and give birth to live young.

FACT: Arthropods have an open circulatory system.

Arthropods have an internal cavity called a hemocoel. The hemocoel provides a cavity in which the animal's internal organs are bathed in body fluids. This bathing enables the exchange of nutrients, waste material, and gases.

FACT: When arthropods grow, they must molt their exoskeleton.

Since the exoskeleton of an arthropod is a rigid structure, it must be shed and replaced periodically to enable growth. This shedding process is known as molting or ecdysis and consists of a series of four stages. First, enzymes are secreted that loosen the old exoskeleton layers. Second, the new exoskeleton layers are secreted. Third, the old exoskeleton splits and falls off. Finally, the new exoskeleton hardens and sets.

FACT: The Subphylum Trilobitomorpha is an extinct subgroup of arthropods.

The Subphylum Trilobitomorpha is a group of arthropods that were a dominant marine animal between 345 and 600 million years ago. Trilobitomorphs had two furrows that ran the length of their body and divided it into three regions: a left lobe, an axial lobe, and a right lobe. The most familiar of the trilobitomorphs were the trilobites. Trilobites (Class Trilobita) had three body sections, a cephalon, thorax, and pygidium. Trilobites are present in the fossil record from the Early Cambrian through the Late Permian. The lifestyles of triolobites were varied. Some species lived on the sea floor and others swam in the water column.

Integumentary System

Arthropods are invertebrates that have an exoskeleton. The exoskeleton is their integumentary system. The skin is hard, and made up of a tough polymer called Chitin. It acts like an armor, and protects them against predators. It also prevents their bodies from drying up, which helps them to survive in extreme dry conditions like the deserts.

The most common arthropods are insects, lice, mites, spiders, crustaceans and centipedes.

The exoskeleton has four layers, namely epicuticle, procuticle, epidermis and basement membrane. The topmost layer, or the epicuticle, serves to lock in water, while the procuticle is the layer that gives strength to the exoskeleton. The third layer, or the epidermis, is responsible in secreting the procuticle.

The muscles of the arthropods are attached to the exoskeleton. The exoskeleton is divided into head, thorax and abdomen. In spiders, their head and thorax are fused and is called Cephalothorax, as the exoskeleton is divided into cephalothorax and abdomen.

The exoskeleton has one disadvantage. It does not grow with the rest of the body. So, it has to molt or shed its skin when it gets too small for the body. Before the shedding process, the cuticle separates from the epidermis in the process called as Apolysis. Then the epidermis secretes a new cuticle, which is soft and takes time to harden. Hardening occurs with dehydration of the cuticle. The new skin will be pale, and it darkens as it becomes harder. In this molting stage, the arthropod has to be careful as it is vulnerable to attacks from predators.

Arachnid(Arthropod): Ecology and Habitats

Although most arachnids are inconspicuous free-living terrestrial forms, some ticks and mites are parasitic, a few spiders live on or near water, and some mites are aquatic. Most arachnids lead solitary lives, coming together only briefly for mating. Even though they possess a chitinous exoskeleton, most arachnids are subject to drying out (desiccation). Many arachnids, especially small little-known forms (ricinuleids, for example), are found only in well-protected habitats or niches. Thriving in the relatively constant, moisture-containing microclimates provided by soil litter, burrows, or caves, arachnids make up a high proportion of the animals found in dark or otherwise hidden environments. Cave-dwelling species often have special adaptations such as long extremities, light colour, and no eyes. Most arachnids, even those adapted to desert areas, avoid excessive heat by adopting a cryptozoic (hidden) habit and by being active only during the cooler parts of the day.

Evolutionary History of the Arthropods

The phylum Arthropoda is closely linked with two other phyla, the velvet worms or Onychophora, and the water bears, or Tardigrada. A nonliving cuticle coverers members of both phyla, and they have appendages. However, the cuticle is not hard, and the appendages are not jointed. They resemble worms in some ways – they are elongate, and their body regions are not highly differentiated. Examine the specimens of velvet worms on display. Living velvet worms are terrestrial, found in moist environments in the tropics. They are carnivores, tangling up their prey in a kind of sticky goo they secrete.

How Human Activities Affected Land Arthropods' Ecological System

In the olden days, farming activities went on at a pace that was natural to man; hence, there seemed to be a balance of existence between human beings, plants, and animals.

However, as world populations grew, the human's need for food increased and was demanded at a faster rate. Technological developments and advancements provided solutions that could increase food production in the agricultural sector. Land became widely used for commercial farming and grew increasingly important as a source of crops and vegetation.

However, the animals' ecological system was being disrupted and the habitats of most insects, arachnids, centipedes, and millipedes were being destroyed. Hedgerows were brought down to expand the crop fields to far reaching realms.

These land arthropods' natural instincts were to migrate and seek food elsewhere, where it could be found abundantly. It wasn't long before they came back and discovered their old habitats but in a new form. The area became a vast region of field crops, orchards, and vegetations where they could resume their ecological communities and perform their natural functions.

Perhaps these arthropods initially rejoiced at the full concentration of food supply in a single location. They came in swarms because there was enough food for everyone, but their arrival was considered as plagues and infestations. Bees, wasps, and locusts provided the most problem to the farmers, since they were quite aggressive and antagonistic in attacking human beings that tried to drive them away from their new found habitats.

Different insects and worms were considered pests, since they fed on crop yields, which lessened the expected agricultural production. Man’s solution was to introduce pesticides and herbicides concocted from chemicals that could eradicate the pest's existence. This modern solution was successful as they slowly eliminated crop infestation as a widespread agricultural problem.


Kingdom Animalia
Phylum Arthropoda
Class Chilopoda

Class Diplopoda

Class Insecta
Order Blattaria

Order Phthiraptera

Order Hemiptera
Family Reduviidae

Family Cimicidae

Order Coleoptera

Order Siphonaptera

Order Diptera

Family Culicidae

Subfamily Anophelinae
(Anopheles sp., Bironella sp., Chagasia sp.)
Subfamily Culicinae
(Aedes sp., Culex sp., Culiseta sp., Coquillettidia sp., Deinocerites sp.,Psorophra sp., Mansonia sp., Ochlerotatus sp., Orthpodomyia sp., Wyeomyia sp., Uranotaenia sp.)
Subfamily Toxorhynchitinae
(Toxorhynchites sp.)
Family Simuliidae

Family Tabanidae
Subfamily Chrysopsinae

Subfamily Tabaninae

Family Muscidae

Family Glossinidae

Family Calliphoridae

Family Sarcophagidae

Family Ceratopogonidae

Family Psychodidae

Order Lepidoptera

Order Hymenoptera

Class Arachnida
Order Araneae

Order Scorpionida

Subclass Acari

Order Ixodida
Family Argasidae

Family Ixodidae

Order Prostigmata
Family Trombiculidae

Family Democidae

Order Astigmata
Family Sarcoptidae

Family Pyroglyphidae

Order Mesostigmata