8.28.4 Lesson: Arthropods and Other Invertebrates

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Arthropods and Other Invertebrates


H.1 – Identify the characteristics of Kingdom Animalia and its various phyla.


Book Lungs

Pair of organs used by terrestrial spiders for respiration


Hard shell of an arthropod


Region that contains a fused head and thorax


Pair of pincher-like mouth parts found in arachnids

Complete Metamorphism

Four progressive stages of insect development including the egg, larva, pupa, and adult


During the day


Internal skeleton of insects that is made up of calcium


Rigid outer covering of an arthropod’s body that provides support and protection


Fluid equivalent to blood that is transported through an open circulatory system

Incomplete Metamorphism

Three progressive stages of insect development including the egg, nymph, and adult


During the night


Series of plates that compose the endoskeleton and are covered by tough skin


Elongated mouthpart in an insect that siphons nectar or other liquids

Phylum Arthropoda

As we continue our examination of invertebrates, we move to another large, diverse group of organisms known as arthropods. Within this phylum are nearly 900,000 species! This includes spiders, insects, centipedes and millipedes, crabs, and shrimp.

An arthropod is characterized by its segmented body, jointed appendages/feet and exoskeleton, which is a rigid outer covering that provides support and protection.

The left-hand image below depicts the main segments of an arthropod’s body (head, thorax, abdomen); the middle image displays an example of a jointed appendage; the right-hand image shows the exoskeleton of a female scorpion.


Another characteristic of arthropods includes an open circulatory system with a dorsal heart and arteries. This means that the heart and arteries are located on the upper side (back) of their bodies. Once blood is pumped by the heart, it moves through the body cavities to reach the internal organs. The blood is considered hemolymph, implying that it is sent straight to the tissues and does not get transported back to the heart due to the lack of veins.

On the flip side of the arthropod’s body is the ventral nervous system, which receives and sends information using sensory organs.

Organisms are further differentiated within the phylum based on the shape of their wings, size/shape of their bodies, parts of their mouths, or by developmental stages.

Subphylum Crustacea

The body plan of a crustacean, like all other arthropods, is segmented. However, the head and thorax can be combined into a single region called the cephalothorax. This portion of the organism is covered by a single plate known as the carapace. Animals in this subphylum are also identified as possessing gills. Examples include shrimp, lobsters, crabs, and pill bugs.

Subphylum Chelicerata

Representatives of the next group include spiders, scorpions, and horseshoe crabs. The cephalothorax region of the next subphylum is known as the prosoma. Each segment on their bodies has a pair of legs. Though these organisms lack antennae, they possess pincher-like mouth parts, called chelicerae.

The respiratory system of these animals differs between aquatic and terrestrial forms. Aquatic forms use gills to exchange gases while the terrestrial forms use both trachea and book lungs. Book lungs are very thin membranes that are located on the abdomens of arachnids. They are plate-like in structure and allow for the circulation of air and exchange of gases.

Class Arachnida

Spiders, scorpions, ticks and mites form a category of arachnids that is comprised of nearly 100,000 identified species! Characteristics of these organisms include:

  • Four pair of walking legs
  • Cephalothorax and abdomen
  • Lack of antennae
  • Use of book lungs for gas exchange
  • Four pair of simple eyes

Other characteristics that are specific to spiders: one pair of chelicerae with poison fangs, one pair of pedipalps (sensory receptors) used by the male to transfer sperm, and the ability to create a web for trapping prey and laying eggs.

Other species-specific characteristics include the poisonous nature of scorpions, the tick’s ability to carry disease (such as Rocky Mountain Spotted Fever) and the mite’s role in causing scabies.

Class Insecta

Insects outnumber all other animals on Earth! Included in this class are butterflies, moths, flies, grasshoppers, beetles, and centipedes just to name a few. Each organism has an exoskeleton, six legs, three body segments (head, thorax and abdomen), and two sets of wings (if wings are present). Insect mouth parts are dependent upon the insect’s diet. Herbivores have chewing mouth structures while nectar-drinking insects have siphoning mouth structures.

Insect Digestion

Insects have a three-part digestive system that consists of a foregut, midgut, and hindgut. The food enters the animal through the mouth where salivary glands aid in moisturizing it as it moves through the esophagus into the crop. The food then proceeds to the gizzard, which uses plates for grinding it into smaller particles.

The gizzard opens to the stomach (the midgut) which is lined with small organs called gastric ceca. The ceca provide the stomach with digestive juices to further break down the food particles. Anything that is not digested moves to the hindgut, or intestine. The waste is passed out of the body through the anus after the removal of water.

Insect Respiration

Insects do not breathe through their mouths, rather they have a complicated system of internal tubes called tracheae that run the length of their bodies. The tracheae open to the outside environment through pores called spiracles along the insect’s sides. Air enters the tracheae and gases move through system into the tissues by diffusion.

Order Orthoptera

Grasshoppers, crickets, locusts, and katydids can be further differentiated by incomplete metamorphism. Incomplete metamorphism is when the insect develops in three stages: the egg, nymph, and adult. During the nymphal stages, the animal looks like a smaller version of the adult.

Members of this class have long jumping legs and straight wings. Their diet consists of plants, giving them the potential to have major economic impact considering the consumption of crops.

Order Coleoptera

Beetles form the most diverse order of insects and are characterized by complete metamorphosis. Complete metamorphism is a four-stage developmental cycle that includes the egg, larva, pupa, and adult.

These organisms are predatory insects that have chewing mouth pieces, an outer set of shell-like wings and an inner set of fragile wings.

Order Lepidoptera

Moths and butterflies are members of the next order, both of which undergo complete metamorphosis. The larval form of these organisms is the caterpillar, which has chewing mouth parts. The adult form is the moth or butterfly. Adult butterflies possess a proboscis mouth structure, which is a coiled tube used to siphon nectar and other liquids.

Moths are primarily nocturnal, or active at night, while butterflies are diurnal, or active during the day. Due to this, moths are generally less colorful than butterflies.

Order Odonata

Order Odonata consists of carnivorous insects. These animals exhibit incomplete metamorphosis with a nymph form called the naiad. Examples include dragonflies and damselflies, both of which have membranous wings.

Order Hemiptera

“True bugs” compose the next order of insects. These animals exhibit incomplete metamorphosis, with a nymph stage that looks similar to the adult but without wings. However, not all adult members have wings. Another characteristic of these insects is the presence of mouth parts that are used for piercing or sucking. Examples include cicadas and aphids (pictured below).

Order Diptera

Flies and mosquitoes undergo complete metamorphosis with a larval form known as a “maggot.” If wings are present on the adult forms, they are found in a single pair. The mouth parts on these animals are used for piercing and contribute to the ability to carry diseases such as malaria and dysentery.

Order Hymenoptera

Wasps, sawflies, bees, and ants undergo complete metamorphosis and have bodies that are covered with small hairs used for pollination. Depending on the species, the insect can have mouth parts for chewing or for siphoning.

Other Invertebrates

As we wrap up our discussion of invertebrates, we will examine one last phylum: Phylum Echinodermata. Organisms in this group include sea stars, sand dollars, sea cucumbers, and sea urchins. These animals have bilateral symmetry during the larval stage of development and radial symmetry during the adult stage of development. Unlike all previously discussed organisms, they have an endoskeleton that is made of calcium. This internal support structure is made of ossicles, which are a series of plates covered by tough skin in the form of an epidermis. The exterior of the organism is spiny in structure and appearance.

Echinoderms have an interesting mechanism used for movement and feeding. Tube feet, which have sucker-like tips, protrude out of the ossicles and are used to move along the bottom of the ocean as the organism uses its teeth to eat algal matter.

Gender is separated for these organisms. A female’s eggs can be fertilized by the male’s sperm either internally or externally. Several forms of echinoderms can also reproduce asexually by methods of regeneration.

Class Asteroidea

Starfish are the most well-known echinoderms. They lack cephalization and instead exhibit radial symmetry by possessing five or more rays around a central disc. A starfish can regenerate from small pieces of itself as long as the central disc is present.

The starfish has a top side and backside. The top side, also known as the aboral side, is completely covered with spines used for protection. An opening, called the madreporite, is used for the intake of water and the removal of waste through the tube feet as the organism moves. This side also contains the eyespots.

The back side, also known as the oral side, contains the mouth towards the center of the starfish’s body. The diet of a starfish includes oysters, clams, sand dollars, mussels and small fish. Their methods of feeding prove interesting: the starfish first opens a clam by using its rays and then extends its own stomach out of its mouth and eats the contents of the mollusk whole. The starfish then re-swallows its stomach and the digestive process continues within its own body.

Class Echinoidea

Sea urchins and sand dollars lack rays/arms. Ossicles on these organisms are fused, functioning more towards protection rather than movement. Sea urchins are found in rounded shapes and have tube feet and spines all over their bodies. Sand dollars are found in flattened shapes and have spines covering their bodies, though the spines are typically much smaller. Like the starfish, these organisms have an oral surface that contains the mouth.

Class Holothuroidea

Sea cucumbers are long slug-like animals that lie on the ocean floor and eat small food/waste particles from the bottom of the sea. They can reproduce sexually, through the fertilization of an egg by male sperm, or asexually by regeneration. Like other members of this phylum, they have an endoskeleton but their plates are widely spaced.

These animals have an interesting defense mechanism. When threatened, they expel their own internal organs from their anus. See the video clip below to watch this form of defense in action.



Class Ophiuroidea

The final class of echinoderms is composed of brittle stars and basket stars. These animals have slender arms that surround a central disc. Though the arms are rather thin, they can be regenerated if broken. Brittle and basket stars are capable of quick movement, aiding in the capture of small fish and other organisms for food.

The video clip below shows two brittle stars fighting over a shrimp.



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