Differences Between Insects and Spiders: Lesson for Kids

Differences Between Insects and Spiders: Lesson for Kids

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»Mom, Mom,» bellowed Ted, »Eugenia threw a nasty insect on me!»

»I did not,» said Eugenia. »It was a spider.»

Is a spider classified as an insect or not? How can you tell?

Arthropods

All bugs are not the same. There is a group of animals called arthropods. Arthropods are invertebrate animals that have a segmented body, jointed limbs, and an outer body that the animals occasionally shed.

Here are a few key points:

  • All insects are arthropods, but not all arthropods are insects.
  • Also, all arachnids are arthropods, but not all arthropods are arachnids.

If this confuses you, let’s try to un-confuse you with the first diagram. See, insects and arachnids are both arthropods, but not all of the arthropods are arachnids or insects.

Arthropods include insects and arachnids.

Insects

Insects are the largest group of arthropods there is, and in fact, the largest group of any animal on Earth. There are over one million different types of insects on Earth. But how can you tell if a bug is an insect?

  1. All insects must have three pairs of legs. That means they have a total of six legs.
  2. Insects can live almost anywhere on Earth.
  3. At some time in their life, an insect must have wings. Maybe not when they are young, but they may have wings when they are adults.
  4. All insects have three parts, or segments, to their bodies.

Parts of an Insect

The head for eating, seeing, smelling, hearing, and any other senses they may have. All the heads of insects have two antennas and two compound eyes. That means they have two eyes that each see many of the same images, which makes it easier to see motion.

The thorax helps insects get around. Their legs are attached to the thorax.

The abdomen is what insects use to reproduce.

Some examples of insects are ants, butterflies, flies, grasshoppers, and lightning bugs.

Arachnids

So, then what is a spider? A spider is in a group of arthropods called arachnids. Arachnids are a group of animals with the following traits:

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  1. Arachnids have four pairs of legs, for a total of eight legs. (Remember that insects have three pairs of legs or six legs in total)
  2. Arachnids never have wings. (Insects do at some point in their lives.)
  3. Arachnids do not have antennas. (Insects do!)
  4. Arachnids have two body sections. These are the cephalothorax (head and thorax together), and the abdomen. (Insects have three body sections.)

Parts of an Arachnid

The cephalothorax has the mouth, eyes, and is also where the legs attach.

Like insects, the abdomen is used in mating.

Other than spiders, arachnids also include scorpions, mites, and ticks

Lesson Summary

Arthropods are invertebrate animals that have a segmented body, jointed limbs, and an outer body that occasionally sheds. Arthropods include both insects and spiders. The differences in the two types of animals are found in the number of body sections, the number of legs, and if they have antennas and wings.

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Tick (Dermacentor sp.)

Detailing the physical features, habits, territorial reach and other identifying qualities of the Tick.

Updated: 8/23/2019; Authored By Staff Writer; Content В©www.InsectIdentification.org

Ticks are tiny, but their reputation for disease and animal irritation make their recognition an important skill.

Tick Videos

A large, flat Tick walking along door trim

Ticks are parasites that feed off the blood of a host. Almost any warm-blooded animal will suffice. As they feed, they release anticoagulants, special chemicals that prevent blood from clotting. Their mouths are so tiny that most hosts like people and dogs do not feel the bite. They might not even realize they were a host even after the tick leaves. Fortunately, the American Dog Tick is not known to carry Lyme Disease, the most popular affliction associated with ticks.

Ticks are not bugs, they are arachnids, which means they have 8 legs. Some ticks are round (like beans) while others are flat with festoons (folds that look like a ridge) around their abdomen. All ticks are parasitic. Their body shape color can greatly change after a blood meal. Flat, hungry ticks become inflated like a balloon after a full meal. Color may change also.

Some, but not all tick bites can develop a red ring, like a target, around the bite site. Complete and speedy removal of a tick from a host can help avoid the spread of tick-borne diseases. Tweezers are helpful in grasping the tick by the head so all the mouth parts are removed. Grabbing the tick by the body may result in breaking it off, leaving the head and mouth embedded in skin.

Some species of tick are capable of spreading disease to humans like Lyme disease and Rocky Mountain Spotted Fever. Although the infectious agent does not affect the tick, once it is passed through the tick’s saliva into the human bloodstream, it can cause an array of symptoms including fatigue, headaches, and general malaise. If you suspect you have been bitten by a tick, seeking the advice of a medical professional as soon as possible is prudent.

www.insectidentification.org

Arthropod

Arthropods Definition

An “arthropod” is an invertebrate animal that has an exoskeleton, a segmented body, and jointed appendages. The following families of organisms are all examples of arthropods:

    Insects such as ants, dragonflies, and bees Arachnids such as spiders and scorpions Myriapods (a term which means “many feet”) such as centipedes and milipedes Crustaceans such as crabs, lobsters, and shrimp
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It may help to remember that the term “arthropod” comes from the Greek words for “jointed foot.” If the organism has an exoskeleton with joints between its feet and its body, it is probably an arthropod!

Arthropods are a lineage of life that developed skeletons on the outside – their hard shells, made of a material called “chitin” – instead of on the inside for structural support.

Arthropods’ bodies also have other important differences from those of vertebrates like ourselves – their organ systems are simpler and less efficient, which limits the size arthropods can attain.

An ant the size of a human, for example, would not be able to pump oxygen through its blood to feed all its tissues, since the arthropod circulatory system is simpler and less efficient than a humans’.

All arthropods are thought to have evolved from a single common ancestor, though scientists are not sure what this common ancestor looked like, or exactly when it lived.

Arthropod Characteristics

Characteristics shared by all arthropods include:

    Exoskeletons made of chitin Highly developed sense organs Jointed limbs (the limbs must be jointed like the joints in a suit of armor, since the exoskeleton is rigid and cannot bend to allow movement) Segmented bodies Ventral nervous system. “Ventral” means “in front,” so this means that arthropods’ nervous systems run along the front of their bodies, near their stomachs, instead of along their backs like the spinal cords of animals. Bilateral symmetry. This means that the left and right sides of an arthropod are the same – it will have the same number and arrangement of legs, eyes, etc. on the right side of its body as on the left.

Types of Arthropods

Trilobites

Trilobites were an ancient family of marine arthropods that went extinct during the Permian-Triassic extinction event. Today, they are known to us mostly through fossils like the one below.

They lived on the ocean floor and occupied ecological niches similar to those occupied by crustaceans today.

Asaphus platyurus

Chelicerates

Chelicerata are a branch of the arthropod family tree that, at first glance, may not appear related to each other.

This family includes arachnids (such as spiders and scorpions), sea spiders (which look similar to arachnids but have some important differences), and horseshoe crabs (which, despite their name, have important differences from other crustaceans).

Myriapods

The term “myriapod” means “many legs” – so it is not surprising that centipedes, milipedes, and other many-legged creatures are part of this family.

Myriapods can have anywhere from less than ten legs – to over 750! That just seems excessive.

Myriapods are typically found in forests and other ecosystems where there is lots of decaying plant and animal material for them to feed on.

Crustaceans

Crustaceans are a family of primarily aquatic arthropods that include lobsters, crabs, shrimp, crayfish, barnacles, and the odd one out – wood lice, also known as pill bugs or “roly polys.”

Unlike their aquatic cousins, wood lice live mostly on dry land and are found in environments such as gardens and forests, where they survive by eating decaying plant and animal material.

It may also surprise you to see barnacles included on this list: adult barnacles develop hard shells that stick them to their surroundings, such as the bottoms of boats or other underwater surfaces.

But earlier in their lives before they freeze in place, barnacles have bodies with legs much like the other crustaceans!

Hexapods

The term “hexapod” literally means “six feet.” It might not surprise you to learn that insects – which all have six legs – are hexapods.

Insects include most “bugs” with six legs, such as flies, ants, termites, beetles, dragonflies, mosquitoes, cockroaches, butterflies, and moths.

There are also three much smaller groups of animals in the “hexapod” category. Collembola, Protura, and Diplura were all once considered to be insects, but later found to have small differences that set them apart from other insects.

Examples of Arthropods

When you think of a stereotypical arthropod body, you probably think of an ant. Ants have hard exoskeletons and jointed legs. They also have bodies which are clearly segmented into a head, thorax, and abdomen.

Ants show one type of social organization that has been developed by arthropods. Ants, bees, and termites are all what is called “eusocial” organisms – organisms living in extreme degree of cooperation, with “colonies” that almost operate like a single organism themselves.

Most arthropod species are not eusocial, but eusocial colony life is one of the fascinating roads that arthropod evolution has taken.

Spiders

Spiders are also arthropods, possessed of hard exoskeletons, segmented bodies, and jointed limbs.

Spiders typically eat smaller arthropods, such as gnats and flies – though they will eat any living thing they can catch, and some particularly huge spiders have been known to eat birds or rodents!

Spiders have evolved a variety of strategies for catching their prey – some spin sticky, nearly invisible webs that prey animals wander into and get stuck. Others are active hunters, including jumping spiders which can jump at extreme speeds using special mechanisms in their legs.

Some spiders combine these two strategies, such as “trap door” spiders, which set traps by creating hiding places for themselves – and then jumping out to grab unsuspecting prey animals that wander by!

Lobsters

With lobster being considered a luxury food today, it’s easy to forget that lobsters are in the same family as spiders and ants.

Crustaceans can grow bigger underwater than on land – and lobsters can grow to weigh nearly 50 pounds!

Lobsters’ body design has changed little in the last 100 million years, and their anatomy is spectacularly weird. The lobster’s kidneys are located in its head, its brain in its throat, and its teeth in its stomach. Its “ears” for picking up sound are located in its legs, and its tastebuds, like those of insects, are in its feet.

Butterflies

Butterflies are the most famous example of arthropod metamorphosis.

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At some point in their lifecycle, all arthropods go through a drastic change from their larval stage to their adult form. But butterflies are the only ones whose adult forms are so beautiful that we pay attention to this change.

The common features of exoskeleton, jointed limbs, and segmented body can be seen in adult butterflies.

Facts About Arthropods

    Arthropods colonized land about 100 million years before vertebrates did. It’s thought that colonizing land was easier for them for several reasons – including the fact that they had already evolved legs, which they used for walking on the bottom of the sea. About 80% of all animal species are arthropods! We don’t see them very often in our daily lives, but all the species of bugs and crustaceans on Earth add up! All arthropods undergo metamorphosis – a process where their bodies change radically as they pass from their larval to adult stages. Butterflies are the best-known for entering cocoons as caterpillars and coming out quite different, but all arthropods do something similar! When arthropods outgrow their old exoskeleton, they have to molt – leaving behind their former skin and growing a new one. All arthropods have to do this at least once in their lives. Crustaceans and arachnids – two types of arthropods – have blue blood instead of red blood!This is because their blood uses a blue copper compound to carry oxygen, instead of the red iron compound used by animals. Arthropods’ hard exoskeletons is made of chitin – which is made of a derivative of the sugar glucose! But chitin would not taste sweet, and you wouldn’t be able to eat it; to make it hard and strong, the glucose is modified so that our bodies no longer recognize it as sugar.

Related Biology Terms

  • Common ancestor – A common ancestor is an individual or species from which multiple individuals or species evolved.
  • Evolution – The process by which populations change over time, due to random mutation and the pressures of natural selection.
  • Extinction – The process by which a species ceases to exist after the death of its last member. Most species that have lived on Earth to date are now extinct.

1. Which of the following is NOT true of arthropods?
A. They have exoskeletons made of chitin.
B. They are symmetrical, having the same features on one side of their body as the other.
C. They colonized land long before vertebrate animals did.
D. None of the above.

2. Which of the following is NOT a type of arthropod?
A. Hexapods
B. Crustaceans
C. Cephalopods
D. Myriapods

3. Which of the following is NOT an arthropod?
A. A scorpion
B. A snail
C. A dust mite
D. A crab

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THE EXCRETORY SYSTEM


Table of Contents

Cells produce water and carbon dioxide as by-products of metabolic breakdown of sugars, fats, and proteins. Chemical groups such as nitrogen, sulfur, and phosphorous must be stripped, from the large molecules to which they were formerly attached, as part of preparing them for energy conversion. The continuous production of metabolic wastes establishes a steep concentration gradient across the plasma membrane, causing wastes to diffuse out of cells and into the extracellular fluid.

Single-celled organisms have most of their wastes diffuse out into the outside environment. Multicellular organisms, and animals in particular, must have a specialized organ system to concentrate and remove wastes from the interstitial fluid into the blood capillaries and eventually deposit that material at a collection point for removal entirely from the body.

Regulation of Extracellular Fluids | Back to Top

Excretory systems regulate the chemical composition of body fluids by removing metabolic wastes and retaining the proper amounts of water, salts, and nutrients. Components of this system in vertebrates include the kidneys, liver, lungs, and skin.

Not all animals use the same routes or excrete their wastes the same way humans do. Excretion applies to metabolic waste products that cross a plasma membrane. Elimination is the removal of feces .

Nitrogen Wastes | Back to Top

Nitrogen wastes are a by product of protein metabolism. Amino groups are removed from amino acids prior to energy conversion. The NH 2 (amino group) combines with a hydrogen ion (proton) to form ammonia (NH 3 ).

Ammonia is very toxic and usually is excreted directly by marine animals. Terrestrial animals usually need to conserve water. Ammonia is converted to urea, a compound the body can tolerate at higher concentrations than ammonia. Birds and insects secrete uric acid that they make through large energy expenditure but little water loss. Amphibians and mammals secrete urea that they form in their liver. Amino groups are turned into ammonia, which in turn is converted to urea, dumped into the blood and concentrated by the kidneys.

Water and Salt Balance | Back to Top

The excretory system is responsible for regulating water balance in various body fluids. Osmoregulation refers to the state aquatic animals are in: they are surrounded by freshwater and must constantly deal with the influx of water. Animals, such as crabs, have an internal salt concentration very similar to that of the surrounding ocean. Such animals are known as osmoconformers , as there is little water transport between the inside of the animal and the isotonic outside environment.

Marine vertebrates, however, have internal concentrations of salt that are about one-third of the surrounding seawater. They are said to be osmoregulators . Osmoregulators face two problems: prevention of water loss from the body and prevention of salts diffusing into the body. Fish deal with this by passing water out of their tissues through their gills by osmosis and salt through their gills by active transport . Cartilaginous fish have a greater salt concentration than seawater, causing water to move into the shark by osmosis; this water is used for excretion. Freshwater fish must prevent water gain and salt loss. They do not drink water, and have their skin covered by a thin mucus. Water enters and leaves through the gills and the fish excretory system produces large amounts of dilute urine .

Terrestrial animals use a variety of methods to reduce water loss: living in moist environments, developing impermeable body coverings, production of more concentrated urine. Water loss can be considerable: a person in a 100 degree F temperature loses 1 liter of water per hour.

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Excretory System Functions | Back to Top


  1. Collect water and filter body fluids.
  2. Remove and concentrate waste products from body fluids and return other substances to body fluids as necessary for homeostasis .
  3. Eliminate excretory products from the body.

Invertebrate Excretory Organs | Back to Top

Many invertebrates such as flatworms use a nephridium as their excretory organ. At the end of each blind tubule of the nephridium is a ciliated flame cell . As fluid passes down the tubule, solutes are reabsorbed and returned to the body fluids.

Excretory system of a flatworm. Image from Purves et al., Life: The Science of Biology , 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Excretory system of an earthworm. Image from Purves et al., Life: The Science of Biology , 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Body fluids are drawn into the Malphigian tubules by osmosis due to large concentrations of potassium inside the tubule. Body fluids pass back into the body, nitrogenous wastes empty into the insect’s gut. Water is reabsorbed and waste is expelled from the insect.

Excretory system of an ant. Images from Purves et al., Life: The Science of Biology , 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Vertebrates Have Paired Kidneys | Back to Top

ALL vertebrates have paired kidneys. Excretion is not the primary function of kidneys. Kidneys regulate body fluid levels as a primary duty, and remove wastes as a secondary one.

The Human Excretory System | Back to Top

The urinary system is made-up of the kidneys, ureters, bladder, and urethra. The nephron, an evolutionary modification of the nephridium, is the kidney’s functional unit. Waste is filtered from the blood and collected as urine in each kidney. Urine leaves the kidneys by ureters , and collects in the bladder . The bladder can distend to store urine that eventually leaves through the urethra .

Human excretory system and the details of the kidney. Images from Purves et al., Life: The Science of Biology , 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

The Nephron

The nephron consists of a cup-shaped capsule containing capillaries and the glomerulus , and a long renal tube . Blood flows into the kidney through the renal artery, which branches into capillaries associated with the glomerulus. Arterial pressure causes water and solutes from the blood to filter into the capsule. Fluid flows through the proximal tubule , which include the loop of Henle , and then into the distal tubule . The distal tubule empties into a collecting duct. Fluids and solutes are returned to the capillaries that surround the nephron tubule.

Filtration of the blood in the fine structure of the kidneys. Image from Purves et al., Life: The Science of Biology , 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

The nephron has three functions:

  1. Glomerular filtration of water and solutes from the blood.
  2. Tubular reabsorption of water and conserved molecules back into the blood.
  3. Tubular secretion of ions and other waste products from surrounding capillaries into the distal tubule.

Nephrons filter 125 ml of body fluid per minute; filtering the entire body fluid component 16 times each day. In a 24 hour period nephrons produce 180 liters of filtrate, of which 178.5 liters are reabsorbed. The remaining 1.5 liters forms urine.

Urine Production


  1. Filtration in the glomerulus and nephron capsule.
  2. Reabsorption in the proximal tubule.
  3. Tubular secretion in the Loop of Henle.

Components of The Nephron


  • Glomerulus: mechanically filters blood
  • Bowman’s Capsule: mechanically filters blood
  • Proximal Convoluted Tubule: Reabsorbs 75% of the water, salts, glucose, and amino acids
  • Loop of Henle: Countercurrent exchange, which maintains the concentration gradient
  • Distal Convoluted Tubule: Tubular secretion of H ions, potassium, and certain drugs.

Kidney Stones

In some cases, excess wastes crystallize as kidney stones . They grow and can become a painful irritant that may require surgery or ultrasound treatments. Some stones are small enough to be forced into the urethra, others are the size of huge, massive boulders (or so I am told).

Kidney Function | Back to Top

Kidneys perform a number of homeostatic functions:

  1. Maintain volume of extracellular fluid
  2. Maintain ionic balance in extracellular fluid
  3. Maintain pH and osmotic concentration of the extracellular fluid.
  4. Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.

Hormone Control of Water and Salt | Back to Top

Water reabsorption is controlled by the antidiuretic hormone (ADH) in negative feedback . ADH is released from the pituitary gland in the brain. Dropping levels of fluid in the blood signal the hypothalamus to cause the pituitary to release ADH into the blood. ADH acts to increase water absorption in the kidneys. This puts more water back in the blood, increasing the concentration of the urine. When too much fluid is present in the blood, sensors in the heart signal the hypothalamus to cause a reduction of the amounts of ADH in the blood. This increases the amount of water absorbed by the kidneys, producing large quantities of a more dilute urine.

Aldosterone , a hormone secreted by the kidneys, regulates the transfer of sodium from the nephron to the blood. When sodium levels in the blood fall, aldosterone is released into the blood, causing more sodium to pass from the nephron to the blood. This causes water to flow into the blood by osmosis. Renin is released into the blood to control aldosterone.

Disruption of Kidney Function | Back to Top

Infection, environmental toxins such as mercury, and genetic disease can have devastating results by causing disruption of kidney function. Many kidney problems can be treated by dialysis, where a machine acts as a kidney. Kidney transplants are an alternative to dialysis.

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