Arthropods

Spiders and Insects

The phylum arthropoda includes insects (hexapoda), spiders (arachnids), crustaceans, and myriapods. These animals have bilateral symmetry, segmented body parts, hard exosketletons, jointed legs, many pairs of legs.

The class arachnid includes spiders, ticks, mites, and scorpions.  These animals have two body parts, eight legs, and do not have wings or antennae. 

Crustaceans include sow bug (also called pill bugs and roly-poly) shrimp, crayfish, crabs, lobsters, and barnacles.  They have two body regions, 10 or more legs, two pair of antennae, a segmented body, hard (chitinous - like a grasshopper) exoskeleton, paired jointed limbs, and no wings. 

Myriapods include the class chilopoda and diplopoda. 

The chilopoda include the centipede with worm like bodies, segmented bodies, one pair of legs on each body segment and one pair of antennae or none. 

The diplopoda include the millipedes with worm like bodies, segmented bodies, two pairs of legs on each segment, and one pair of antennae or none.

The insects have segmented bodies, jointed legs, an exoskeleton and three body regions.  1) Head which includes the mouth parts, eyes, and antennae.  2) Thorax which has three pairs of legs and usually 1-2 pairs of wings.  3) Abdomen which is many segments and includes the digestive, excretory, and reproductive systems.

Insects are useful for pollination, produce useful substances like honey, silk, wax, dyes, and pigments.  They control other pests, are scavengers, provide food, and are used in genetic studies.  Four-fifths of all animals on the Earth are from this class.  There are over one million species of insects.  There may be from 500-2000 insects on one square meter of land.  This make a total of 4,000,000 on one acre.

Insects have many harmful effects.  One of the most recognized is the damage they do to crops.  They eat millions of dollars of crops every day.  As incredible as this seems, most insects will feed only on certain plants and some only on one variety.  One example of an insect that has been in Nebraska for millions of years and not done much damage until recently is the Colorado potato beetle.  This insect normally feeds on the buffalo burr plant, but has adapted to feed on potatoes.  This bad news for the potato growers and good news for the buffalo burr.  Besides eating crops insects spread disease.  Flies infect many animals.  The screw worm infects cattle, the mosquito infects humans with sleeping sickness, yellow fever, malaria, and encephalitis.  Fleas infect people with bubonic plague and lice with typhus.  Some insects also harm humans by stinging and may cause allergic reactions.

The growth changes insects pass through are called metamorphosis (change in form).  There are four types of metamorphosis.  1) Growth without metamorphosis is when the insect looks exactly like the adult form when it is hatched from the egg.  Insect orders of this type include thysanura (silverfish and firebrats), collembola (springtails), mallophaga (chewing lice), and anoplura (sucking lice).  The three growth changes are called egg, young, and adult.  2) Gradual metamorphosis where insects change shape gradually through three stages with each stage looking more like the adult.  Orders of this type include orthoptera (grasshoppers, crickets, preying mantids, walking sticks, and cockroaches), isoptera (termites), corrodentia (book lice), thysanoptera (thrips), hemiptera (true bugs), homoptera (scales, cicadas, and aphids), and dermaptera (earwigs).  The three stages are egg, nymphs, and adult.  3) Incomplete metamorphosis where the insects change shape gradually but do not look like adults until they shed their last skin and then they change fast.  Orders of this type include ephemeroptera (May flies), odonata (dragon and damsel flies), and plecoptera (stonefly).  The three stages are called egg, nymph, and adult.  4) Complete metamorphosis is when the insect changes shape through four stages of growth.  The young do not look like the adult and there is a great change in shape from the pupa stage to the adult stage.  Orders of this type include neuroptera (aphid lions, ant lions,dobson flies), coleoptera (beetles), strepsiptera, mecoptera (scorpion flies), tricoptera (caddis flies), lepidoptera (butterflies and moths), diptera (flies), siphonaptera (fleas), and hymenoptera (bees, wasps, and ants).  The four stages are the egg, larvae, pupa, and adult. 

Insect eggs vary greatly in color and shape.  Only a few may be produces or masses of a thousand or more deposited.  The embryo usually develops from a fertilized egg but in some cases (wasps and bees) the unfertilized egg produces a male.  The caterpillar bites its way out of the egg.  The flea has a hatching spine that splits open the shell.  Larvae is the stage that the insect does much damage to plants.  They eat very much as they are storing enough energy to become an adult.  Some types of larvae are caterpillars, grubs, wire worms, and maggot.  The pupa is the resting stage of the insect before it becomes an adult.  In this stage there may be a cocoon, a case that looks like a seed, or it may have a small immature bug like shape.

Insects protect themselves by secreting poison, stinging, and protective coloration.  Insects control their body temperature by the way they position themselves relative to the sun.  The temperature 38-40 degrees Celsius is the temperature they must warm their wings to for optimum flight.

Insect orders

  • Coleoptera - beetles   
  • Lepidoptera - butterflies and moths  
  • Hymenoptera - bees, wasps, and ants 
  • Diptera - flies 
  • Orthoptera - grasshoppers, crickets, preying mantis, walking sticks, and cockroaches
  • Hemiptera - true bugs 
  • Homoptera - scales, cicadas, and aphids
  • Odonata  - dragon and damsel fliesMallophaga   chewing lice
  • Tricoptera - caddis flies
  • Neuroptera - aphid lions, ant lions, and dobson flies
  • Siphonaptera - fleas and mosquitoes
  • Corrodentia - book lice 
  • Ephemeroptera - may flies
  • Dermaptera - earwigs
  • Thysanoptera - thrips
  • Anoplura - sucking lice 
  • Mecoptera- scorpion flies 
  • Collembola - springtails
  • Thysanura - silverfish and firebrats
  • Isoptera - termites 

Arachnids

The word spider is related to the Old English word spinnan which means to spin. While most people associate spiders with spinning webs wandering spiders build none at all.  They do use silk in other ways.

Most spiders release a line of thread as they walk called a drag line.  As they move they attach the drag line to the surface on which they walk.  If they happen to fall or jump they are supported by the line.

The spider also uses the silk to wrap its prey, build a nest for eggs; molting; hibernating; or to rest in a protected area, females for egg sacs, and males for sperm webs.  The most familiar use of webs is for snares.  Snare webs are made as sheet webs, funnel webs, orb webs, and combinations of theses.  Another interesting use of the web is as a floatation device.  The spider climbs on a plant and releases threads from its spinnerets and when there are enough to support the spider it will let go of its support and float away in the air.

The eggs are always laid in silk and the number of eggs varies from one to two, 25-30, and 100-300 in most.  Once there were 2652 counted in one sac.  Some spiders make more than one sac and some up to as many as twenty or more.

Spiders molt (shed their outer covering) as they become older.  If they loose a leg they may be able to regenerate it if the molt is not too near. They regenerated leg will be somewhat smaller than normal.

Spiders are carnivorous and are attracted to the movement of their prey.  Most live on insects.  There are two ways spiders ingest their food.  Those with weak jaws puncture the body of the insect with their fangs and then inject digestive juices into this hole and suck the digested liquid back into their body until there is only an empty shell left.  The tarantulas, wolf spider, large orb weavers, and others with strong jaws mash the insect to a pulp with their jaws and regurgitate the digestive juices over it.  Ingesting the digested material.  Most spiders can survive long periods of fasting and many can go long periods without water.  However, some species may die if deprived of water for only a few days.  Spiders may feed on other spiders and this tendency of cannibalism does not make them a social animal.

The male spider will become a wanderer to find a mate.  When he meets a female he will many times dance to attract her attention.  It is not true that the female will always kill the male and eat him.  This is only true in a very few species.  In some species the male will leave and seek another mate.  In some cases the male will stay and share the same web for a period of time.  The male can be expected to die sooner than the female and even though many females die soon after laying their eggs some species will live two or three years more after laying their eggs.

Spiders have been found almost everywhere.  Near water, in or on top of the ground, from caves to the top of mountains (Mount Everest 22,000 feet) and some have actually been collected 5000 feet in the air from planes.  The number of spiders per acre has been found in one instance to be 407,000 in a clay meadow and 2,200,000 in a grassy field. 

Summary of spiders and insects characteristics:

  Spider Insect
Body region two three
Antennae none one pair 
Legs four pair three pair
Pedipalps one pair six segments  none
Poison apparatus on fangs If present on abdomen
Wings none most adults have
Eyes Simple ocelli commonly 8-6 compound plus some have 2-3 ocelli
Silk apparatus always present on the end of the abdomen only in some larvae at the lower lip
Food digestion always before swallowing by regurgitation  usually after swallowing
Development direst no larval stage spiderlings look like parents may have metamorphosis with larval and pupa stage or nymphs 

Arthropod classification check list

  Animal Bilateral symmetry Segmented body Exoskeleton Leg joints Pairs of legs
Spider            
Tick            
Fly            
Dog            
Centipede            
Pill bug            
Lego Exo-Force Supernova or Bionicle            
             
             

Dr. Robert Sweetland's Notes ©