An Online Introduction to the Biology of Animals and Plants





Key Concepts




Section 3

Chapter 7







Another Extremely Successful Group



There was an earlier discussion about how the success of groups could be determined and compared.  The arthropods, animals that have taken over almost all of the environments of the earth, could easily be given the top spot on the success pile.  One piece of supporting evidence you might run across is that there are more species of arthropods than of all of the other multicelled animals combined, but that is only true for the described species, and more hobbyists have added to that list for insects (which are arthropods) than for any other group.  No one knows how many species of roundworms there are, in contrast, because only a tiny group of researchers even care.

Probably the  most important feature of the arthropods in their exoskeleton, a complex rigid outside covering that gives the muscles something to pull against.  Unlike a simple shell such as those of mollusks, an exoskeleton has many flexible joints, allowing a wide range of movement.  Skeletons in our group are the "opposite" kind, endoskeletons.  There are advantages and disadvantages to the two skeletal systems: 



More protective, covering the muscles and other soft tissues.

Less protective, usually being under the muscles and soft tissues.

Produces better leverage for the pull of muscles.

Leverage is worse.

As animals size increases, becomes impractically heavy - imposes size limitations, especially on land.

Being internal, can support great size and weight without becoming itself too heavy.

Once formed, does not grow;  must be shed (molted), and animal must then rapidly "grow" (usually by taking in water or air) and produce a new, larger exoskeleton to "grow into."  During molts, animals are extremely vulnerable.

Grows fairly smoothly and evenly, no molting.

As small animals, arthropods gain great advantages from their exoskeletons, including protection almost as good as an actual shell.  As anyone familiar with arthropod-based super heroes knows, arthropods are very strong for their size, but this is deceptive - this proportionate strength is a combination of better system leverage but mostly due to the fact that smaller muscles are proportionately stronger  (a muscle half the size is much more than half as strong), and the basis of comparison, lifting X times ones own body weight, is easier if the body weight is very low.  A spider or ant the size of a human would not even be strong enough to support its own weight.  Dont tell the people who make science-fiction-horror movies.  Land arthropods often produce exoskeletons out of materials that will still give support but which are relatively light, but even that keeps them from getting very big.

So arthropods are very good as little animals, but have serious size limitations, which they make up for in numbers.  Wherever you are, you are probably outnumbered by the arthropods in your immediate vicinity by hundreds, maybe thousands, to one. 









It may be that arthropods are descended from some sort of segmented worm-type ancestor;  in fact, a small phylum called the Onychophorans have many features of both groups, a few surviving species that appear as a blend or "link" might be expected to.  Arthropods are segmented, at least as embryos, and commonly have antennae and chitinous mouthparts (chitin is a sort of hard but light protein-based material), also common in segmented worms.  The two groups also have the same sort of nervous system.

But where segmented worms have setae, arthropods have legs, sometimes lots and lots of them, sometimes modified to do some very un-leggy jobs, like manipulating food or pinching predators.  Arthropod legs and other leglike appendages show a great deal of serial homology, a visible remnant of segmentation.  It is common for subgroups of the arthropods to have characteristic numbers of legs, such as the decapods (10 legs), including crabs and lobsters, or the arachnids (8 walking legs), including spiders and scorpions, or the insects (6 legs).

There are not a lot of features that all arthropods have in common;  beyond the couple just discussed, maybe there is only their open circulation system, where blood pushed by a heart just sort of percolates around through open spaces called sinuses.  Such a system is somewhat limited for circulating oxygen, and higher-metabolism arthropods often have some sort of enhanced oxygen-carrying set-up:  most insects, for example, have a tracheal system of tubes that connect every cell in the body to the outside air.







The arthropod phylum is divided into two subphyla, named for a difference in mouthparts but with several other differences as well.  

The chelicerates, named for mouthparts derived from front legs and called chelicerae, do not have the mandibles or antennae found in the other subphylum.  Chelicerates typically have six pairs of appendages:  one pair is the chelicerae, followed usually by a pair of pedipalps, legs adapted for purposes other than walking, and then four pairs of walking legs.

The chelicerates include -

 - The horseshoe crabs, a very old group who have five pairs of walking legs and no pedipalps.  From above, a horseshoe crab looks like a shield with two eyes and a tail;  turned over, it looks a bit like the creatures that jump on peoples faces in the Alien movies (but is really much less nasty).  Horseshoe crabs live in sandy tidal zones, where they have existed for a very long time looking much the same as they do today.  Evolutionary theory would suggest that a creature very well-suited to an environment that has remained stable for a very long time would change very little, and this is certainly true of horseshoe crabs.  In fact, when dosed with radiation or chemicals that would mutate most animals DNA, a horseshoe crab just fixes the damaged molecules - they actually have evolved systems that resist the changes that animals in less stable environments require to continue to survive.  They also have some odd commercial uses.

 - The arachnids, a land group made up of mostly predators, includes the scorpions, whose pedipalps are adapted into claw-like grabbers, chelipeds;  the spiders, whose pedipalps are blunt and used as "feelers," and who make silk (even the ones that dont live in webs) and inject a combination of venom and digestive enzymes into their prey;  the harvestmen, or "daddy longlegs," which are not really spiders although many folks think they are;  the ticks and mites, small round-bodied animals.   The ticks are virtually all parasites, and many of the mites are as well, although many mite species are free-living.  Ticks are significant carriers of human diseases, and many humans develop allergies, "to dust," which are actually to mite droppings in the dust. 

The mandibulates, which have mandibles and antennae similar to those of segmented worms, are the other subphylum.  The number and uses of their legs follows no set rule for the whole group, although subgroups of the mandibulates may have characteristic numbers of legs.

The mandibulates include -

 - The crustaceans, a huge and varied group that is mostly marine, but has many fresh water species and a handful of land species.  One shared feature of this group is that their legs and leg-derived structures are somewhat forked (a trait called biramous appendages).  The crustaceans include some familiar "shellfish," such as lobsters, crayfish, crabs, and shrimp, as well as many types of planktonic animals which float in the surface layers of large bodies of water.  There is one species of tiny crustacean, with individuals distributed in the top layers of all the the worlds oceans, that is probably the species with more living individuals than any other.  The land-living crustaceans are limited to damp environments, but include those animals commonly called pillbugs, potato bugs, wood lice or armadillo beetles.

 - The centipedes, a land group of poisonous predators with many legs, one pair on each segment.  These often have warning colors such as red or orange and scurry off when youve turned over a rock.

 - The millipedes, a land group of plant-eaters with many legs, two pairs on each segment.  These slow-moving animals often have contrasting black warning markings, because when threatened they release poisonous cyanide gas.

 - The insects, a huge group, considered a land group although with a few fresh water species.  These are such a part of human life that they get their own devoted section.








Insects have the traits already discussed for the arthropods and the mandibulates - exoskeleton (of chitin, a light material but very supportive if a structure is small enough - in some ways like fiberglass), open circulatory system, segmented worm-type nervous system, mandibles, antennae - and a few specific traits of their own.  Insects typically are divided into three sections:  a head, thorax, and abdomen.  The thorax is where all of the structures used for movement are located:  six walking legs (several types of mouthparts and perhaps other structures are built from what used to be legs, but only six real legs remain), and typically four wings.  Insects, unlike other winged animals, have not evolved their wings from pre-existing appendages, but rather from outgrowths of the thorax exoskeleton, possibly flaps that had been used to cover the gills of fresh water insects, or from the gills themselves.  The progression probably went from blunt gliding structures, through steerable glider wings, to flappable wings, to systems that can beat wings up to at least a thousand times a second.  Many types of insects use only the back pair of wings for flying;  the front pair in those insects have generally evolved into protective covers for the back wings.

Insects typically go through a limited number of molts (6 is the typical number) between hatching and adulthood, and with very few exceptions only develop fully-functional wings during the last molt.  If you are looking at an insect with full wings, its almost certainly an adult and never going to get any bigger.  Insects also go through two different pathways of development.  In direct development (sometimes called simple or incomplete metamorphosis), young insects look like miniature, wingless versions of the adults (these young are typically called nymphs).  Crickets and grasshoppers develop this way.  In indirect development (sometimes called complete metamorphosis), the larva is physically quite different from the adult.  They often occupy different niches than adults as well, allowing more individuals to exist together in an ecosystem.  Larvae grow, molt, grow, molt, et cetera, until they encase themselves in a pupa form and completely reorganize themselves, a process called metamorphosis (sometimes breaking down to a "soup" of cells that then completely build a new individual) and emerge as adults.  The terms "simple metamorphosis" and "complete metamorphosis" mentioned above can be confusing, and "metamorphosis" is usually just applied to animals that go through a major change.  This is what butterflies do, as you probably know, but also beetles, bees, ants, and flies develop this way.  In some cases, the adult acts almost exclusively to reproduce and spread offspring, not even being able to feed during their short time in that form.



There are still some species of insects around descended from ancestors who never evolved wings.  Most of these are small and, although youve been around them your whole life, you probably have never noticed them,  except for maybe the insects commonly called silverfish, which encounter humans usually around drains.


Flying insects use two different drive systems in their wings - the more primitive system attaches directly to the inner ends of the wings, while the more modern drive a lever-and-pivot system built into the structure of the thorax.  The primitive winged insects include the dragonflies, mayflies and damselflies (Odonata). 


These use the thorax drive system, which can produce wing beats so rapid that a special type of muscle had to evolve just to keep things moving.  This group includes (these are only the well-known groups, there are several more) -

 - The grasshoppers and their relatives, which are obvious insects such as crickets and katydids and insects maybe not so obviously related, such as cockroaches, mantises, and walking sticks.  These are direct developers.  Orthoptera.

 - The "true bugs," which includes leaf hoppers and cicadas.  These insects often seem more "boxy," with angles and straight lines.  They are  direct developers.  Hemiptera.

 - The beetles, which have more named species than any other insect group.  These typically have hard shiny coverings, including front wings that cover and protect the back wings.  They are indirect developers, often going through a grub or wormy larval stage.  Coleoptera.

 - Butterflies and moths.  Although usually moth wings fold back over the abdomen and butterfly wings don't, that is not a reliable way to tell them apart - there isn't really an easy way to do it, since there are some very mothlike butterflies and butterflylike moths.  These are indirect developers, often with caterpillars as larvae.  Lepidoptera.

 - Flies.  These roundish insects have only a single pair of wings, which often beat at extremely high speeds.  The other pair of wings have evolved into short knobby structures (halteres) that beat opposite to the wings and counterbalance those forces.  These are indirect developers, often with soft wormy larvae, including maggots.  Diptera.

 - Termites.  These ant-like insects include the famous wood-eaters, which depend upon a whole ecosystem of unusual protozoans that live in their guts and actually digest the wood, but some termite species are mound-builders.  They are indirect developers, with usually only reproductive adults having wings, and commonly exist in social colonies.  Isoptera.

 - Ants, Bees, and Wasps.  Sometimes combined with the termites, these also are indirect developers and live in social colonies.  Ants are ground foragers, bees are mostly plant eaters and wasps are mostly meat eaters.  Hymenoptera.


These insects show evidence of wings, so it is assumed that their ancestors were winged and evolution has led to the loss of the wings.  Both of the types of insects in this group are commonly parasites for whom wings might interfere with staying on their host.

This group includes the lice (Phthiraptera and Psocoptera) and the fleas (Siphonaptera).









Building air conditioners and bridgesWaging war and taking slaves (and slavery revolts).  Ranching and farming.   Communicating precise directions to a food source.  Awarding promotions based upon merit.  All of these abilities have been observed in one or more of the social insects.

Now, individual insects don't seem that brainy, but when you put them together in an organized colony,  the collective seems to be smarter than the individuals.  This has been compared to how individual small bits of a large brain can work together to solve problems that each cannot handle on its own.   It seems a very different system than the one found in larger animals, but may work more-or-less the same.  And it may point the way to designing truly intelligent computers.

In case you
were wondering, termites construct elaborate structures to air condition their mounds, and bees use workers to both heat and cool hives.  Ants will build bridges (and boats), sometimes out of the ants themselves, to cross obstacles.  Ants also will send out purposeful raiding parties to other nests, bringing back larvae and pupae that as adults are permanently put to work on the "lowly" jobs that only the newly-emerged local adults do.  They also confine, protect, and milk sugary food out of aphids (the ranching) and cultivate and fertilize fungus to eat (the farming).  Bees returning from a foraging expedition perform a sound-and-touch "dance" to tell other bees where (in relation to the suns position) and how far away the food source is.  There is even some evidence that when another'bee picks up this information, it has some idea of where exactly its going - if a researcher places a food source out in the middle of a pond, many bees seem to decide its not worth leaving the hive for.  Although termites are born into their jobs and hold them their whole lives, several types of ants have a sort of promotion-track, where the more experienced ants are given more complicated jobs.






Informational Links



Lots and lots of tick pictures.

Lots and lots of dragonfly and damselfly links.

Lots of links to photographs of gorgeously-colored insects.

Several pages of flying insects caught in flight with high-speed film.

Many many flies.

A blog entry on the movement of ancient arthropods onto the land.

More great photos of arthropods.

Photos of millipedes and centipedes.






Click on term to go to it in the text.
Terms are in the order they appear.



Success and the Arthropods  
Size Limitations in Arthropods  
Arthropod Features  
Arthropod - Segmented Worm Similarities  
Serial Homology  
Leg Numbers - Decapods  
Leg Numbers - Arachnids  
Leg Numbers - Insects  
Open Circulation System  
Tracheal System  
Arthropod Subgroups  
Horseshoe Crabs  
Evolution through Non-Change  
Ticks and Disease  
Mites and Allergies  
Insect Features  
Head, Thorax, Abdomen  
Insect Wings - Evolution  
Development - Direct  
Development - Indirect  
Insect Subgroups  
Wingless Insects, Primitive  
Winged Insects, Primitive  
Winged Insects, Modern  
Grasshoppers and Relatives / Orthoptera
Walking Sticks  
Bugs, True / Hemiptera
Leaf Hoppers  
Beetles / Coleoptera  
Butterflies and Moths / Lepidoptera  
Flies / Diptera  
Termites / Isoptera  
Ants, Bees, and Wasps / Hymenoptera  
Wingless Insects, Modern  
Lice / Phthiraptera & Psocoptera 
Fleas / Siphonaptera  
Intelligence in Insects  
Social Insect Behaviors  
Termite Behavior  
Ant Behavior  
Bee Behavior





Go On to Next Chapter - Echinoderms






Online Introduction to the Biology of Animals and Plants.

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