Biology of Animals & Plants - Arthropods
Arthropods
 
 
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: 
EXOSKELETONS
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).  Th e             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.
 
 
 
Insect s 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.  Insect s, 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.
Insect s 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             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.
PRIMITIVE WINGLESS INSECTS -
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.
PRIMITIVE WINGED INSECTS -
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). 
MODERN WINGED INSECTS -
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 dont , that is not a reliable way to tell them apart - there             isnt really an easy way to do it, since there are some very mothlike butterflies and butterflylike moths.  These are             indirect developers, often 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, with 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.
MODERN WINGLESS             INSECTS .
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 bridges.  Waging             war and taking slaves.  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 dont             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.