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ARTHROPODS
in General

tickJoint-legged animals without backbones are called arthropods and are placed in the phylum Arthropoda. Human legs are jointed at the knees, but we have backbones, so we're not arthropods. On the tick at the left (found climbing my screen door, so I scanned him!) you can very clearly see the joints.

How do you know this tick isn't an insect? The easiest thing to notice is that insects have 3 pairs of legs (6 of them) while this tick has 4 pairs (8 legs). Also, insects have 3 body regions (head, thorax and abdomen) while this tick appears to just have a head and a body. This tick is a good arthropod, but it never could be confused for an insect. Both ticks and insects are members of the phylum Arthropoda, but ticks are members of the class Arachnida, while insects belong to the class Insecta.

ARTHROPODS ARE SUCCESSFUL ANIMALS

arthropod pie chartFrom nature's point of view, the combination of being joint legged and not having a backbone seems to be a good one, because arthropods are the most successful group of animals on earth! The chart shows just how successful.  It shows that the number of arthropod species on Earth is far greater than the number of species of all other kinds of animals. Scientists believe that 4-6 million arthropod species exist.

The word "successful" needs to be defined, for many humans would consider mammals to be earth's most successful group of animals, since humans are mammals, and humans are obviously Earth's "top dogs." However, the human standard for "success" isn't necessarily the only one. From a biological perspective, animal groups are "successful" if...

  • they include a large number of species
  • the species occupy many different kinds of habitats
  • the species eat many kinds of food
  • the species are good at defending themselves from their enemies

WELL-KNOWN SUBDIVISIONS
OF THE ARTHROPOD
PHYLUM

  • Class Arachnida (arachnids)
    • Order Scorpiones (scorpions)
    • Order Opiliones (daddy-longlegs)
    • Order Acari (mites & ticks)
    • Order Araneae (spiders)
  • Class Merostomata (horseshoe crabs)
  • Class Chilopoda (centipedes)
  • Class Diplopoda (millipedes)
  • Class Insecta  (insects)

To see a much more complete, and confusing, breakdown, visit the NCBI taxonomy database

In these terms, humans don't look particularly successful. For one reason, there's just one species of us. If we do something really dumb and end up going extinct, there won't be another human-like species to evolve into the brain-using "niche" we're presently occupying. Current thought is that during the evolution of the human species there have been several species of the genus Homo -- several kinds of humans, such as the Neatherthals -- but the other human kinds went extinct (like the Neatherthals), possibly because we Homo sapiens killed them off.

Moreover, there's still a big question as to whether we'll ever overcome our enemy -- which is ourselves. Shall we destroy ourselves in nuclear or biological war, or by continuing to poison the earth with our pollution? It's often said that if anything can survive a nuclear holocaust or a poisoned earth, it's a cockroach. And cockroaches are insects, which are arthropods!

ARTHROPOD EVOLUTIONARY ADVANCES

What makes arthropods so successful? Here are some of the most obvious evolutionary advances that arthropods show over the simpler, more "primitive" mollusks and segmented worms:

  • Jointed legs give arthropods more mobility on dry land than mollusks and worms have
  • Arthropod body segments are more specialized than annelid segments, so arthropods enjoy a greater range of possible activities and behaviors than do segmented worms
  • The arthropods' exoskeleton reduces water loss from their bodies much better than the annelids' cuticle and the mollusks' thin skin

ARTHROPOD EXOSKELETONS

We need to talk more about this exoskeleton, because it's one of "nature's greatest inventions" -- one of the main features responsible for arthropods being so successful.

ecdysis of pill bug
Pillbug shedding exoskeleton

If you're a land animal wanting to move around faster than a snail or an earthworm, you need muscles and you need stiffness. Stiffness is important because muscles must be attached to something secure, and the appendages they move must be stiff, or else they'll flop and flap, instead of taking steps, or raising hands or feet. Unskeletonized flesh can artfully ripple, wiggle, and elongate and shorten, but that's nothing compared to a cockroach's ability to skitter across the kitchen floor.

Arthropod exoskeletons serve the same functions as our human skeletons, which are inside-the-body endoskeletons. The question arises, then, "If exoskeletons are so great, why don't humans and elephants have them?"

It happens that, because of the general physics of large and small bodies, endoskeletons work better in land animals over, say, three inches long, while exoskeletons are best for smaller land animals. Anyone who has seen an oversized ripe watermelon burst during a modest mishandling knows how fragile rigid coverings are for large, heavy bodies. On the other hand, it's hard to imagine a Ladybird Beetle crammed with the numbers of highly specialized bones a human has.

Moreover, exoskeletons have at least two major drawbacks, even for small animals.:

  • First, to grow, animals contained within exoskeletons must periodically split through their coverings, then produce new, larger ones to accommodate their larger sizes. During the time between breaking from the old exoskeleton and when the new, larger shell has hardened, the animal is often vulnerable to its predators. The shedding of an exoskeleton or outer layer of skin is referred to as ecdysis.
  • The "general physics of large and small bodies" spoken of earlier precludes animals with exoskeletons from growing as large as animals with internal skeletons. Therefore, animals small enough to be well served by exoskeletons can't evolve into species with brains large enough to be creative in the manner that humans are. Future arthropod species seem doomed to live lives guided more by instinct than by reason.

You can review books about arthropods in general, available at Amazon.com, by clicking here.

ARTHROPOD BOOKS:

Cite this page as:
Conrad, Jim. Last updated . Page title: . Retrieved from The Backyard Nature Website at .