Introduction to Invertebrates and Arthropods
All Life
| This page is meant to introduce you to a large group of organisms collectively called arthropods. In order to place arthropods within the context of life we will very briefly review all life on Earth. None of the following is overly complicated, but you will encounter a bunch of new words. Don’t panic. A long time ago, when you were much younger, you ran into Tyrannosaurus rex and it did not inconvenience you in the least. |
|---|
All life on Earth is related and even the most dissimilar organisms share many basic biochemical attributes (such as the use of DNA and RNA).[7] This relatedness means that life falls within a hierarchy[8], where “all life on Earth” is the most inclusive category and the “species” is the most exclusive.
A hierarchy based on relatedness is very helpful because it carries information. The closer things are on the hierarchy the more characteristics they tend to have in common. Consider three organisms: a pine tree, an earthworm, and a great white shark. The worm and the shark are both in the animal kingdom and have more in common with each other than with the tree. If we add a dog to the mix we see that the shark and dog, both in the same phylum (Chordata), have more in common with each other than with the earthworm. Finally, if we add a housecat, the dog and cat, which are in the same order (Carnivora), have the most characteristics in common.
This hierarchy also provides us with amazing predictive power. For instance, if I say the Greater Whatsit is a close relative of the Robin then you can make some very specific predictions about the morphology, behavior, life cycle, and ecology of the Greater Whatsit. This concept is a very important tool you can use to make educated guesses when working with an organism you are not familiar with.
We will very briefly run through the basic hierarchy of life on Earth (Figure 1.)
Divisions
We can divide all life on Earth into three major groups called Divisions: Bacteria, Archaea, and Eucaryota.[9] The Division Bacteria [10] and Division Archaea (also called Archaebacteria)[11] have very small cells without a nucleus and are always unicellular. Often the Bacteria and Archaea are placed in a single group, Prokaryota [12]), separate from eukaryotes. All prokaryotes are single celled (although some can grow in clumps) and are very small. The world’s most gargantuan bacteria is Thiomargarita namibiensis[13] which is about as big as the period at the end of this sentence.
The Division Eukaryota [14] has bigger cells with specialized sub-cellular structures enclosed in membranes. Some Eukaryotes are unicellular and these were historically placed in one group called Protista (which includes amoebas, etc.).[15] However with new molecular techniques this huge group of diverse animalcules has been split into many groups with long complicated names.
The rest of the Eukaryota are plants, fungi, and animals. Basically every living thing you can see with the naked eye is a eukaryote.
Kingdoms within Eukaryota
Within the Division Eukaryota there are several kingdoms.[16] Many of these kingdoms are unicellular and there is much heated debate about what they should include and what they should be called (the Protists mentioned above).[17] They will not be discussed here.
The only kingdoms with multicellular organisms are Plantae, Fungi, and Animalia:
The Kingdom Plantae includes red algae[18] (5000+ species), and green plants[19] which include green algae, ferns, conifers, and flowering plants (300,000+ species). Most plants get their energy from sunlight (photosynthesis) and construct organic molecules from nonorganic molecules (although some are parasitic![20]) The major structural molecule in plants is cellulose (e.g., the solid component of wood).
The Kingdom Fungi includes mushrooms, yeasts, (half of) lichens, rusts, smuts, etc.[21] (100,000+ named species but many more are undescribed). Traditionally fungi were thought of as plants, but evidence has shown that they are much more closely related to animals. Fungi are mostly decomposers (although some are predatory![22]) and are a very important part of a functioning ecosystem. The major structural molecule in fungi is chitin (e.g., the hard part of shelf fungi).
The Kingdom Animalia includes sponges, jellyfish, echinoderms (sea stars, etc.), vertebrates (fish, amphibians, reptiles, birds, mammals), nematodes, horsehair worms, flatworms, tapeworms, velvet worms, water bears, arthropods (insects, spiders, etc.), mollusks (snails, clams, squid, etc.), segmented worms, brachiopods, etc. [23] More than 2 million species of animals are described and about 1.2 million are arthropods. An important structural molecule in Animalia is chitin [24] which makes up the exoskeletons of arthropods (insects, crustaceans, etc.), the radulas of mollusks, and beaks of squid and octopuses.
Phyla within Kingdom Animalia
| Vertebrate vs. Invertebrate | The animal kingdom is popularly divided into two parts: 1) vertebrates, those animals with a backbone; and 2) invertebrates, animals without a backbone.[25] This division is not based on relatedness. Vertebrates are a group within the phylum Chordata (but not all chordates are vertebrates![26]) Invertebrates are just everything that isn’t a vertebrate. Because this isn’t a natural division strange things happen, for instance some invertebrates are much more closely related to vertebrates than to other invertebrates. For example starfish are much more closely related to birds than they are to sea snails!
Here is an analogy to help explain this strange situation: Imagine if the post office divided all addresses on Earth into two groups, residents of DeKalb County, Illinois, and non-residents of DeKalb County, Illinois. DeKalb County is in the U. S., and so is every address in Indiana, but Indiana addresses would be lumped in with addresses from Japan and Australia. Same as “vertebrate” and “invertebrate”: sea squirts[27] (“invertebrate”) are in the same phylum as fish (“verteberate”), but are grouped with earthworms[28] (“invertebrate”) in the popular classification! This doesn’t mean that vertebrate and invertebrate aren’t, at times, useful distinctions, most vertebrates are larger (and sometimes tastier) than invertebrates. But this distinction doesn’t carry as much information as groups based on relatedness. |
|---|
The Kingdom Animalia is divided into about 36 major groups called phyla (phylum, singular).[29] Some of the major phyla include the sponges (5000+ species), jellyfish (11,000+ species), mollusks (112,000+ species, including the largest invertebrate at 1000+ pounds[30]), segmented worms (16,000+ species), echinoderms (7000+ species), nematodes (28,000+ species), and flatworms (25,000+ species). Each of these phyla are fascinating (the interaction of horsehair worms and crickets, for example[31]) and many a pleasant afternoon can be spent learning about the amazing world of the overlooked.
We’ll go into greater detail about two animal phyla, Chordata and Arthropoda.
Phylum Chordata[32]: This phylum is highlighted to help put the others into perspective. Chordata consists of animals with a dorsal nerve cord and contains tunicates (3,000+ species), lancelets (30+ species), and the vertebrates. The vertebrates[33] are what many people think of when they hear the term “animal.” The vertebrates are animals with backbones and a skull, they include: “fish” (30,000+ species)[34], amphibians (6,000+ species)[35], reptiles (8,000+ species)[36], birds (9,000+ species)[37], and mammals (5,800+ species, including humans)[38]. Vertebrates only make up about 5% of all animal life. Most people are surprised to learn that there are more types of jellyfish than birds!
The Phylum Arthropoda (arthro, joint or segment; poda, foot or appendage) [1] is the largest group of organisms on Earth.[39] They tend to have a segmented body and jointed legs (thus the name). This body plan of repeating parts and appendages means that parts can be modified in different species to perform different functions. For example, a praying mantis has raptorial forelimbs it uses to catch and hold prey, while the hind legs are used for standing and walking.[40] Arthropods are the Swiss Army knives of the animal kingdom.
Phylum Arthropoda
The Phylum Arthropoda[41] is an enormous group of organisms consisting of a diverse array of beasts ranging from horseshoe crabs to scale insects (Arthropod Family Tree: #1). Virtually every mode of life is represented within the Arthropods: free living, parasitic, herbivore, carnivore, fungivore, flier, walker, runner, swimmer, social, solitary, marine, aquatic, terrestrial, long lived, short lived, big, little, deadly to humans, annoying to humans, benign to humans, monogamous, polygamous, sexual, asexual, etc., etc.
Important features of arthropods include: internal and external body segmentation (tagmosis[42]); a hardened exoskeleton (often used to reduce water loss in terrestrial forms); body segments with paired (one on each side), jointed appendages (sometimes these are reduced or lost from some body segments); paired compound eyes; an open circulatory system (the “blood” of insects, called hemolymph, is not in arteries or veins, but bathes the organs directly); complete digestive tract; a ventral nerve cord (the opposite of us, we have a nerve cord that runs along our back (dorsal), but theirs runs along their belly); and growth by molting.[43][2]
The largest arthropods that ever lived were “sea scorpions” (Eurypterids)[44] that could grow to more than 8 feet long. Today the largest arthropod is the Japanese Spider Crab[45], weighing more than 41 pounds with a leg span of more than 12 feet. The largest terrestrial arthropod is the Coconut Crab[46] which can have a body more than a foot long and can weigh 9 pounds! The smallest arthropod is a type of parasitic crustacean Stygotantulus stocki [47] that is less than 0.1 mm (0.004 in) long.
The arthropods can be broken into three major subgroups: the Lace Crabs (Marellomorpha)[48]; the Trilobites, Horseshoe Crabs, Arachnids, and relatives (Arachnomorpha[49]); and the Crustaceans, Myriapods, and Hexapods (including Insects) (Mandibulata)[50].
Marellomorpha
The Lace Crabs are an ancient, extinct group of arthropods known from the Burgess Shale.[51] (Not shown in Figure 2.)
Arachnomorpha: Trilobites, Horseshoe Crabs, Arachnids, and relatives
The Arachnomorpha (arachno, spider; morpha, form) contains the Trilobites, Horseshoe Crabs, and Arachnids (among others) (Arthropod Family Tree: #2).
Trilobites
The name “trilobite” means three lobes, referring to the three lobes when viewed in cross section, not, as commonly thought, the three major body segments from front to rear (Cephalon, Thorax, Pygidium). [52][53] Trilobites were some of the first arthropods and existed from about 540 to 250 million years ago, when they became extinct. More than 15,000 species of trilobites have been described.[54] They ranged in size from a few millimeters to 71 cm (28 inches) long.[55] Growth was achieved by molting. In general most trilobites hatched with only a head (cephalon) and tail (pygidium).[56] As they grew they added segments in the middle (throax). After the appropriate number of molts they no longer added segments but could still increase in size with each molt. Molting meant that any given trilobite had the potential to leave a lot of evidence of its existence (fossils). Most fossil evidence consists of molted exoskeletons, with intact specimens much less common. Trilobites are important Index Fossils[57] and are often used to estimate the relative ages of rocks they are found in.
Horseshoe Crabs
Horseshoe Crabs (Xiphosura)[58] aren’t really crabs at all, but are more closely related to trilobites and arachnids (Arthropod Family Tree: #3). Horseshoe crabs have been around for about 300 million years and today’s species share many characteristics with ancient fossils. Four species are alive today, one on the east coast of North America and the Yucatan Peninsula and three in the Indo-Pacific region.[59]
Horseshoe crabs have three main body segments, from head to tail: prosoma, opisthosoma, and telson[60] (pro, before; opistho, behind or back; soma, body; telson, tail).[61] They grow by molting and can increase in size by 25% with each molt. Horseshoe crabs leave the water in large groups to lay eggs (up to 12,000 per female) and this “orgy” is an important part of the diet of many migratory birds.[62] Additionally, the blood of horseshoe crabs, which uses copper instead of iron to hold oxygen, is used in medical tests to detect bacterial endotoxins in infected humans.[63] To get the blood the crabs are dredged from the bottom of the ocean, some blood is removed, and then the crab is tossed back. About 15% of the crabs die from this procedure. Female horseshoe crabs have to be about 10-12 years old before they are mature enough to reproduce so care must be taken not to over exploit this unexpectedly valuable natural resource.
The arthropods are continued below:
Intro. to Arthropods: Arachnids (Spiders, Mites, etc.)
Intro. to Arthropods: Mandibulates (Crustaceans, Insects, etc.)
Print References
First Detector Entomology Training Project
