THE WORLD OF LIFE with Douglas Drenkow

Many arthropods are marine or freshwater species. Many arthropods are terrestrial. Many arthropods are aerial (Insects are the only flying invertebrates, although some lightweight spiders can "balloon", on strands of silk). Many arthropods live on or in plants or other animals. And some arthropods live in shelters of their own choosing or construction (such as hermit crabs in the empty shells of gastropods, caddisworms in their silk-and-sand tubes, or wasps in the pulped-paper cells of their nest). Virtually any habitat on Earth that can support life does support a thriving population of arthropods.

Arthropods, especially the "planktonic" (microscopic and drifting) crustaceans in the sea and the omnipresent spiders and insects on the land, are the dominant group of animals on Earth.

DESCRIPTION

An arthropod is encased within an "exoskeleton", which shapes, supports, and protects its segmented body. In the embryos and presumed ancestors of arthropods, the body is supported by "hydrostatic (water) pressure" within the "coelom" (the membrane-lined body cavity outside of the gut); however, in adult insects, only bits of the coelom remain -- support is provided exclusively by the exoskeleton.

The exoskeleton is composed of plates consisting primarily of hardened, horny "chitin", a "mucopolysaccharide", a polymer (chain-like molecule) of sugars, found also in the cell walls of fungi and chemically related to the "cellulose" of plant cell walls.

The exoskeleton must be shed (in "molts") from time to time as the arthropod grows; and because of its sheer bulk, the exoskeleton ultimately limits the size of the creature, typically to less than an inch long (although such marine arthropods as crabs, buoyed-up by the salt water, may grow to several feet wide, from legtip to legtip).

The typically small size of arthropods is not necessarily a disadvantage -- it can help the creatures defend themselves, as by simply drifting in great groups in the open sea or by hiding in terrestrial nooks and crannies.

Related segments in the exoskeleton of an arthropod are grouped together as a "tagma", a body region such as a head or an abdomen. In general, the evolutionary trend has been for fewer segments, grouped into more multi-purpose tagmata.

Jointed "appendages" -- such as antennae, jaws, or legs -- as well as wings (in various insects) are formed from the exoskeleton of an arthropod. The appendages (but not the wings) represent segmented, fleshy outgrowths of the body covered by exoskeleton. Wings are flap-like outgrowths of the body wall, the veins in a wing remnants of living tissue inside the double-layer of exoskeleton.

Arthropod appendages are often highly adapted. For example, the body of a honey bee is covered literally from head to "tarsus" (foot) with hooks; combs; brushes; baskets; and other antenna-, mouthpart-, and leg-modifications for gathering pollen and nectar from carefully selected flowers.

Trilobites -- long extinct and, thus, found only as fossils -- were perhaps the first arthropods. Their body was "dorso-ventrally" flattened (flattened from top-to-bottom) and composed of three major parts: The "cephalon" (head), a many-segmented "thorax", and a short "pygidium" (tail section). The cephalon bore one pair of eyes, one pair of antennae, one pair of mouthparts, and several pairs of gilled/legged appendages. The thorax bore many pairs of gilled/legged appendages. Note that the name "trilobite" does not refer to these three major portions of the body, from head to tail, but rather to the three apparent regions of the body, from side to side, set off by lengthwise furrows.

The body of chelicerates is typically composed of a "cephalothorax" (bearing no antennae, one pair of typically fang- or pincer-like "chelicerae", and four pairs of true legs) and an "abdomen" (bearing no large appendages).

Horseshoe-crabs, up to two feet long, are actually more closely related to arachnids than crustaceans. Covered by a shell-like "carapace", their "cephalothorax" (front body section) bears one pair of eyes, no antennae, one pair of small pincer-like "chelicerae", and five pairs of legs -- the first pair actually false "pedipalp" legs and all but the last pair having pincers. Composed of fused segments, the "abdomen" (rear body section) of a horseshoe-crab bears five pairs of covered gills, also under the carapace, and a long "telson" (tail), which sticks out from under the carapace and is used by the animal to flip itself over whenever it has been flipped upside-down, as by the surf.

For further information, please see Arachnids, Crustaceans, Myriapods, and Insects.

FEEDING HABITS

Almost anything edible is eaten by at least one species of arthropod.

Various arthropods are scavenging, filter-feeding, herbivorous, carnivorous, "omnivorous" (eating almost anything), or parasitic.

MOTION

Arthropods have well-developed muscles, well-arranged in bundles, taking advantage of leverage to pull into action specific segments of the body or parts of the jointed appendages. Note that the muscles of arthropods pull on the inside of their exoskeleton, whereas the muscles of us vertebrates pull on the outside of an "endoskeleton".

For further information, please see Arachnids, Crustaceans, Myriapods, and Insects.

DIGESTION

The gut of an arthropod is "complete", with both a mouth and an anus.

The mouthparts of many arthropods are highly modified, for various foodstuffs and means of feeding (chewing, sucking, lapping, etc.)

For further information, please see Arachnids, Crustaceans, Myriapods, and Insects.

The gut of an arthropod is typically composed of a "foregut", "midgut", and "hindgut". Typically, the foregut functions as both a food-grinding "gizzard" and a food-storing "crop"; the midgut typically digests and absorbs food, as in special pouches or with the aid of "digestive glands"; and the "hindgut", composed of an "intestine" and a "rectum" in insects, absorbs water and forms feces in most arthropods.

RESPIRATION

Gases simply diffuse by osmosis, through selective cell membranes, across the body wall of some small arthropods.

In aquatic arthropods, gases are usually "exchanged" (between the body and the environment) through typically external, feather- or comb-like gills.

In such arachnids as scorpions or spiders, gases are exchanged through "book lungs", which open through slits on the outside of the body and have many "leaves" inside.

In millipedes, centipedes, insects, and many other arthropods, gases are exchanged through "tracheal" tubes, opening through "spiracle" pores on the outside of the body and branching throughout the inside of the body.

CIRCULATION

Dissolved gases (in arthropods without a trachael system) and other materials are typically carried by an "open" circulatory system. Blood is pumped by the dorsal tubular heart (in the upperside of the body) through "arteries" (Ex. There is a single "aorta" in insects, through which the blood usually flows "anteriorly", towards the head). The blood in the arthropod body returns through spaces within the "hemocoel" (the blood-filled body cavity), not through membrane-bound veins, and then back into the heart through small holes in its sides (which are closed by one-way valves when the heart pumps).

Such crustaceans as many copepods and ostracods and all barnacles have no circulatory system at all -- dissolved materials simply diffuse throughout the tiny body.

EXCRETION

Arthropods have no true kidneys. "Nitrogen-rich" and other soluble wastes (as in our urine) are typically collected throughout the body by little sacs or dead-end tubes (such as the "Malpighian tubules" within insects); these then excrete the wastes outside the body, into such areas as near the base of the antennae (of many crustaceans) or into the gut (of insects and spiders). Freshwater and terrestrial species of arthropods typically have salt- and water-balancing systems, involving these excretory organs and/or the gut.

An exoskeleton is invaluable in preventing the evaporation of bodily water from terrestrial species.

COORDINATION

The growth, development, and activities of arthropods are under genetic control, influenced by the environment. If this is indeed the age of insect and human beings, then this is also the age of instinct and intelligence. As evidence of this, note that some ants have an apparently inborn, genetic instinct to "herd" aphids like dairy cattle and to "milk" them for their sweet honeydew; and some wasps even have an instinct to use primitive tools -- sticks or pebbles -- to tamp dirt over the eggs in their burrows!

The growth, development, and activities of arthropods are also under hormonal control. In addition to hormones within the body (such as those controlling the molting of the exoskeleton), many arthropods (and other animals) emit "pheromones" into the environment, such as the hormonal perfumes of female moths that, even in minute concentrations, can attract mates of the appropriate species far downwind.

The activities of arthropods are also under control of the nervous system. There is a dorsal brain -- within the upperside of the typically "well-cephalized" (highly sensory) head. Like their possible annelid ancestors, arthropods typically have a "double ventral nerve cord" (composed of two strands within the underside of the body) with "ganglia" (nerve knots) inside the body segments.

In the sense organs of arthropods, nerves are typically connected to bristles or membranes in the exoskeleton -- the part of the body in closest contact with the environment. The sense organs of arthropods are typically highly developed, especially in highly mobile species. Crickets and grasshoppers use their fine sense of hearing in recognizing species-specific mating calls. Fireflies recognize the instinctively coded messages sent by the flashing, chemically generated lights of their mates. The often beautiful markings on the wings of butterflies help identify individuals by sight to potential mates of the same species. The scent trails laid-down by scout ants can direct entire armies of their fellow workers to sources of food. Certain beetles can "mimic" (imitate) the touch-signals used by ants in order to get fed by them. The taste of nectar regurgitated by foraging honey bees returned to the hive can identify to their fellow workers particular species of flowers; and the properly directed and paced tail-wagging dance of a foraging honey bee on the honeycomb can direct other workers (in the dark, buzzing hive) out to specific pollen- and nectar-sources (The worker bees also rely upon magnetic compasses, polarized- and ultraviolet-light sensors, biological clocks, and other navigational and flight controls built-into their truly amazing little bodies).

In addition to the light-sensing, "simple" eyes typical of arthropods, most crustaceans and insects (and a relative few millipedes and centipedes) have a pair of "compound" eyes, composed of many hexagonal "facets" (six-sided parts), each contributing a small portion to the overall (fairly crude, wide-angled, fixed-focus) image, like pixels on a computer screen.

Finally, the growth, development, and activities of some arthropods are under the control of "societies": Please see Termites and Wasps, Ants, & Bees for further information.

REPRODUCTION

Arthropods reproduce sexually and usually have separate sexes. The typically enormous reproduction rates of arthropods are keys to their success.

Male horseshoe-crabs grab onto females and "externally fertilize" their eggs; that is, the sperms and eggs are released into the surrounding water.

Crustaceans and terrestrial arthropods typically "copulate" (mate with physical contact); typically, the penis of the male (accompanied by "claspers" in male insects) introduces sperms into the vagina of the female.

As an arthropod grows, its exoskeleton is shed from time to time, in a process known as "molting" -- the new exoskeleton is soft, hardening only after expanding to a larger size.

The young of most arthropods look like small adults, although often with fewer body segments or appendages. However, the young of some crustaceans and the higher orders of insects are often very different, in form and habits, from the adults -- the young and adults thus exploit and compete in different ecological "niches".

The young of a horseshoe-crab looks remarkably like a triblobite (Often, the young of an animal develops through stages that look like its presumed evolutionary ancestors -- for example, a human embryo, developing in its mother's womb, has a long, fish-like tail).

For further information, please see Arachnids, Crustaceans, Myriapods, and Insects.

Representatives

Animals (Metazoa)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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