Arthropod characteristics

• jointed appendages
• segmented body
• exoskeleton
• ventral nervous system
• open circulatory system
• sensory receptors

The exoskeleton is composed of a tough compound called chitin (KITE - 'n). Chitin is an unbranched polymer of N-Acetyl-D-glucosamine. It may be regarded as a derivative of cellulose, in which the hydroxyl groups of the second carbon of each glucose unit have been replaced with acetamido (-NH(C=O)CH₃) groups. This allows for increased hydrogen bonding between adjacent polymers, giving the chitin-polymer matrix increased strength.

In its unmodified form, chitin is translucent, pliable, resilient and quite tough. In arthropods, however, it is often modified, becoming embedded in a hardened protein matrix, which forms much of the exoskeleton. In its pure form it's leathery, but when encrusted in calcium carbonate it becomes much harder. The difference between the unmodified and modified forms of chitin can be seen by comparing the body of a caterpillar (unmodified) to the body of a beetle (modified).

The exoskeleton is composed of three layers, secreted by the epidermis that lies just beneath:

• A thin, waxy epicuticle containing no chitin. It repels water and keeps terrestrial arthropods from drying out.
   
• The thick, hard exocuticle is rich in calcium and provides protection for the inner body parts.
   
• The endocuticle consists of laminated layers of protein and chitin fibers, creating an extremely tough and flexible substance allowing the animal to move freely.

The molting process

• The epidermis secretes an enzyme that digests the inner layer of the exoskeleton.
• At the same time, the epidermis uses the digested material to begin building a new exoskeleton.
• When the inner layer is weak enough, the old exoskeleton splits along specific lines allowing the the animal to "shed" the old exoskeleton.
• At first, the new exoskeleton is soft. The animal will draw in air or water, depending on its habitat, to swell the new exoskeleton larger than its body.
• Once the new exoskeleton hardens, the animal releases the excess air or water allowing the body to shrink back to normal size. This leaves some space for growth between the exoskeleton and the soft body parts.

All living members of the Phylum Arthropoda are in these three subphyla:

1. Subphylum Crustacea

Members are distinguished for having chewing mouthparts called mandibles and branched antennae.

Crustacea is the only arthropod subphylum that contains mostly aquatic organisms. Most crustaceans in Oklahoma are small. Although small, the animals in these two classes are an important link in the aquatic food chain. The pictures of the cyclops, Class Maxillopoda, (left) and water flea, Class Branchiopoda, (right) show their relative size. Both animals are barely visible with the naked eye.

The largest crustacean in Oklahoma is the crayfish. It is mostly a scavenger, found in most bodies of water in the state. The crayfish needs calcium and other minerals for its heavy exoskeleton. There are a few streams that are lacking these minerals where the crayfish is not found. The animal is suseptable to pollution and can be used as an indicator for pollution problems. The only completely terrestrial crustaceans, sowbugs and pillbugs, Armadillidium vulgare, are in the Class Isopoda. The pillbug is known to most children as the "roly-poly" because of its defensive mechanism of rolling into a tight ball when threatened. While the two animals are quite similar, the sowbug has two projections on its tail and does not roll into a ball when threatened.

Even though they are terrestrial, these animals have gills instead of lungs. For this reason, they are commonly found under objects in moist dirt.

 
2. Subphylum Chelicerata

This subphylum is represented by the Class Arachnida, the spiders.

The study of spiders is Arachnology.

Recognized by the eight legs attached to the cephalothorax, spiders are very useful and should not be killed. They eat only insects and other small pests and deserve a better reputation than they have.

While ALL spiders kill their prey by injecting poison, only two Oklahoma spiders have the ability to actually harm most people. The Black Widow, Latrodectus mactans and the Brown Recluse, Loxosceles reclusa produce dangerous bites.

The Black Widow is more common than the Brown Recluse in Oklahoma. Neither is overly aggressive and only bite when they feel threatened. Both spiders hunt at night and prefer out of the way places to hide during the day. Most are encountered when people reach into small, protected places. A favorite habitat for both is a wood or brush pile.

It has been said that the venom of the black widow spider is 15 times as toxic as the prairie rattlesnake. This is probably an exaggeration! Always remember this about venomous bites − the severity of the bite depends on the amount of venom injected, the distance the bite is from vital organs, and the sensitivity of the person to the toxin. Obviously, a spider would inject a lot less venom than a rattlesnake.

The largest Oklahoma spider, the tarantula, is not considered harmful. The bite of this mild-tempered spider is no worse than a bee sting.

Another Arachnid in Oklahoma is the scorpion. This picture shows a female with her newly hatched young.

While the "sting" of a scorpion is more painful than most spider bites, scorpions are also predators of pests and should be protected. Scorpions catch prey with their enlarged pedipalps and tear their prey apart. The stinger on the tail is used mostly in defense.

While not "venomous" animals, ticks are a more serious threat to health than spiders and scorpions in Oklahoma. Two Oklahoma ticks are carriers for very serious disease.

• Spotted fever is carried by the American dog tick, Dermacentor variabilis.
• Lyme disease is carried by the black-legged tick, Ixodes scapularis.

While both sexes take blood, it is the female that engorges herself, after mating, to aid in egg formation. The male takes a shorter blood meal, before mating, and will release itself when finished.

Chiggers are actually the immature form of a mite. They do not "bite"! Digestive enzymes are secreted into a hair follicle or skin pore and the liquified cells are sucked up for food. This is the process that causes the intense itching associated with chiggers. Scratching the area often leads to secondary infections.

You can see from the picture that the nymph has only six legs. The engorged nymph will drop off and molt, becoming the adult harvest mite, Trombicula alfreddugesi, with eight legs.
 
3. Subphylum Uniramia

This subphylum contains three important Classes.

Class Chilopoda - Centipedes

Centipedes in Oklahoma range in size from one to ten inches. They are fast moving preditors with one pair of strong legs per body segment. Centipedes have fangs to inject poison and will attack animals much larger than themselves.

Both centipedes and millipedes have a problem with water loss. Their exoskeleton does not have the waxy covering of other terrestrial arthropods. They are limited to moist environments and will usually be found in moist dirt under rocks and logs.

Class Diplopoda - Millipedes

Millipedes are slow moving with two pairs of small legs per body segment. They eat plant material and do not have poison glands. Their bodies are more rounded than centipedes. When threatened, many of them will coil up to protect their softer underbelly. Some produce a strong-smelling secretion to discourage predators.

Class Insecta - The Insects