Spiders

Spider Facts

Classification:

Kingdom: Animalia

Subkingdom: Bilateria

Phylum: Arthropoda

Class: Arachnida

Order: Araneae

Suborders: Mygalomorphae (the primitive spiders); Aranaeomorphae (the modern spiders); Mesothelae, with one family of spiders, the Liphistiidae

Geographical location: Every continent except Antarctica

Habitat: Diverse; mostly on land, inside and outside buildings, on or close to the ground, under stones, logs, litter, low or medium foliage, tall shrubs and trees, under bark; some live in freshwater and very few in salt water

Gestational period: Varies; some female spiders can carry sperm for some time after mating until ready to produce an egg sac

Life span: Most spider species live for up to two years on average; a few species, such as the tarantula, have much longer life expectancies

Special anatomy: Eight walking legs, four on each side of the thorax; eight simple eyes, each with a single lens, that is particularly sensitive to movement; an extensive nerve system; silk-making spinners; muscular jaws equipped to inject poison into prey

The major classes in the phylum Arthropoda include the insects (class Insecta), the spiders and their relatives (class Arachnida), and the crabs, shrimps, and their relatives (class Crustacea). Arthropods are everywhere, cohabiting with humans, and including ants, yellow jackets, spiders, and dragonflies. It is only fitting to look at the factors contributing to the huge army of arthropods before examining one of their members: spiders.

Traits Contributing to the Success of Arthropods

Several adaptations are responsible for the spread of arthropods to nearly every habitat on earth: an exoskeleton, segmentation, the efficient use of oxygen, and a well-developed body plan, including circulatory, sensory, and nervous systems. The exoskeleton is formed via secretion by the epidermis (the outer layer of skin) and is made up mainly of protein and a polysaccharide called chitin. The exoskeleton protects against predators and is responsible for arthropods’ increased agility over their wormlike ancestors. The exoskeleton allows precision movement, making possible the flight of the bumblebee and the intricate, delicate manipulations of the spider as it weaves its web. By providing a watertight covering for delicate, moist tissues, the exoskeleton also contributed enormously to the arthropods’ invasion of dry land habitats. However, the exoskeleton also creates some unique problems for arthropods. First, it cannot expand as the animal grows and thus must be shed, or molted, periodically and replaced with a larger one. In the process of molting, arthropods are vulnerable to predators. Second, the weight of the exoskeleton increases exponentially as the animal grows, placing a cap on the size arthropods can reach. No wonder some of the largest arthropods (crabs and lobsters) reside in buoyant watery habitats.

In general, arthropod segmentation is less distinct than the worms (annelids). Three main segments are evident: the anterior segment forms the head, and the posterior segment holds digestive structures as the abdomen. Between the head and the abdomen is the thorax, consisting of structures used for movement, such as wings and legs.

The efficient gas exchange in arthropods is accomplished by gills in aquatic forms and by either tracheas or book lungs in terrestrial forms. Arthropods also have a well-developed open circulatory system, by which blood not only travels through vessels but also bathes the internal organs directly.

A well-developed sensory and nerve system is another feature of arthropods. Most have compound eyes and acute chemical and tactile senses. The nervous system consists of a brain with fused ganglia in the head and a series of ganglia running through the body, coordinating movement and other complex behaviors.

Endangered Spider Species

Factors that affect a species becoming endangered might include climate changes, loss of habitat, or human causes. In July 2016, the International Union for Conservation of Nature (IUCN) listed 616 endangered arthropod species and 393 critically endangered arthropod species. Some examples of endangered spiders include the Kauai cave wolf spider (Adelocosa anops) of Kauai, Hawaii, and the Spruce-fir moss spider (Microhexura montivaga) found only on mountain peaks in North Carolina and Tennessee. These spiders are listed on the federal Endangered Species Act and are frequently listed as under pressure. The Dolloff cave spider (Meta dolloff), native to California and Texas, is listed as vulnerable by the IUCN.

Common Characteristics of Spiders

Spiders, scorpions, mites, ticks, and their relatives are members of the class Arachnida. The arachnids comprise about seventy thousand species of terrestrial arthropods. Animals in this class have eight walking legs; most are carnivorous, and many subsist on a liquid diet of blood or predigested prey. Spiders are the most numerous arachnids, accounting for more than forty-three thousand species belonging to more than one hundred different families. New species are continually being identified. In 2017, seven new species of Peacock spiders were found in Australia. In 2023, scientists discovered a rare giant species of trap door spider, Euoplos dignitas. The number of spiders can be larger than anyone expected. A study in Great Britain counted spiders in a meadow, coming up with 131 spiders per square meter. Within the area of 36,150 square kilometers that composes the Netherlands, there are approximately five trillion spider inhabitants. Put together, these spiders could consume all fifteen million Dutchmen in merely three days.

Many people confuse spiders with insects. Although many similarities exist between spiders and insects, such as the presence of an exoskeleton, the gas exchange system, and the circulation system, three conspicuous traits can serve to distinguish spiders and insects. First, spiders usually have four pairs of legs compared to insects’ three. Second, insects have compound eyes, whereas spiders have singular eyes with lenses. Third, insects have antennae, while spiders do not.

The body of a spider has two distinct parts: the cephalothorax, consisting of the head and breast, and the opisthosoma, or abdomen. The back of a spider is referred to as its dorsal side, and the bottom is its ventral side. The eight legs, two jaws, and two feelers (palps) are connected to the cephalothorax. The males have a bulb at the end of their palps, which is used to store and inject semen into the sexual organs of the female. There are usually eight eyes on the cephalothorax, although the number may vary from none to twelve. An extensive nerve system comprises a brain located in the cephalothorax and ganglia (the equivalent of nerves in mammals) that run through various sections of the body. The heart is situated at the front upper side of the abdomen. The silk-making spinners are found at the rear of the abdomen. These spinners are linked to glands that produce a variety of proteins, which, when mixed, polymerize to form silk. As the fluid silk is pressed through the spinners, a thread is made. The reproductive organs are located between the book lungs and the spinners. Running through the whole body is the alimentary canal, at the end of which is the excretory system.

Various Body Parts and Their Functions

Some spiders are equipped with poison glands to kill prey. The jaws are used to grab and crunch the prey. A pair of syringelike structures, which are hollow and extremely sharp, are found at the end of the jaws. They are used to puncture the body and inject poison into prey. The venom is produced in special glands and stored in a special bladder, around which is a spiral muscle. This muscle contracts to eject the poison through the syringe into the victim. The poison, made up of proteins, amines, and polypeptides, causes paralysis by disrupting the communication between the nervous system and the muscles. The poison and digestive enzymes cause the death of cells and dissolve the contents of the prey. The spider then sucks the prey empty, leaving a shell behind. Many spiders can give a nasty bite comparable to the stinging of a wasp; a spider sting can even be fatal to children and persons with weak constitutions.

Spiders use a trachea, a slit above the spinners that can be opened and closed, for the admission of oxygen. Long small tubes run from this slit into the body. Gases are exchanged with the blood by diffusion. Many spiders also have book lungs, which are hollow, leaflike structures through which the blood flows. Many modern spiders have both tracheas and book lungs. With these two systems together providing extra oxygen, the modern spider has an advantage in having quicker and more sustained reaction times than the primitive counterparts with only book lungs. The circulating blood in a spider’s body is colorless and called hemolymph. It transports nutrients, hormones, and cells in addition to oxygen. It is also used locally to raise blood pressure during molting and stretching of the legs. Spiders have an open blood circulation system, with the heart located in the back of the abdomen. Blood vessels transport the blood to the heart, but thereafter the blood flows freely in the open spaces between organs. The heart is an open tube with valves which is hung in a cavity. Elastic muscles around this cavity contract, enlarging the tubes and forcing blood to flow in only one direction. The size of the heart is closely correlated with the size of the trachea system.

A number of nerves extend from the brain to the legs, eyes, and the rest of the body. The brain occupies about 20 to 30 percent of the cephalothorax volume. Spiders have several sensory organs with which to sense and react to their surroundings. They have simple eyes, each with a single lens, which are particularly sensitive to movement. Spiders have neither ears nor a sense of taste. However, they can detect smell with scent-sensitive hairs located on their legs. With the brain and all sense organs, spiders are sharp hunters.

With its enormous strength, spider silk is an extraordinary material. A thread of silk the thickness of a pencil has enough strength to stop a Boeing 747 flying at full speed. Humans simply do not yet know how to duplicate such material. Silk threads are produced by several glands located at the spider’s abdomen. Every gland produces a thread for a special purpose: glandula ampullacea for the silk of the walking thread, glandula pyriformis for the attaching threads, glandula acinoformes for the encapsulation of prey, glandula tubiliformes for the thread of cocoons, and glandula coronatae for the adhesive threads. A thread is made up of polymerized protein molecules. The smallest measured thread was only two one-hundredths micrometers, yet a web made up of it can stop a bee flying at full speed. The thread is also very elastic and can be stretched 30 percent without breaking. Spider webs take various shapes and function to trap prey, produce cocoons, and provide hiding places for the spiders.

Male spiders are often smaller and more colorful than the females. Males can also be recognized by what appears to be a fifth pair of legs. These are actually palps with bulbs for injecting their sperm into a female during mating. During breeding season, males search for females. Once the female is found, the male has to avoid being mistaken for prey by the female. Male spiders of different species use different ways to announce to the female that they are interested in mating. If the signals are right and the female is ready, mating occurs. After the mating, the males of some species must be extremely careful, or they will become an easy meal for the female. The females lay their eggs and tend the young. The kingdom of spiders goes on.

Principal Terms

Book Lungs: specialized respiratory structures of arachnids, such as spiders

Class: the taxonomic category composed of related genera; closely related classes form a phylum or division

Compound Eyes: eyes that are made up of multiple lenses or light detectors

Exoskeleton: an external skeleton that encloses the body like a suit of armor

Molting: in animals with an exoskeleton, the process of shedding the old shell to allow the increase in body size due to growth

Phylum (pl. Phyla): the taxonomic category of animals and animal-like protists that is contained within a kingdom and consists of related classes

Trachea: a network of narrow, branching respiratory tubes

Bibliography

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