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Spiders
are superb predators in the garden and occur in a wide variety of shapes,
colorations and adornments (such as hairs, spines, etc.). Even though we have
not yet identified the spider pictured at left, we have observed several in our
demonstration garden. It is a commonly seen spider in the Galveston-Houston
region.
Most gardeners would agree that spiders are not the most aesthetically pleasing of beneficials. Spiders may even be scary with most species having eight eyes, hairy bodies and fiendish fangs as well as eight legs. We propose you look on spiders with thanks, not concern, as they are definitely one of our friends.
Role of Spiders as Biological Control Agents
Spiders are abundant and widespread and, best of all, a natural controller of insect pests. Spiders are beneficial predators and serve a significant role in keeping populations of many insect pests in check. Spiders are oftentimes the most important biological control of pests in and around homes, yards, gardens and crops.
Spiders use various tactics to capture prey. Web building spiders use their webbing to ensnare; other species are hunters that actively search for their food. Consequently, destruction of most spiders should be avoided because without spiders our world would be over-run by insects. And spiders are a food source for birds and other small mammals, especially during winter and spring.
Of course, some folks are afraid of spiders. Most spiders are small, inconspicuous arthropods, which are harmless to humans. There are only four species found in the United States whose bites cause serious reactions in humans and only two consist of the nearly 900 species of spiders in Galveston County.
Basic Spider Biology
Spiders are arachnids, not insects; however both belong to the largest group of animals on earth—the arthropods. These are animals with hard external skeletons and jointed limbs. Arthropods come from the Greek arthro meaning “joint” and podos meaning “footed.” So what’s the difference between spiders and insects, you ask?
Simply put, spiders have two main body parts (a cephalothorax, which is the fused head and thorax, and an abdomen on which the tip has a group of small spinnerets that produce silk) whereas insects have three body parts (head, thorax and abdomen). Spiders have eight walking legs and insects have six. Spiders have six or eight “simple” eyes and insects’ eyes are “compound.”
Spiders have a piercing jaw and fangs (the jaw-like structures are called chelicerae, each of which ends in a hollow fang through which venom can be ejected) and insects simply chew. Spiders can't fly, but many insects can.
Spiders come in unusual body shapes and colors, which are helpful to deceive and ambush prey, as well as to attract mates. The spider size is somewhat limited as their respiratory systems become less efficient as their size increases.
Yet there are spiders that are quite tiny and found in hidden areas such as damp, cool forest leaf litter and moss. This is because their small bodies will lose water quickly in dryer habitats. On the other hand, some spiders grow so large that their legs can span a dinner plate. These spiders usually take more than a decade to reach full maturity.
Spiders lay their eggs in a silken, egg-shaped sac. The egg sac can be hidden in a web, attached to a surface, or carried by several of the female species (wolf, cellar and nursery web spiders). Spiders may produce several egg sacs, each containing up to several hundred eggs. Young spiders, known as spiderlings, emerge from the egg sac and disperse. Many climb to the top of a nearby object, produce long filaments of silk (known as gossamer) and are carried by the wind. This method of dispersal is known as ballooning.
Young spiders (spiderlings) resemble adults except for their smaller size and coloration. A spider grows by shedding its skin between four to twelve times before maturity. Adult male spiders are smaller than females, sometimes dramatically so. Males are identified by an enlarged pair of palps (mouthparts), which have been compared to miniature boxing gloves or a fifth pair of legs. These palps are used to transfer sperm. Male spiders are often found in homes as they tend to wander during the mating season in search of females or during the early fall when cooler outdoor temperatures force them to find shelter.
Though some species of spiders (widows and some wolf spiders) may live for a few years, most only survive for a season. However, tarantulas will often survive a decade or more.
Another thing that sets spiders apart from other animals is their ability to spin silk. All spiders produce silk, which is secreted as a liquid through their spinnerets and hardens on air contact. Spiders use silk for a variety of purposes, such as making egg sacs, capturing and holding prey, making shelters or retreats and transferring sperm during mating. Spider silk is necessary to some species of birds for nest building; for example, hummingbirds steal spider webs and use them to bind their nests.
The venom of most species is not particularly toxic to humans, usually resulting in no more than a slight inflammation or itching sensation. Most spiders’ fangs are too small or weak to puncture human skin. Spiders usually will not attempt to bite unless accidentally trapped against the skin or grasped, although some species actively guard their egg sacs or young. The black widow spider (Latrodectus hesperus), the brown recluse (Loxosceles reclusa), the hobo spider (Tegenaria agrestis) and the sac spider (Cheiracanthium trachelas) are the quarrelsome quartet in the US.
As previously stated, some spiders actively search for their prey such as jumping spiders, nursery web spiders and wolf spiders. Any webs they construct are used solely as resting areas. You will only see these spiders when they are in search for food.
Passive hunters are spiders that lay in wait for their target rather than searching for it. When their quarry approaches, they may jump or pounce to seize it. Crab spiders are purely passive hunters, though tarantulas and other spiders use this technique. Many spiders use webbing to ensnare their prey. Their web designs vary and may or may not be elaborate.
Web building spiders include cellar spiders, cobweb spiders, funnel web spiders
and orb weaver spiders. All spiders produce a venom that is poisonous to their
food source and once this venom is injected, it immobilizes their victim and
then begins the digestion process.
One other group of spiders are the spitting spiders (Scytodes). A spitting
spider has long, spindly, banded legs and a spotted pattern on its raised
cephalothorax, the front body region. Spitting spiders are slow-moving, common
in window sills and considered harmless.
The class Arachnida includes spiders and some other arthropods that are closely related to them. Close relatives of spiders are scorpions, pseudoscorpions, mites, ticks and daddy-long-legs (also called harvestmen). Daddy-long-legs are very commonly confused with spiders due to their general appearance and eight legs, but these brown creatures belong to the order Opiliones and are not spiders.
Influence of Spiders on Human History & Mythology
So much for science. Let’s talk history. Interestingly, variations on the same spider theme changed the lives of characters, as well as history, in such diverse cultures as in Christianity, Japanese folklore and Islam.
When King Saul pursued David, he hid in a cave near Jerusalem. After David entered, a spider made its web across the cave’s entrance. Saul saw the web and called his men away, for the undisturbed web showed no one had been there. David's life was saved and he became the King of Israel.
In Japanese mythology, the warrior Yoritomo, with six of his most faithful followers, hid from enemies inside a large hollow tree. As in the story of David, a spider then built its web across the opening of the hollow and the bad guys decided Yoritomo was not inside because the web was intact. Yoritomo escaped to become the founder of the Shogunate and the first Japanese Mayor of the Palace.
Fourteen hundred years ago, enemies were chasing the prophet Mohammed, who hid in a cave. Suddenly, an acacia tree sprang up out of the ground in front of the cave. Then a spider made its web from the entrance of the cave to the acacia tree. Needless to say, his enemies saw the web, left and Mohammed escaped to become the Prophet of Islam.
King Edward I of England battled six times with Robert the Bruce attempting to drive Robert out of Scotland. The Scots had been badly beaten and Robert was forced to hide in a barn. In his discouragement, he raised his eyes heavenward and noticed a spider hanging by a long silvery thread from one of the wooden beams. The spider was trying to swing itself to another beam.
The spider tried again and again, failing every time. 'Six times,' thought Robert to himself, 'have I fought against the English and failed.' And six times the spider tried to reach its goal. As Robert watched, the spider swung itself again with all its tiny strength. On the seventh try, it succeeded. It swung on to the beam and fastened its thread.
"Try, try and try again" was the motto Bruce adopted from watching the spider. A seventh battle was fought and this time the King of England was forced to retreat back to his own country. To this day, the victory and independence of Scotland is traced to a spider that kept trying again and again to spin her web in a cave and inspired the king of Scotland, Robert the Bruce.
History of Spiders
Spiders have a long and interesting history. They have been around for longer than we can imagine and still continue to fascinate us today. Spiders were among the earliest of animals to live on land. Entomologists believe the Attercopus fimbriungus lived 380 million years ago during the Devonian Period, evolving from a thick-waisted arachnid ancestor that had just emerged from life in the water.
Most of the early segmented fossil spiders belonged to a group of (probably) ground-dwelling predators, living in the giant clubmoss and fern forests of the mid-late Paleozoic period. It is believed they existed on cockroaches, giant silverfish, slaters and millipedes. (Are you liking them better?)
The use of their silk may have begun simply as a protective covering for the eggs, a lining for their den or even as a ground web. As the spider’s development progressed, so did its silk. By the Jurassic Period, the orb weaving spiders had developed a maze-like, aerial web to trap flying insects.
The hunting spiders living in litter, bark and foliage seemingly adapted to the onslaught of new prey and dense habitats. There are a remarkable number of whole spiders caught in fossils from tree resins showing that, during the Tertiary Period, the spider of 30 million years ago is structurally the same as today.
Summary
While there are a few species of spiders that can be hazardous to our well-being, the vast majority of spider species clearly provide a beneficial impact by serving as very effective predators of insects pests.