Spiders

Spiders are omens of good luck, and the bigger they are, the more luck for you.  If you see a spider on Halloween, it means that a deceased loved one is watching over you.  According to folklore, spiders have mystical powers with their ability to spin both physical and magical webs.  The name spider is from “spinder” a reference to its spinning ability.  The Greek goddess Arachnea was jealous of her rival, the goddess Athena and challenged her to a silk spinning contest.  When Arachnea lost, Athena turned her into a spider destined to spin silk forever.   There are more than 600 species of spiders in Illinois with about 60 of those common to northern Illinois.  Let’s take a closer look at spiders and their abilities.

Spiders are part of the largest phylum of animals on earth: arthropods, which include crustaceans, insects, myriapods (millipedes and centipedes) and arachnids (spiders, scorpions, mites and ticks).  Spiders are characterized by having eight legs and two body parts, the cephalothorax and the abdomen, connected by a narrow portion called the pedicel.

We may characterize the cephalothorax as the head, with the spider’s face in front, mouth at the bottom, and all eight legs attached to its underside.  Internally, the cephalothorax contains the venom gland, the brain and the sucking stomach.  At the top of the face  are six to eight simple eyes arranged in two rows from left to right.  Each eye is a single lens, but all can detect color, movement, shape and size.  Below the eyes are two vertical jaws lined with teeth and tipped with fangs.  The fangs are used to inject both poison to disable and kill a victim and enzymes to break down a victim’s internal tissue to a liquid form.  Located below the jaws is another pair of appendages called pedipalps.  These are used when holding prey and feeding and are used in mating on fully grown adult males. 

The abdomen contains all remaining organs including heart, intestines, book lungs and trachea, ovaries and silk glands.  The abdomen may be long and thin or large and bulbous and is often colorfully marked.  These distinctive colorings are useful for species identification.  At the very back of the abdomen are three pairs of spinnerets.  Each one attaches to an internal silk producing gland.  Silk is a liquid mixture of proteins, which when exposed to air, dries immediately to a threadlike, elastic material.  It can be stretched up to three times its original length, and the tensile strength is greater than that of solid bone.  Each gland may produce a different type of silk.  The spider uses silk, both sticky and non-sticky strands, to form webs.  A web may be designed as a sheet, a funnel, a circular orb or in an irregular jumble of strands.  Other uses for silk are for wrapping prey for later consumption, creating the outer covering of an egg sac, or to create a shelter.  Silk is used in two ways for movement.  One is to create a dragline, which a spider attaches to an anchor point, such as a tree limb or house eave, and uses it to lower themselves.  The second movement, used by immature spiders to leave home, is called a balloon string.  The young spider points its abdomen skywards and shoots out a set of three threads in a triangle pattern designed to catch the wind and fly them to new territory.  Ballooning usually takes the spider several inches or feet to the next branch, but it has been known to transport a spider over many miles.  The last use is to create a sperm web, which I will discuss below.  Silk is reusable, and all spiders consume their webs after each use, returning the proteins to liquid form available to be re-used within minutes.

All spiders are predators and hunt for food.  Favorite foods include other arthropods as well as other spiders.  Adult females are considerably larger in size than adult males who must impregnate the female without getting eaten.  When males reach sexual maturity, the ends of their pedipalps enlarge to form a sperm holding chamber.  A sperm web is formed from silk and sperm is laid on this web to be taken up into the pedipalps where it is stored until needed.  To avoid being eaten, males will court a female to clearly establish his intentions.  For species with poor eyesight, the male will walk onto a female’s web using an intricate pattern of vibrations that indicates he is not prey.  For species with good eyesight, the male will get in front of the female and perform a courtship dance or combination of poses. 

Once the female accepts the male, his sperm must be deposited from each pedipalp into the epigynum, an opening on the bottom of her abdomen leading to the ovaries.  He will climb on the back of her abdomen, reach around and deposit sperm from both pedipalps  and leave.  Males may mate with more than one partner, but most will die shortly after mating.  Eggs are produced and fertilized during the summer, then deposited in an egg sac.  The egg sac may be carried around by the female or attached to the edge of her web.  Egg sacs may contain from one to several hundred eggs.  Eggs will hatch within a few weeks, depending on the species.  Spiders are born blind but will grow functioning eyes within several days of hatching.  The young stay close to the egg sac to use it for nourishment until they can see and begin to hunt for themselves.  The young will molt from four to twelve times, growing functional eye tissue, fangs and spinnerets during subsequent molts.  Any lost body parts, such as a leg, can also be regrown during a molt, however once they are adults, they will never molt again.  The molting process normally takes several months and many species will spend their first winter between molts, having their final molt during spring of the following year.

Following the first or second molt, juveniles are able to feed themselves and will begin to hunt on their own.  As predators, they quickly find that other spiders make good meals, and cannibalism in large hatches is a reality.  In general, spiders are either active hunters, typically found in species with good eyesight, or web weavers, typically those species with poorer eyesight.  For web weavers, all prey is caught in a web.  Prey thrashes about getting more entangled until the prey tires.  Spiders can tell from the vibrations at which point they can safely move in and using their fangs, inject venom to further subdue and kill the victim.  At the same time, enzymes are also injected to break down internal tissues into liquid form.  Active hunters will lie in wait or chase down prey.  Many active hunters use coloration to blend in with flowers and other vegetation while hunting.  Some species, including crab spiders in our area, can change their body colors to match different flower colors.  Spiders eat all meals in a liquid state and have powerful stomach muscles that pull all liquids into their stomach.

All spiders produce venom.  In northern Illinois, there are only two species that may pose a serious threat to humans.  They are the Brown Recluse, Loxosceles reclusa and the Northern Widow, Latrodectus variolus.  Both of these species are native to areas farther south and are not commonly found in northern Illinois.  It is thought that they only occur when brought into an individual home from elsewhere.  Spiders, in general, including these two species, are not known to bite unless provoked.  Most spiders live an average of one to two years, although some species in the tropics may live up to 20 years.  Their most effective predators are found among wasps.  Two wasp families will repeatedly sting the spider until it is paralyzed, then drag it off to their nests.  Instead of eating the spider, the wasps lay an egg on the spider’s abdomen, and when the egg hatches, the larva will feed on the still living spider until it dies and the wasp pupates.  Other predators include fish, frogs, toads, salamanders and other spiders.  Spiders are one of our gardens most beneficial inhabitants, eating many of the garden’s numerous flower and vegetable pests. 

Winter is the time of least activity, but warmer days or when the sun is providing radiant warmth, you can observe spiders being active even on top of the snow.  Most overwinter in leaf litter, under bark or in hollow logs or crevices.  If they decide to move inside with you and you prefer they didn’t, they can easily be captured with a paper cup and a lid (3×5 cards work well) and released outdoors.  However, they will do a good job patrolling insect pests, including aphids on indoor plants, as well.  There are many good books and websites to get more information.  A couple of my favorites include “Good Garden Bugs” by Mary M. Gardiner, Ph.D.;  “Spiders of the North Woods” by Larry Weber, part of the North Woods Naturalist Series; and a video of ballooning on the Science  Magazine website found at watch-ballooning-spider-take-flight.