Minibeast Profiles: Spiders
by Gary A. Dunn, M.S., F.R.E.S., Director of Education
It's not hard to tell spiders apart from other tiny animals such as ticks, scorpions, centipedes, millipedes, isopods, and insects. Spiders belong to the class ARACHNIDA; they have an exoskeleton (like all arthropods), two body parts, eight legs, chelicerae (fangs), 4-8 simple eyes, and a pair of spinnerets on the abdomen. Spiders differ from insects by lacking compound eyes and wings. The world's smallest spider is the tiny Samoan moss spider (1/2 mm, roughly pinhead size); the largest is the goliath birdeating spider (4" body and 10" leg span).
Blonde palomino tarantula, Aphonopelma chalcodes
There are about 35,000 species of spiders around the world. Some spider species have inhabited the planet for about 300 million years (we know this because of the fossil record that spiders have left behind). Spiders are classified (divided into groups) based on various body characteristics. There are three large groupings of spiders (known as suborders): the primitive spiders (Mesothelae), the tarantulas and relatives (Orthognatha), and the true spiders (Labidognatha). The true spiders are the most commonly encountered, and are represented by over two dozen families including the orb weavers, wolf spiders, jumping spiders, funnel web spiders, crab spiders, and widow spiders.
Spiders can be found in most major habitat types: forests, deserts, jungles, grasslands, and even aquatic situations. Spiders are adapted to their environments. Adaptations are inherited abilities that help animals carry out their life functions and survive in the natural world. Adaptations include physical structures (size, shape, and special body features), as well as internal processes (egg production, food digestion and protection against hazards). Spiders have special adaptations for breathing, feeding, escaping enemies, sensing changes in the environment, and mating.
Because spiders live inside a hardened, non-living exoskeleton, they can only grow by periodically shedding their skeleton and replacing it with a larger one. This process is called molting. The baby spiders, known as spiderlings, hatch from special egg sacs, and molt several times before becoming adults. Unlike most insects, when spiders molt they are able to regenerate lost limbs and other appendages.
Why do spiders do so many strange and amazing things? Behavior is one of the ways that spiders adapt to their environment and become more successful. Spiders have many ways of sensing changes in their environment and of responding to those changes. Behavior is the outward, visible activities of spiders. Spider behavior requires a stimulus, an urge to satisfy a special need. Spiders have three basic needs that determine almost all of their behavior: finding food, survival, and finding a mate. One of the most unusual abilities of spiders is their production of silken thread from a special gland located at the tip of the abdomen next to the spinnerets. This silk is stronger than a steel thread of the same thickness. It is used to catch prey, wrap prey for storage, wrap and protect eggs, make a shelter, use as a safety rope when climbing, and for ballooning (floating on the wind). Spiders are capable of producing tremendous amounts of silk during their lifetimes. In fact, some webs, if unwound and stretched out, would be 300 miles long! Humans have also found spider silk useful, and it has been used in the manufacture of bandages and rifle scope crosshairs. It is not, however, used to weave cloth (silk cloth comes from silk threads produced by the silkworm caterpillar).
Spiders are often extremely abundant in terms of their populations. Their role as predators of small invertebrate animals (especially insects) and as food for other animals (including frogs and toads, lizards, birds, shrews, mice, beetles, centipedes, wasps, flies, and even other spiders) are very important to the ecology of our planet. Most all spiders are considered beneficial to humans because they help people by controlling unwanted pests. Spiders also produce silk for products such as bandages, bullet-proof vests, and telescope crosshairs. A few spiders have aesthetic and entertainment value. Some spiders are harmful to humans because they hurt people or domestic animals. For example, spiders may annoy and injure man and other animals by biting. The bites of a very few spiders (widow, recluse, and funnelweb) can be quite serious, even deadly. The bite of a tarantula, despite what most people think, is rarely more serious than a bee sting (generally painful, only rarely serious).
Because of people's activities and destruction of spider habitats, some spiders have become rare, threatened or endangered. Several tarantulas are now threatened with extinction. Spiders are not without natural enemies, and frogs, toads, lizards, birds, shrews, mice, beetles, parasitic flies, wasps, centipedes, and even other spiders prey upon spiders. Many spiders are also killed by fungus diseases and adverse weather conditions, and of course many humans quickly crush (or spray) any spiders found in or around the house and yard.
REFERENCES
If you would like to learn more about spiders, I recommend the following books:
| This educational resource was prepared by the Young Entomologists Society, 6907 West Grand River Ave., Lansing MI 48906-9131, phone/fax 517-886-0630, e-mail YESbugs@aol.com. Support minibeast youth education - join Y.E.S. today! |