Characteristics of Spiders

Spiders are one of the most essential carnivorous groups of animals on the planet. Without spiders, insects would reach pest proportions throughout the entire world and cause massive ecosystem imbalances. The physical characteristics, diet, and predatory skills of spiders set them apart from other arachnids and allow them to be as successful as they are.

Spider Classification and Physiology

Spiders are not insects. However, like insects and crustaceans, they belong to a subgroup within the phylum arthropod. Arthropods are invertebrates with an exoskeleton.

Spiders belong to the class Arachnida, joined also by scorpions, daddy longlegs, and ticks. Like all arachnids, spiders have just two body regions, a cephalothorax, and an abdomen. These two body regions are joined by a narrow tube at their waist called a pedicel. The abdomen is soft and unsegmented, while the cephalothorax is hard and includes a spider's infamous set of eight legs. Most spiders have eight eyes, though some have less or even none at all and all have rather poor vision.

Diet and Feeding Habits

Spiders prey on many different organisms and employ a wide range of strategies to capture prey. They may trap prey in sticky webs, lasso it with sticky balls, mimick it to avoid detection, or chase and tackle it. Most detect prey mainly by sensing vibrations, but active hunters have acute vision.

Spiders can only consume liquids because they lack chewing mouthparts. They use chelicerae, pointed appendages such as the fangs at the front of their cephalothorax, to grasp prey and inject venom. Digestive juices break the food down into a liquid, which a spider can then ingest.

Prey

Spiders may prey on any of the following:

• arthropods (such as insects and other spiders)
• small birds
• frogs
• reptiles
• amphibians
• small mammals
• sometimes: pollen and nectar

If an organism is small enough for a spider to catch and consume, it will.

Habitat

It has been estimated that more than 40,000 species of spiders inhabit the earth. They are found on every continent except for Antarctica and have become established in nearly every habitat, excepting only the air. The vast majority of spiders are terrestrial, with only a few specialized species able to live in freshwater.

Spiders decide where to live based largely on the availability of prey and potential for reproduction. They will usually construct a web to scope out a potential nesting location as they try to determine whether there will be enough food and a place for them to lay their eggs. Some spiders tend to judge an area based on the presence (or lack) of other spiders and may even force their competitors from their webs, claiming them for themselves, if they deem a location sufficient for nesting.

Silk

Almost all spiders produce silk. Silk-producing spinnerets are usually located under the tip of a spider's abdomen, which enables them to spin a long strand of silk behind them. Silk production is no simple endeavor for spiders as it requires great time and energy. Because of this, some species have been recorded consuming their own silk when they are finished with it to store for later use.

There are many different types of silk and each type serves a different function for the spider.

Types of Silk and Their Functions

• Attachment: clinging to surfaces
• Cocoon: forming a protective case for eggs
• Dragline: web construction
• Glue-like: capturing prey
• Minor: web construction
• Viscid: capturing prey
• Wrapping: wrapping prey in silk to allow for consumption

Spider silk is highly regarded as a marvel of engineering by scientists for its structural properties. It is fine yet strong, resistant to many solvents, and even possesses thermal conductivity properties. Researchers have been studying spider silk for years in the hopes of understanding it well enough to manufacture a synthetic version for human use.

Species

Common Species

• Orb weaver
  • Known for weaving large, circular webs.
• Cobweb spider
  • This species includes the venomous black widow spider.
• Wolf spider
  • Large nocturnal spiders that hunt at night
• Tarantula
  • These huge, hairy hunting spiders make great pets.
• Jumping spider
  • These are tiny spiders with big eyes and the tendency to leap.

Extraordinary Spiders

There are less common species of spiders with interesting features that distinguish them from the rest.

Female flower crab spiders, also known as Misumena vatia, transform from white to yellow camouflage into flowers, where they lie in wait for pollinators to eat.

Spiders of the genus Celaenia resemble bird droppings, a clever trick that keeps them safe from most predators.

The ant spiders of the family Zodariidae are so named because they mimic ants. Some even use their front legs as pseudo-antennae.

The magnificent spider, named the Ordgarius magnificus, lures its moth prey with pheromones into a silk trap. The pheromone mimics the moth's own reproductive hormones, thus making it attractive to males seeking a female.

Sources

• Glover, N. “The Habitat Preferences of Web Building Spiders.” The Plymouth Student Scientist, vol. 1, no. 6, 2013, pp. 363–375.
• Marshall, S. A. Insects: Their Natural History and Diversity with a Photographic Guide to Insects of Eastern North America. Firefly Books, 2017.
• Saravanan, D. “Spider Silk - Structure, Properties and Spinning.” Journal of Textile and Apparel, Technology and Management, vol. 5, no. 1, 2006.