A Look At The Characteristics of Spiders

Spiders are generally feared by people, something that is common for many of us, when we see any kind of insect, not just this one. Knowing about something we fear helps liberate us though, so take a look at the information we have on spiders below.

Most arachnids share certain characteristics including a body divided into two parts, the cephalothorax or prosoma and the abdomen or opisthosoma; no antennae; chelicerae; a unique pair of appendages in the front of their bodies called pedipalps involved in feeding and / or mating; four pairs of walking legs; external digestion; indirect sperm transfer; elaborate courtship behaviors; a strict predatory lifestyle.

However, not all arachnids share all these characteristics. For example, the cephalothorax and abdomen of opilionids and acarines have become fused giving them the appearance of having only a single body part rather than the two characteristic of the other arachnid groups. Some species in these two orders also differ in having direct sperm transfer.

Some opilionids can digest solid particulate food and, therefore, do not rely solely on predigested, or externally digested, material. Opilionids and acarines also differ in that none group is a strict predator. Opilionids are, for the most part, scavengers, whereas acarines can be predators, ectoparasites, or plant feeders.

Silk Production

Silk is characteristic of all spiders. Spider silk emerges from structures at the posterior end of the spider's abdomen called spinnerets. The proteinaceous material is produced in silk glands in the abdomen. As the silk protein is exuded through the tiny opening of the silk spigot on the surface of the spinneret, most of the water is re-absorbed.

The protein configuration completes its transformation from liquid to solid as the droplet of silk is stretched or put under tension. This can occur as the spider pulls the silk out of the spinnerets with a rear leg, or as the spider attaches the droplet to the substrate and walks away – these movements create enough tension to change the protein structure from liquid to solid.

Most spiders have three pairs of spinnerets – the anterior, the median, and the posterior pairs. However, it is thought that ancestral spiders had four pairs: an anterior lateral pair, an anterior median pair, a median pair, and a posterior pair.

Spiders in the suborder Mesothelae still have four pairs of spinnerets, but one pair (the anterior median) is nonfunctional and residual.

The anterior median pair has been lost and a rudimentary, non-functional structure called the colulus remains, positioned between the anterior lateral pair of spinnerets. Other species of araneomorphs, called cribellate spiders, have a fattened spinning plate, called the cribellum, positioned between the anterior lateral pair of spinnerets.

It is thought that the cribellum and the colulus are homologous structures and both are thought to be evolutionary derivatives of the anterior median pair of spinnerets. The cribellum is also thought to be an ancestral, or plesiomorphic, characteristic among the araneomorph spiders. The cribellum is covered with hundreds to thousands of individual silk spigots. In some cribellate species, the cribellum is divided into two small plates; in others, it is a single continuous plate.

Cribellate spiders also have a specialized comb, called a calamistrum, on the metatarsal segment of the last leg. The calamistrum is used to comb the cribellate silk out of the cribellum. Both cribellate and ecribellate spiders are found in the Haplogynae and in the Entelegynae. Therefore, it is currently thought that the cribellum is a polyphysical characteristic.