Anatomy and Physiology of Spiders

Introduction

There are around 37,000 species of spiders can be found in the world. All spiders have the similar body structures. Their bodies are divided into two main parts, prosoma (cephalothorax) and opisthosoma (abdomen). The cephalothorax consists of four pairs of legs, poison glands, eyes, nervous system, and a pair of pedipalp. For the abdomen, there are the digestive system, book lungs, Malpighian tubules, and silk glands.

Key Terms 

Chelicerae
Pedipalps
Book lungs
Spinnerets
Malpighian tubules
Silk glands

External Anatomy

Figure S.1 External anatomy of a stereotypical spider

Spiders and other arachnids are not insects because they have four pair of legs and a pair of chelicerae. Chelicerae, are the unique structures of this Subphylum Chelicerata, which have the poison ducts. All spiders have poison glands in order to attack and digest their preys. Most likely, human body is immune from those toxic substances; however, the minor of the populations is still allergic to those substances. Importantly, two genus of spiders should be paid attention, Genus Lactrodectus (black or brown widow) and Genus Loxosceles (brown recluse).

Similarly, spider's legs are named as human, there are eight main parts, coxa, trochanter, femur, patella, tibia, metatarus, tarsus, and tarsal claws. Each of this sections plays an identical roles as human and other mammals (see Figure S.1).

The pedipalps of spiders are used as an assistance tool for diets. However, they play a very important in mating because the male spider picks up and stores the sperms in the pedipalps. When the male meets the female, the pedipalps are used to insert sperms into a female genitals. There are usually eight simple eyes on spiders which are used to detect the environment more quickly. However, the numbers of eyes can be different from species to species.


Internal Anatomy

Another unique feature in spider and other arachnids is the book lung, which is the parallel sockets connected to the blood (Barron 2012). They all use book lung and trachea to perform respiration. The book lung usually located under the ventral side of abdomen and diffused air through trachea from the spiracles (see Figure S.2).

Figure S.2 Internal anatomy of a stereotypical spider. After Foelix 1996

The digestive tract of spider is relatively simple. which consists of the esophagus, sucking stomach, digestive cecum, intestines, digestive tubules, and anus. Most spiders have a strong pharynx in order to suck the fluid parts of the preys and digest a portion with the digestive cecum. Moreover, the Malpighian tubule in spider is used as a kidney to transport the waste products or "urine" into the intestine and released through the anus.

It is important to distinguish the spinneret and anus in spider carefully. Most species have the spinneret located under the anus (see Figure S.2). There are two or three spinnerets eject the silk produced from the silk gland in order to make the webs. Silk in spider initially is scleroprotein secretion (liquid) and then solidified by a single touch of the spider legs (silk threads) (Barron 2012). Scientists have proved that the pattern of making web is strongly depended on the spider psychology.

The Biochemistry of Spider Silk

References

Barron, Blake. 2012. Phylum Arthropoda, Subphylum Chelicerata. Lecture in Animal Biology. 27 September 2012. Santa Barbara: Santa Barbara City College

Foelix, Rainer F. 1996. Biology of Spiders. np.