Spider Anatomy

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Arachnids have two body parts as opposed to insects with three. The first part is the prosoma and the abdomen. usually referred to as the cephalathorax, as it consists of a fused cephalic (head) and thorax which may be separated by a distinct or indistinct cervical groove and the fovea.

The prosoma,  usually referred to as the cephalathorax, as it consists of a fused cephalic (head) and thorax which may be separated by a distinct or indistinct cervical groove and the fovea, a depression to which the stomach sucking muscles are attached. The shape of the fovea and cervical groove are important diagnostic features for the identification of spiders into their respective taxa. Chitin is responsible for this part of the exoskeleton being hard and inflexible.

The prosoma houses various external appendages:

1. A pair of chelicerae with hinged fangs
2. A pair of pedipalps similar to the legs but with metatarsus absent. The palps are often used as a pair of arms and in adult males the tarsus develop secondary genitalia that give males the appearance of having boxing gloves. These copulatory organs are vary important in the identification of genera and species.
3. Four pairs of legs consisting of seven segments. apically is the coxa and trochanter (together forming the hip), the femur (thigh), the petalla (knee), the tibia ( shin or lower leg), the metatarsus (foot) and the tarsus (toes) with tarsal claws (toenails) at its end. The legs are use for locomotion and prey capture.
4. Dorsally the plate covering the prosoma is the carapace which contains the simple eyes, up to eight in various positions, an important  identification character. The fovea a depression to which the stomach sucking muscles are attached is an important diagnostic feature for the identification of spiders into their respective taxa.
5. Ventrally the endites (pedipalpal coxa) and the labium (lip) form the mouth. The sternum (breast plate) with the dorsal carapace holds everything together. the two are joined by a pleurae, an elastic membrane. It is along the pleurae that the first crack stars the moulting process.

Internally the prosoma houses the central nervous system (brain), venom glands, the sucking stomach with part of the intestine and the muscles to control the appendages

The opisthosoma (abdomen), unlike the hard prosoma is soft and pliable. this allows the abdomen to expand when saturated with food or to house the developing eggs. Externally the abdomen contains:

1. Booklungs (a primitive breathing apparatus) are situated anterio-ventrally. Primitive mygalomorphae (baboon and trapdoor spiders) have two pairs, Araneomorphae one pair and a tracheal spiracle (a breathing apparatus as used by insects) and some advanced spiders have on tracheal spiracles. the posterior edge of the booklungs is situated on the epigastric furrow (fold) which house the spiders genitalia. Situated centrally on the epigastric furrow  the adult female has a blackish scleritised  epigyne. this epigyne is used in the identification of female spiders to generic and species level.
2. Posteriorly spiders have four to six spinnerets for the production of silk.
3. Above the spinnerets is the anal tubercle for the excretion waste products.

The opisthosoma contains the respiratory organs, heart, various spinning glands, the midgut, and ovaries for egg production.

Left: External ventral view of Araneomorpha. Right: External ventral view of Mygalomorpha.

Copulatory organs: Male palp (entelegyne). lateral view.

Copulatory organs: Male palp (entelegyne) ventral view

Copulatory organs: Male palp (haplogyne)

Illustrations from DIPPENAAR-SCHOEMAN, A. S. and JOCQUÉ, R.1997. African Spiders. An Identification Manual. Plant Protection Research Institute, Handbook No.9. 392 pp. Used with permission.

Text by Norman Larsen ©