Arachnids consist 2 major body parts, the prosoma (cephalothorax) and opisthosoma, as opposed to insects with 3 tagma (body parts). Both are attached to each other by a thin and flexible pedicel, through which circulatory, digestive and nervous systems are canalized, surrounded by a complex musculature. This pedicel creates the ability to move the opisthosoma independent from the prosoma, making the art of webbing far more easy.
On the dorsal side of the prosoma we find the carapace, subdivided in the anterior cephalic region (containing the eyes) and the posterior thoracic region, which might be seperated by a distinct or indistinct cervical groove and fovea. The shape and the surface texture of the carapace and its components, such as hairiness and the shape of the fovea, are proven to be taxonomic features of big importance. Except for a few cave-dwelling species, Sinopoda scurion for example, all spiders are equipped with a set of eyes and named after their relative positions on the carapace. The number of eyes may vary within a single family, but that’s rather uncommon. Theraphosids contain 8 eyes located on the ocular tubercle, which are small compared to their body sizes. Nevertheless spiders often suffer poor eyesight. Far more posterior on the carapace the fovea is located, a distinct depression that comes in a variety of shapes and sizes on which the stomach sucking muscles are attached internally. Some species show an incredible conical horn on top of the fovea (example). The prosoma houses several external appendages, such as legs, pedipalps, chelicerae and other mouth parts. Each spider has 8 legs as opposed to insects with 6 legs. Due to the fact typical features may vary from leg to leg, but spiders are built symmetrical, people often speak of leg pairs, numbered from I to IV, anterior to posterior. Every leg consists 7 leg segments. The basal segment, which attaches the legs to the spider’s body, is the coxa, chronologically followed by trochanter, femur, patella, tibia, metatarsus and tarsus. Remarkably the tarsi end up in 2 or 3 little tarsal claws. Hunting species in general have 2 claws, whereas all web-building species need 3 claws to hold a grip on the silk strand. Some theraphosids might show a third undeveloped claw. The leg-like features next to the chelicerae are the pedipalps. They’re almost the same as a traditional leg, except for the fact metatarsi are absent, they’re mainly used for probing, digging and feeling (not locomotion) and they transform to show palpal bulbs on mature males. Together with these palpal bulbs most, but not all mature males will show tibial spurs on the first leg pair. Chelicerae are located anterior to the carapace, protecting the fangs which are used to penetrate and inject venom and enzymes inside the prey’s body. In combination with the scopula pads on legs and pedipalps and the cheliceral teeth the spider keeps a grip on the prey in order to suck the liquids inside. Spiders can’t chew on their prey, so they use venom to paralyse the prey and enzymes to liquify as much as possible outside the spider’s body. On the ventral side of the prosoma, the basal segments (coxae) of the pedipalps, termed as maxillae, and the labium assist the spider’s feeding. Surrounded by the legs’ coxae, the sternum keeps everything together ventrally. Sternum and carapace are joined by an elastic membrane, the pleurea. Along this membrane the molting process will start. The exoskeleton of the prosoma contains high doses of chitin, as it needs to be inflexible and way thicker than the opisthosoma in order to survive. Internally the prosoma houses the central nervous system (the tarantula’s brain), venom glands, sucking stomach and muscles for moving the attached appendages.
In contrast with the prosoma, the opisthosoma is flexible in order to be able to absorb nutrition and produce eggs. Externally it shows the abdomen and associated appendages. All theraphosids have an equal elliptical, globose or oval abdomen, which is in contrast with the adorned abdomen of, for example, Gasteracantha and Micrathena species. Colors and patterns on the abdomen are different and may vary throughout the stages of life. Some species are equipped with urticating setae on the abdomen, others do not own those type of hairs and very few, such as Ephebopus spp. have the urticating setae on the pedipalps (read more). Anterior-ventrally on the opisthosoma, on either side of the epigastric region, the book lungs are located. This is a rather primitive breathing system. Different to all other spiders primitive mygalomorphae, such as theraphosids, possess 2 pairs of book lungs. Araneomorphae have 1 pair and a tubular trachea. Some spiders rely on solely tracheae for respiration. The epigastric region contains an opening, housing the spider’s genitalia. In males (the gonopore) this leads to the testes, in females (the gonoslit) it hides the ovaries. Adult females of entelegyn Araneomorphae show a sclerotized external structure centrally of the epigastric region which is referred to as the epigyne. Mygalomorphae, incl. Theraphosidae, and haplogyne Araneomorphae lack epigyne! The vulva, in entelegyne Araneomorphae located underneath the epygine, usually consists spermathecae, fertilization and copulatory ducts. Usually, because (for example) some theraphosid spp. are known not to possess spermathecae (ref. Encyocratella olivacea). The structures of the epigyne in entelegyne Araneomorphae (not in theraphosid spp.), the vulva and the pedipalp morphology on mature males are proven to be the most powerful tools on taxonomic level. Near the posterior end of the abdomen spinnerets are situated. These are silk-producing organs and obviously every spider owns a few. Most spiders have 6 spinnerets, which is different to theraphosids owning solely 2 pairs of which 1 pair is nearly invisible. However not every spider will construst a web as we know it, all spiders will need to make a web to construct retreats, traps, spermwebs and/or egg sacs. Slightly above the spinnerets there is the anal tubercle for the excretion of feces. Internally the opisthosoma houses the heart, respiratory organs, the midgut, ovaries for egg production and various spinning glands.
Theraphosid species are known for their size and hairiness, but not all species are as big and hairy as you might think. Tmesiphantes mirim, the smallest theraphosid known to date, is with a body size of 5,5mm way smaller than the more common Tegenaria domestica. Theraphosidae belong to the suborder of the Mygalomorphae, which are mainly different to Araneae by the articulation of the chelicerae. Mygalomorphae will always strike downwards. Mesothelae own the same kind of chelicerae as Mygalomorphae, but their abdomen is dorsally armoured, in which they differ from Mygalomorphae. Theraphosidae, however, are only a small part of the suborder of the Mygalomorphae. Theraphosidae possess rather small eyes compared to their body sizes, thick scopula pads, claw tufts next to the tarsal claws, 2 pairs of book lungs, only 2 pairs of spinnerets and different to Barychelidae they have a prominent anterior lobe on the palpal coxae (source). Species within the family of the Theraphosidae differ as well. Taxonomists subdivide the family of the theraphosidae in genera.
THE OCULAR TUBERCLE
Eyes differ in structure and function, as the direct eyes (AME) appears dark when lightened, which is in contrast with the silvery indirect eyes (ALE, PME, PLE) containing a light reflecting tapetum ludicum behind the light sensitive retina. The tapeta in those eyes increase visual sensitivity at low light intensities. These indirect eyes are often enlarged in spiders with good vision, which is not the case in theraphosids. Similar to other arthropods, including some spider families such as Lycosidae and Agelenidae, it might be possible theraphosids are able to detect polarized light as well, playing an important role in their orientation skills.
Clypeus: The small distance between the ocular tubercle and the anterior edge of the carapace.
Ocular tubercle: Small “hill” on which the eyes of the tarantula can be found. Although they possess 8 eyes, eyesight is very bad.
Median ocular quadrangle (MOQ): The area of the 4 median eyes.
THE MOUTH PARTS
Fangs: Used to threat the enemy, to penetrate the prey and inject venom into its body. They’ve been proven very useful to dig as well.
Maxilla: Often referred to as palpal coxa or gnathocoxa.
Labiosternal mounds: A labiosternal groove demarcates the labium from the sternum.
Labium: Covers the mouth and assists manipulation of food during mastication, covered with small sclerotised cupsules.
Sternum: The ventral shield, keeping the opisthosoma together. It shows 3 pairs of stigilla, which are spots for internal muscle attachment.
Cheliceral teeth: Sharp and lined-up hardened points on the chelicerae above the fangs, used to get (and hold) a grip on the prey. Bird-eaters use their cheliceral teeth to rip the prey into pieces and create an unrecognizable ball (bolus) of leftovers.
Except for Uloboridae, all spiders are equipped with venom glands, located in either the chelicerae or under the carapace. In mygalomorph species the venom glands are located in their large chelicerae. The venom goes through the venom canal towards the fangs, from where it’s injected into the prey’s body through the venom holes in order to paralyze the victim. Secretion of digestive fluids, coming from other glands, will start external digestion of prey. Characteristics of the venom of various spiders are determined and is considered to be target-specific. The venom of Latrodectus spp. (black widows), for example, contains neurotoxic proteins that cause severe pain in humans (esp. in the abdominal region), but is usually not fatal. The Australian funnel web spiders from the genera Atrax and Hadronyche (family Hexathelidae), however, can cause severe neurological and cardiovascular envenomation in humans and primates, but usually cause less severe effects in several other vertebrates. These funnel web spiders are considered to be one of the most dangerous spiders to humans around the world. The venom of Loxosceles spp. (known as the American recluse) is considered to be very effective against humans and rabbits, but probably less harmful to some other vertebrates. In this last generalisation we have to keep in mind the venom of Loxosceles (and Sicarius) spp. acts very different to that of all other spiders. In the venom of Sicariid spiders enzymes play a higher -cell destroying- role. These different effects of the same venom on different species, might indicate several species carry antibodies to neutralize the venom before it can do any harm. This statement, however, might not be entirely correct considering the fact peptide toxins mainly act on ion channels. These ion channels in different animals basically work in the same way and are highly conserved. However, there can be few mutations in the amino acid sequence and even single mutations can have huge effects. One very interesting example about this was published: The toxin μ-theraphotoxin-Ae1a (from Augacephalus ezendami) is very active in one cockroach species (Blattella germanica) but is hardly active in another cockroach species (Periplaneta americana). The reason for that is one single point mutation in the amino acid sequence of the sodium channel where it acts on. (Source, Scientific Reports 6:29538). In general, little is known about the effect of spider venom on humans. Often you can see LD50 values show up, typically obtained from mice or other laboratory organisms. These data, however, are highly dependent on the experimental set-up and can’t be extrapolated to humans. Many of the so called “medically-significant” spiders seem to have toxins that are particularly active in vertebrates/mammals and on receptors that are involved in processes maintaining life. Besides the venom potency, the amount of venom being available and injected during bites should also be taken in consideration. For example, unpleasant symptoms often show up after bites from Poecilotheria species, but this is no prove to indicate a higher venom potency. Poecilotheria species, on the other hand, usually injects quite high amounts of venom compared to many other tarantulas. We’re currently working to give you a proper, clear and correct view about spider venom, but due to it’s complexity this will take a while. Special thanks goes out to Tobias Hauke for his constructive contribution on text and information.
Stridulation is the act of making a shrill creaking noise by rubbing special bodily structures together and is used as a warning sign when threatened. Those structures consist of either torn setae, paddle setae, plumose setae (as seen in the photo below) or small pikes (strikers). Forms and combinations differ from species to species and are proven to be a powerful taxonomical tool. In tarantulas those structures are located on the opposing faces of the chelicerae, on the opposing faces of chelicerae and pedipalps, on the opposing faces of the maxillae, or between coxae and trochanter of pedipalps and forelegs. In some species, such as Pelinobius muticus, stridulatory bristles are located on coxa and trochanter of the first two leg pairs. Length, frequency and loudness of stridulation depends on the species.
• About: Stridulation.
Every leg consists 7 leg segments. The basal segment, which attaches the legs to the spider’s body, is the coxa, chronologically followed by trochanter, femur, patella, tibia, metatarsus and tarsus. Remarkably the tarsi end up in 2 or 3 little tarsal claws. Hunting species in general have 2 claws, whereas all web-building species need 3 claws to hold a grip on the silk strand. Some theraphosids might show a third undeveloped claw.
Tarsal claws: Claws at the distal end of the tarsi. Female spiders of some families possess 1 single claw on the pedipalps, but these are no theraphosids. Some theraphosid genera, however, show a poorly developed third claw. All web-building spiders have three claws, as it is used to grab the silk strand, but not all three-clawed spiders build webs.
Scopulae: Dense tufts of hair at the end of the spiders’ legs. Scopulae consist of microscopic hairs, each covered in even smaller hairs called setules or “end feet”. Scopulae increase with each molt and aid the spider in climbing and gripping. Scopulae on pedipalps are mainly there to get a grip on the prey. The size and/or the presence of scopulae on the metatarsi may distinguish one species from the other.
HAIRS AND VIBRATION
In order to survive spiders rely on proprioceptor stimuli and the sense of touch and vibration, due to the fact they lack auditory receptors and often suffer bad eyesight. Most hairs on a spider’s body are there to detect low-frequency vibrations, to protect against parasites and to keep water on a safe distance. Except for spiders from the orders Solifugae, Ricinulei and Opiliones all possess trichobothria, elongate setae found on legs and pedipalps in tarantulas, which are sensitive to air movement and sound waves. This way the spider is able to detect prey and predator. Due to proprioceptive skills the spider detects the position of body appendages and recognizes her position in space. Hair plates of different sizes in the coxae give information to the spider’s nervous system about the legs’ postures.
A SPIDER’S INTERNAL ANATOMY
Caution: This drawing was made by John Henry Comstock a century ago. This is no theraphosid.
A spider’s brain is almost entirely found in the prosoma, with only a few bundles of nerve cells (ganglia) in the opisthosoma. The shape of the brain reflects the spider’s behavior, as the posterior region of the brain is bigger in webbers than hunters due to the fact they need a higher sense of touch to survive. The supraoesophageal ganglion, often relied to as the brain, is the first part of the spider’s brain, situated (as the name suggests) dorsal of the esophagus. It processes sensory information, esp. from the eyes and gives proper impulses to the chelicerae and venom glands. Spiders relying on a good eyesight, probably have a bigger supraoesophageal ganglion. The suboesophageal ganglion is situated below the digestive system and shows some lateral extensions towards legs and palps. Most of the spider’s muscles are controlled by this ganglion and in some species it helps in processing sensory information coming from the legs. Spiders for which eyesight is not that important for survival and webbers in general probably possess a bigger suboesophageal ganglion. From the suboesophageal ganglion the brain goes through the pedicel to the abdomen and ends up in a few abdominal stuctures, such as the heart.
In mammals (and birds) the circulatory system is a closed double circuit, meaning that the blood flows through the heart twice. The blood will first pass through the lungs in order to be oxygenated from which it goes back to the heart to be pumped around the body. When the deoxygenated blood returns in the heart it will have to pass the lungs first in order to be able to supply body tissues again. This system stands in contrast with the open circulatory system, which means its arteries carry haemolymph, the equivalent of blood in arthropods. Haemolymph contains hemocytes and hemocyanin, a copper-based oxygen transporter molecule. It is because of the presence of hemocyanin, haemolymph turns blue-green when oxygenated. Deoxygenated it’s gray or colorless. Hemocyanin, however, is by far less efficient than haemoglobin in mammals. It is assumed giant arthropods from Paleozoic, who lived 300 million years ago, got extinct because of the inefficiency of hemocyanin and the tracheal system of respiration.  Circulation of oxygenated haemolymph in spiders starts by filling up little holes, ostia, in the heart during diastole (relaxation of the heart muscle). The heart is located in a membranous bag, the pericardium, centrally and dorsally inside the opisthosoma.  When the heart muscle contracts during systole, the haemolymph is pushed into the 2 main arteries. The anterior aorta sends oxygenated haemolymph towards the prosoma, whereas the posterior aorta does the same towards the opisthosoma. Valves ensure a flow of haemolymph in the right direction.  There are a few veins with open ends in the system to make sure the blood goes where it needs to be, but no capillaries at all.  Driven by body pressure the deoxygenated haemolymph is collected in lacunae in the ventral area of the opisthosoma, from where it passes the booklungs to be oxygenated to find its way back towards the pericardium to start the journey all over again. Research data suggests there is a circulatory centre within the brain of tarantulas, which exerts neural control over a cardiac ganglion in the first segment of the spider’s heart (source).
• About: Circulation of haemolymph: “The circulatory system of spiders” by Christian S. Wirkner and Katarina Huckstorf.
THE SUCKING STOMACH
Pharynx: Anterior part of the foregut between mouth and esophagus.
Esophagus: A fibromuscular tube through which food passes, connecting the pharynx with the sucking stomach.
Sucking stomach: A widening of the posterior esophagus, located in the prosoma, situated just behind the point where the foregut emerges from the nervous collar and rests upon the endosternite. The gut is lined with cuticle up to the sucking stomach, which makes it technically part of the exoskeleton. Remarkably, the spider will have to pull the esophagus and the sucking stomach through the brain during molt. The sucking stomach is a pump, driven by (dilation and circular) muscles, to aid the spider sucking the fluids out of it’s prey. Valves at entrance and exit ensure the fluids are going one way, directly into the digestive system.
• About: Sucking stomach, pedicel and heart: Coupling between the heart and sucking stomach during ingestion in a tarantula.
• About: Sucking stomach: Spider digestion & food storage.
Arthrordial membrane: Muscle attachments on both sides of the epigynum.
Bursa copulatrix: Lower alcove below the gonoslit. Sperm and eggs meet here before they’re deposited in the egg sac.
Epigastric furrow: A transverse groove or fold across the anterior ventral part of the opisthosoma, delimiting the second and third opisthosomal segments.
Gonoslit: The slit, opening, towards the female’s reproductive organs.
Slit sensilla (plural): Slit sense organs in the exoskeleton of arachnids.
Spermathecae: Haplogyne spiders, such as mygalomorphs and some araneomorphs, possess a more primitive female genital anatomy than the rest of the araneomorph families, which are entelegyne spiders. The small crevice in between the first pair of book lungs houses two small sacs, the spermatheca, used to receive and store sperm from the male. Spermathecae come in a variety of shapes and sizes, but are mainly subdivided in 3 categories: fused, partially fused and paired. In contrast with the last 2 categories, a fused spermatheca is a single structure. Fused spermathecae start as a pair and fuse later as the spider gets older.
Uterus externus: Connects uterus internus and the gonoslit. Some species don’t possess spermathecae and store the sperm in the oviduct and uterus externus. Sickius longibulbi and Encyocratella olivacea are the only 2 theraphosid species without spermathecae.
• About: Spermathecae.
Used to neutralize the fangs of the female during the mating process. Whether or not tibial spurs are present on a mature male, combined with a variety of shapes, is a powerful taxonomical tool to distinguish 1 species from the other (for example: Theraphosa blondi vs. Theraphosa apophysis).
Only present on the distal part of the pedipalps on all mature males. Used as sperm transfer organ during copulation. Once matured, the males will build a triangular web on which they drop sperm. Tapping the pedipalps on the so-called spermweb will fill up the bulbusses. A mature male has to go through this procedure to be ready to mate and successfully copulate.
Cymbium: The pedipalp tarsus of a mature male.
Embolus: The smaller end of the bulb that injects the sperm.
• About palpal bulbs: Male palpal bulbs and homologous features in Theraphosinae (Aranea, Theraphosidae).
• About anatomy: British Tarantula Society.
• About: Spider’s tacticle hairs.