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A slightly in-depth guide to tarantula anatomy and taxonomic terms

Discussion in 'General Tarantula Discussion' started by plessey, Dec 31, 2012.

  1. plessey

    plessey Active Member

    The Beginners Guide to Theraphosid Taxonomy​
    by A.C. Himpanzee​

    The aim of this guide is to hopefully give you a better understanding of the various taxonomical terms and spider anatomy that crop up in description papers and articles. Normally I would suggest to anyone interested in this sort of thing to seek out copies of Andrew Smith's excellent books, The Tarantula Classification and Identification Guide, Baboon Spiders Tarantulas of Africa and the Middle East or Tarantula Spiders Tarantulas of the USA and Mexico. Unfortunately all three books have been out of print for a number of years and where once upon a time you could regularly find them on offer on ebay for around £10 - £20 ($16 - $32) it seems those days are long gone. On the odd occasion that they do crop up now it is usually for a ridiculous sum of money like £250 ($400). So because there seems to be no other books of their kind readily available on the market that offer a stepping stone onto the long path to spider geekdom I have decided to knock up this guide. Now I must warn you I am certainly no taxonomist, I didn't even study biology at school (went for the physics and chemistry option instead), so if anyone who knows better spots any mistakes then please let me know and I shall correct them.

    Basic anatomy

    To begin with i will start with the basic anatomy of a tarantula. You have probably seen loads of examples of these in books or on-line so you should already be familiar with most of the body parts labeled. Later on we will look at some of these parts in more detail.

    This first picture is just a dorsal view of the whole spider

    The next picture is a ventral view of the abdomen showing both pairs of book lungs, the epigastric furrow which is sometimes referred to as a gonopore (if male) or a gonoslit (if female), both anterior and posterior spinnerets labeling the 3 sections of the posterior pair and the anus.

    Ventral view of the cephalothorax. Note it is quite common to see the maxillae referred to as palpal coxa in some papers.

    Inside of the chelicerae showing the fang and cheliceral teeth

    Close up of cheliceral teeth

    The leg consisting of 7 segments. In a lot of old descriptions the length of the patella + tibia compared to the length of the carapace was used as a taxonomic tool.

    On the end of the tarsus there are usually 2 small retractable claws called tarsal claws. In some genera such as Phlogiellus there is a smaller 3rd claw present.


    The pedipalp. Looks just like a small leg but as you can see it lacks a metatarsus.

    Anatomical directions and locations

    Before looking into various body parts in more depth it is probably best to get to grips with some basic anatomical directions. The main ones being anterior, posterior, retrolateral, prolateral, medial, proximal and distal.

    Anterior. Refers to the front or head of an organism or structure. It is derived from the Latin word ante meaning "before" or "in front of". It is shown in the diagram as a black arrow.

    Posterior. Refers to the rear end of an organism or structure. It is derived from the Latin word post meaning "after" or "behind". It is shown in the diagram as a white arrow.

    Retrolateral. Simply means the backwards facing side. It is derived from the Latin words retro meaning "backwards" and lateralis meaning "to the side". The retrolateral sides of the spiders appendages are shown in blue.

    Prolateral. This is the opposite to retrolateral so it is the forward facing side. It is derived from the Latin words pro meaning "for" or "forward" and again lateralis meaning "to the side". The prolateral sides of the spiders appendages are shown in red.

    Medial. Refers to down the middle of an organism or limb and is derived from the Latin word medius meaning "middle". The medial lines of the carapace, abdomen and leg are shown in pink.

    Proximal. Means near to the body and is often used when describing the placement of a structure on a limb or a segment. It is derived from the Latin word proximus meaning "nearest". I have marked proximal points in orangey yellow on different segments of each limb. For example the femur on the palp or the tibia on leg I.

    Distal. This is the opposite of proximal meaning away from the body. It is derived from the Latin word distare meaning "to stand away from". Again it is often used when describing the placement of a structure on a limb or segment. I have marked distal points in green on the same segments the proximal points were marked on.

    Another couple of terms not shown in the diagram are superior and inferior.

    Superior. Means the upper part of a structure and is usually used when describing the placement of keels on a palpal bulb. It is Latin for "above" or "over".

    Inferior. Means the lower part of a structure and again is used when describing the placement of keels on a palpal bulb. It is Latin for "below".
    We will see an example of these later on when we look at palpal bulbs.

    These anatomical terms can be combined together to give an accurate description of the location of certain features such as spines on the metatarsus of leg 4 for example. Using the tibia of a left leg 1 i have numbered various points on the ventral and dorsal side and given the relevant anatomical term of location below.


    1.DPV - Distal Pro Ventral
    2.DRV - Distal Retro Ventral
    3.DMV - Distal Mid Ventral
    4.DPL - Distal Pro Lateral
    5.DRL - Distal Retro Lateral
    6.MPV - Medial Pro Ventral
    7.MRV - Medial Retro Ventral
    8.MMV - Medial Mid Ventral
    9.MPL - Medial Pro Lateral
    10.MRL - Medial Retro Lateral
    11.PPV - Proximal Pro Ventral
    12.PRV - Proximal Retro Ventral
    13.PMV - Proximal Mid Ventral
    14.PPL - Proximal Pro Lateral
    15.PRL - Proximal Retro Lateral
    16.DRD - Distal Retro Dorsal
    17.DPD - Distal Pro Dorsal
    18.DMD - Distal Mid Dorsal
    19.MRD - Medial Retro Dorsal
    20.MPD - Medial Pro Dorsal
    21.MMD - Medial Mid Dorsal
    22.PRD - Proximal Retro Dorsal
    23.PPD - Proximal Pro Dorsal
    24.PMD - Proximal Mid Dorsal

    The cephalathorax

    Now with our new found knowledge of spider anatomy and anatomical directions we can begin to look at various parts of the spider in more detail starting with the carapace.

    As mentioned earlier, many old descriptions relied on comparing the lengths of the patella + tibia of legs I-IV to the length of the carapace but as theraphosid taxonomy progressed this was found to be a very limited tool. There are however other features of the carapace that are of great use such as the foveal or thoracic groove, the ocular tubercle and the clypeus.
    The foveal groove can vary greatly between species. It can be shallow or deep, transverse (straight), procurved (U-shaped), recurved (n-shaped) or in the case of some Ceratogyrus have a horn or small mound rising above the carapace. In the photo below you can see that the fovea is very slightly procurved and quite deep.

    The ocular tubercle or eye turret/eye hill houses the eyes of the tarantula, the layout of which used to be considered a primary taxonomic tool but has been found to be unstable in a lot of species. Tarantulas have 8 eyes which are separated into 2 rows, anterior row and posterior row. The eyes situated on the outside are known as lateral eyes and those on the inside are median eyes. So this gives us anterior lateral eyes (ALE), anterior median eyes (AME), posterior lateral eyes (PLE) and posterior median eyes (PME). Note that in a lot of descriptions they are referred to by their acronyms. Looking at the eyes on the photo below we can see that the anterior eye row is procurved and the posterior eye row is transverse.
    A more reliable taxonomic tool is the clypeus. This is the area between the ocular tubercle and the anterior edge of the carapace. This can be wide, narrow or even absent in some genera such as Anoploscelus or Chaetopelma. In the example below the clypeus is narrow.

    Next we will have a closer look at the labium. As you can see from the photograph below they are black bumps or granules on the labium and ventral maxillae. These are known as cuspules and the layout and number present a useful taxonomic tool. In the example below you can see that there are not many cuspules on the labium and there for can be easily counted. This is not always the case and so in the case of example no.2 a rough estimate is taken by counting the number of cuspules in a horizontal row and multiplying it by the number of cuspules in a vertical row.
    In the suture between the labium and the sternum you can see the labiosternal mounds. These were first noticed by the Australian taxonomist Robert Raven and described in his 1985 work The Spider Infra order as "a uniquely modified labiosternal junction: instead of being a shallow or hardly defined "groove," as in most mygalomorphs, the "groove" is inverted so that two distinct narrow mounds with several peaks or teeth are present. The mounds are best developed in Haploclastus, in which they are bicuspidate, but the mounds are not present in Phlogiellus, Haploclastus, Lyrognathus, or some Selenocosmia species." So the presence or absence of these mounds or, as in the case of the Heteroscodra genus, the distinct shape means it can be a useful tool.

    Example 2

    Sexual organs.

    The morphology of the sexual organs have long been regarded a useful primary taxonomic tool usually more so the male palpal bulb probably due to its easy access compared to the female's spermathecae that has to be dissected out of the dead spider. That said we shall look at the female organs first.
    Spermathecae can come in many different shapes and sizes but the three main types are fused, paired and partially fused (at the base) with twin receptacles. In description papers spermathecae are nearly always drawn or photographed from the dorsal side.
    Fused spermathecae look like a single structure. Named so as they are thought to have started off as paired spermathecae and then fused together as the spider moults. If this is the case then it is done within the first few instars as they are always fused by the time they are large enough to see.

    Paired spermathecae. Named so because there is a pair of them surprisingly enough.

    Fused at the base but with twin seminal receptacles

    To see a whole range of weird and wonderful looking spermathecae have a look at the spermathecae gallery here or visit Mikhail's wonderful website here. Also for more information on various sexual organ related structures why not pay the sexing guide here a visit.

    The palpal organ has been used a taxonomic tool since as far back as the 1870's and since it acts as a key that can usually only fit the epigyne of that particular species it makes for a very useful taxonomic feature. However the downside to it is that it is only useful for helping identify the male of the species and only mature ones at that.
    The picture below shows the palp of a mature male. Note the palpal tarsus is referred to as the cymbium once the male has matured and changes from an elongated spade shape to a short rounded structure (often referred to as "boxing gloves" by many hobbyists).

    The palpal bulb itself is usually a pear shape with a long fine embolus running off it. They are often adorned with ridges which are called keels. In the example below this particular bulb has 2 keels running along the prolateral side. They are the prolateral superior keel (PS) and the prolateral inferior keel (PI). There are other keels that can be found on palpal bulbs such as the apical (A), sub apical (SA) and retrolateral (R) keels but I have nether the material or inclination to show all of these so I will instead point you to this paper that will show and explain them all far better than I ever could *cough*cop out*cough* - Bertani, R. 2000. Male palpal bulbs and homologous features in Theraphosinae (Araneae, Theraphosidae). Journal of Arachnology 28: 29-42.


    Brachypelma bulb retrolateral view

    Prolateral view

    The tibial spurs, sometimes called tibial apophysis, of the mature male are often a useful taxonomic tool on a generic level. The presence or absence of these spurs as well as the shape and spination of them are used for identification purposes. They are located on the distal end of the tibia and can range from one or two spurs to a single spine (often referred to as a megaspine). In the examples below one has a primary segment with a spine on the end and the other has a primary and secondary segment.

    Note the curved metatarsus. This is very prominent in species such as Hapalopus and Haplotremus.



    Tarsal and metatarsal scopula

    The presence or absence of a dividing line in the metatarsal and tarsal scopula of legs III and IV is a taxonomic tool that has been used as far back as 1871 by Anton Ausserer and continued by Eugene Simon. However Reginald Pocock raised doubts about the validity of this tool when he pointed out that tarantulas do not have scopulae on the tarsus when very young. With each moult the scopula increases in number starting from the edge of the segments and working their way in so this tool can really only be used when describing full adult specimens. However Andrew Smith does state in his Baboon Spiders book that it is an important taxonomic character for Ischnocolinae and Eumenophorinae. In the examples below we have the metatarsus and tarsus of leg IV belonging to 3 different species.

    1. A Chilobrachys species showing the metatarsus and tarsus divided by a line of setae.

    2. An Avicularia species showing tarsal scopula undivided and metatarsal scopula divided by setae.

    3. A Cyriopagopus species showing tarsal and metatarsal scopula undivided.

    Another taxonomic use of the metatarsal scopula is the measurement of the amount of scopula found on the metatarsus. Usually legs I and II have complete coverage in most species so it is mainly used for the metatarsus of legs III and IV. In the example below we can see that the metatarsus is half scopulate.

    Stridulating organs

    The primary stridulating organs are located on the retrolateral chelicerae and on the prolateral maxillae. The usually take the form of either small spikes on the chelicerae (known as strikers) with paddle and clavate setae on the maxillae or a patch of plumose setae (sometimes referred to as a scopulate pad) on the chelicerae with thorn like setae on the maxillae. The shape and pattern that these modified setae form are used as a taxonomic tool that is very useful in the identification of species. By rule they are usually only found on old world species but as usual there is always an exception to the rule and in this case it is the Psalmopoeus genus which posses lyra on the maxillae similar to those found in Selenocosmiinae but no strikers on the chelicerae although there are a few longer thicker set setae at the base.

    Strikers on the retrolateral chelicerae

    Close up

    Prolateral maxilla with paddle setae

    Close up

    Retrolateral Chelicerae with plumose setae. Note the longer stout plumose setae at the base.

    Prolateral maxilla with thorn setae

    Secondary stridulating organs

    In some genera instead of having stridulating organs on the chelicerae and maxillae they have lyra on the retrolateral coxa and trochanter of the palp and the prolateral coxa and trochanter of leg I.

    Plumose setae on retrolateral trochanter of palp

    Plumose setae on prolateral trochanter of leg I

    Lyra on the retrolateral palpal coxa of G. rosea

    Lyra on the prolateral coxa of leg I. G. rosea

    For more examples of stridulating organs check out the gallery here

    Urticating hair

    At the present moment there are six different types of urticating hair which are only found in the subfamilies Theraphosinae and Aviculariinae. The presence or absence of each of these types of hair can be a useful tool in identifying genera specific to these subfamilies.

    Types I-IV

    Type V are so far only found on the prolateral femur of the pedipalp of Ephebopus species as opposed to the abdomen where urticating hair is usually found.

    Type VI hairs were discovered by Fernando Perez Miles in 1998 on Hemirrhagus cervinus which you can read about here - Pérez-Miles, F. 1998.
    Notes on the systematics of the little known theraphosid spider Hemirrhagus cervinus, with a description of a new type of urticating hair.Journal of Arachnology 26: 120-123.

    Hopefully by now you will have a better understanding of the anatomy and terms used in theraphosid taxonomy and all those description papers in the tarantula literature thread won't seem so daunting and alien. Either that or you are now fast asleep. On the off chance that you have managed to get this far without falling asleep or have clicked onto another thread containing something slightly more interesting then i have compiled a glossary of terms used here and in various papers that you are likely to come across.

    Glossary of terms

    Apex - The tip/top of an organ.

    Apophysis - An outgrowth or projection of an organ. The most common example being the tibial apophysis of a mature male ie the tibial spur or hook.

    Anterior - Situated at the front.

    Bacilliform - Rod shaped. Used to describe stridulating setae.

    Bicuspidate - Having two points or prominences.

    Caput - The raised anterior portion of the carapace. The head part as it were.

    Clavate - Club shaped. Used to describe stridulating setae.

    Clypeus - The area between the ocular tubercle and anterior edge of carapace.

    Conical or sub conical - Cone shaped (obvious really). Tends to crop up when describing the apical segment of Idiothele spinnerets.

    Conical process - Refers to the horn found on the carapace of some Ceratogyrus and Sphaerobothria.

    Cracked tarsus - A weakening of the exoskeleton of the tarsus that gives an effect that looks similar to a dried river bed.

    Cuspules - Small bumps found on the labium and posterior ventral palpal coxa.

    Cymbium - The palpal tarsus of a mature male.

    Digitiform - Finger like in shape.

    Distal - Situated away from the body.

    Dorsal - The back or upper surface.

    Embolus - The tip of the palpal bulb that injects the sperm.

    Filiform - Thread like in shape. Used to describe stridulating setae.

    Fovea - A point of muscle attachment in the cephalothorax.

    Foveal groove - Sometimes referred to as the thoracic groove. A straight or curved indentation on the carapace.

    Habitus - Overall physical appearance.

    Holotype - The unique specimen designated to represent the concept for a named species. Also referred to as type specimen.

    Homology - The recognition across taxa of identity among structures, behaviors, and other attributes, on the basis of similarity of form and position.

    Incertae sedis - Of uncertain position. Applied to taxa, which for some reason, cannot be placed with certainty in a classification.

    Incrassate - Thickened or swollen. Often used to describe leg segments.

    Inferior - The lower part of a structure. Used to describe the placement of keels on palpal bulbs.

    Keel - A ridge running along the palpal bulb or embolus.

    Lancet - Lance shaped. Like the surgical blade or a church window. Used to described stridulating setae.

    Labium - Plate on the ventral side of a spider that forms the back of its mouth.

    Lectotype - A specimen, serving the function of a holotype, as designated from the members of a syntype series.

    Lyra - Stridulating setae.

    Medial/Median - Down the middle.

    Neotype - A specimen selected to serve the function of the primary type, this being required because the holotype, lectotype or syntypes have been lost or destroyed.

    Nomen dubium - Latin for doubtful name. A scientific name that is of unknown or doubtful application.

    Nomen nudum - Latin for naked name. A name of a new taxon published without a description or diagnosis or reference to a description or diagnosis.

    Ocular tubercle - Also known as the eye turret or eye hill. A raised bump on the anterior carapace upon which the eyes are situated.

    Opisthosoma - The posterior portion of the body. ie the abdomen.

    Palpal bulb - male sex organ used to store and transfer sperm.

    Plesiomorphy - A primitive character, not group defining at the level at which it is being observed.

    Plumose - Feather like in appearance. Used to describe stridulating setae.

    Posterior - Situated at the rear.

    Procurved - U-shaped (horns point to the anterior). Most commonly used to describe the shape of the foveal groove or eye rows.

    Prolateral - Inside edge or forward facing side.

    Prosoma - Anterior portion of the body. ie the cephalothorax.

    Proximal - Situated near to the body.

    Recurved - n-shaped (horns point to the posterior). Most commonly used to describe the shape of the foveal groove or eye rows.

    Retrolateral - outside edge or rear facing side.

    Sclerite - Seperate sclerotized structures connected to others by membranes.

    Sclerotized - Hardened.

    Scopula - A dense brush-like tuft of hairs. Usually referring to the hairs on the ventral surface of the tarsus and metatarsus or the plumose pad on the retro-lateral chelicerae of some species.

    Setae - A stiff hair or bristle.

    Sigilla - Oval shaped points of muscle attachment on the sternum. Usually 3 pairs near the outside edge of the sternum.

    Stridulating organs - Specialised setae found on the chelicerae and maxillae or on the coxa and trochanters which when rubbed together create a hissing sound.

    Subtegulum - Sclerite located at the base of the palpal bulb.

    Superior - The upper part of a structure. Used to describe the placement of keels on palpal bulbs.

    Suture - A line of junction. Usually refers to the line running through the prolateral maxilla.

    Synapomorphy - Shared, derived, group defining trait.

    Synonym - Two or more different names applied to the same taxon.

    Syntype - Two or more specimens examined by the original author of a species, none of which was uniquely designated to serve as the name bearer for the taxon.

    Taxon - A grouping of organisms at any level in the systematic hierarchy.

    Tegulum - Sclerite located at the base of the palpal bulb. The main lower portion of the bulb.

    Transverse - Straight (-). Most commonly used to describe the shape of the foveal groove or eye rows.

    Trichobothria - Specialised hairs on the legs of spiders that are used to detect air currents and vibrations.

    Truncate - Square or broad at the apex almost as if it has been cut off.

    Type specimen - See holotype.

    Ventral - Underneath or lower surface.

    This presentation was brought to you by the letters T,D,S and by the number 2. [​IMG]

    Oh and these publications most of which can be found here

    Bertani, R. 2000. Male palpal bulbs and homologous features in Theraphosinae (Araneae, Theraphosidae). Journal of Arachnology 28: 29-42.

    Cooke, J.A.L., V.D. Roth & F.M. Miller. 1972. The urticating hairs of Theraphosidae spiders. American Museum Novitates 2498: 1-43.

    Foelix, R. F. 1996. Biology of Spiders. Oxford University Press

    Gallon, R.C. 2002. Revision of the African genera Pterinochilus and Eucratoscelus (Araneae, Theraphosidae, Harpactirinae) with description of two new genera. Bulletin of the British Arachnological Society 12: 201-232.

    Hancock, K. & Hancock, J. 1989. Sex determination of immature theraphosid spiders from their cast skins.

    Pérez-Miles, F., S.M.Lucas, P.I. da Silva Jr., & R. Bertani. 1996. Systemaic revision and cladistic analysis of Theraphosinae. Mygalomorph 1: 33-68.

    Pérez-Miles, F. 1998. Notes on the systematics of the little known theraphosid spider Hemirrhagus cervinus, with a description of a new type of urticating hair. Journal of Arachnology 26: 120-123.

    Pérez-Miles, F. 1994. Tarsal scopula division in Theraphosidae (Araneae, Theraphosidae): Its systematic significance. Journal of Arachnology 22(1): 46-53.

    Pérez-Miles, F. & L. Montes de Oca. 2005. Surface ultrastructure of labial and maxillary cuspules in eight species of Theraphosidae (Araneae) Journal of Arachnology 33(1): 43-49.

    Raven, R.J. 1985. The spider infra order Mygalomorphae (Araneae): cladistics and systematics. Bulletin of the American Museum of Natural History 182: 1-175.

    Schuh, R.T. 2000. Biological Systematics. Principles and Applications. Cornell University Press

    Smith, A . M . 1988. The Tarantula: Classification and Identification Guide. Fitzgerald Publishing. London

    Smith, A . M . 1990. Baboon Spiders: Tarantulas of Africa and the Middle East. Fitzgerald Publishing.London,

    West, R. C., S. D. Marshall, C. S. Fukishima & R. Bertani (2008): Review and cladistic analysis of the Neotropical tarantula genus Ephebopus Simon 1892 (Araneae: Theraphosidae) with notes on the Aviculariinae. Zootaxa (1894): 35-58

    Greek and Latin Roots in English - Oh no I've blown any credibility with the use of a dreaded wikipedia article.

    Anatomical Directions - Bugger i did it again.

    And a big thanks to Phil Rea for helping ensure this didn't become the ramblings of a total retard.
    Nada and Bast like this.
  2. Bast

    Bast Well-Known Member

    Calgary, AB
    Very awesome!!! Thanks for all the hard work and info Plessey! This should be stickied!
  3. DalilahBlue

    DalilahBlue Moderator Staff Member

    GA, USA
    Amazing. Wonderful work.
  4. harleyqueen

    harleyqueen Well-Known Member

    Omg that must have taken forever , really interesting thanks for going to all that trouble I will enjoy learning from this.
  5. plessey

    plessey Active Member

    Thanks. IIRC it took me 4 days to get all the photos and write it up.
    I've done a bit of tidying up on it today and redone a few of the photos as the writing wasn't particularly clear on some of them. I'll probably add some more bits and pieces to it over time but not really anything that anyone would notice.
  6. harleyqueen

    harleyqueen Well-Known Member

    I have got a lot to learn as I want to breed some of mine when they are bigger, this is so fascinating and just the sort of things I want to learn.
  7. Nada

    Nada Moderator

    Arizona USA
    All Hail Plessey!!!! Amazing!

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