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Last updated 19th August 2022. See Update Record link (above) for details of all updates since April 2019.
Introduction
Tetragnatha montana male (bottom) and female (top) in a stand-off
This website attempts to illustrate the distinguishing features of some British spiders, as imaged using a Scanning Electron Microscope (SEM). Normally, spiders are identified by examining key features using a stereo light microscope, and in particular by studying the morphology of the epigyne and pedipalp (female and male genitalia, respectively). Comparing these features with sketches in the reference material for UK spiders (such as References 1-3) the experienced arachnologist can normally determine the mature spider to species level - eventually.
Using an SEM provides a different way of looking at the genitalia, and also enables one to visualise other features in more detail using the higher magnification and greater depth of field of an SEM compared with a stereo microscope. Despite the advanced features of an SEM, however, this instrument cannot always show all the same information visible using a light microscope. In particular, many distinguishing features of an epigyne are beneath a transparent membrane. The SEM will image the surface of the membrane rather than the structure beneath it. (Compare, for example, the electron micrograph of the epigyne of Metellina segmentata with the sketch in the reference material.) Similarly, it is not always easy to arrange the pedipalps in exactly the same orientation as is used in the references.
It is not suggested that this website should be used instead of conventional methods of identifying spiders. For a start, few arachnologists would have access to an SEM, and in any event this site will only ever show images from a small number of the 650 or more species that have been identified in the UK. Rather, users may care to browse it to view features (such as tarsal claws) that are not easily distinguished using a stereo microscope, or simply to enjoy the images in their own right.
Using these Pages
The species list, below right, identifies which spiders have been imaged so far. Clicking on the species name will open a "Species Page" with somewhere between five and 12 micrographs of key features of the species, normally selected from the features described below. Clicking on an individual micrograph will bring up a larger version of the photo in a new, floating, window. These windows may be closed individually, or if a number of windows have been opened from the same page, then the green button "Click to close floating windows" may be clicked on. Once the Species Page has been closed, any individual windows opened from it will need to be closed one at a time. Clicking on "Back to Species List" will take you back to this page, scrolled to position the Species List at the top right of the screen. "Open Spiders home in a new window" is self-explanatory. Not all browsers work the same. I find that Chrome and Firefox work best. Even so, the floating windows (other than the most recent one) tend to be hidden behind the Species Page, and stacked directly on top of each other. If your browser is set to prevent scripts running, you will need to temporarily allow this to take advantage of the larger floating windows for the features. The reason the windows are floating is to enable a feature (for example head) of one species to be compared side by side with that of a different species.
If you are not careful, you may end up with rather a lot of open windows!
Please note that this site is under continuous development, and that missing information and illustrations will be added as they become available. Please refer to the Update Record (reached through the button at the top of this page, and opens in a new tab) to see what changes or additions have taken place since your last visit. Feedback is welcome via the contact button, as are offers to provide photographs for each Species Page where one is not present, and preserved specimens of spiders not yet included on the species list.
Epigyne
This is the female sex organ, and is located on the ventral side of the abdomen. In mating, the male will insert the tip (embolus) of one palp into one of two copulatory ducts in the epigyne and discharge the previously loaded sperm. He will (female willing) repeat the process using the other palp and copulatory duct.
Spinnerets
The spinnerets are the organs from where the silk is spun. There are various types of silk depending on the function required of it. In addition to building webs, spiders use silk for functions such as making a safety line, wrapping prey, wrapping eggs, "ballooning", the web for charging the pedipalps and so on. The cribellate spiders (e.g. Amaurobius sp.) have an additional silk-producing area called the cribellum, used for spinning very fine silk to entrap prey near the opening of the hole in the wall or other structure where the spider lies in wait.
Pedipalp (female)
Females also have pedipalps, but they are not as developed as in the males. On some species, a single tarsal claw can be discerned. The female will bring its pedipalps into use for handling prey and possibly also for defence.
Tarsal Claw
This is the end part of each leg, similar to a foot. It is used for locomotion on the web and for climbing vertical surfaces, as well as more conventional movement. Web-dwelling spiders (such as the ARANEIDAE) have two combed claws on each tarsus along with a third hook-like claw that is used for grabbing and hanging on to threads of silk. Other spiders that do not rely on webs for their food do not have the central hook, but instead have a collection of matted hairs on each tarsus, called a scopula after the latin for a brush, which allow them to climb on vertical surfaces.
References
Lockett, G.H. and Millidge, A.F. 1951 British Spiders. The Ray Society Volumes 1 to 3
Roberts, M.J. 1985, 1987 The Spiders of Great Britain and Ireland. Harley Books, Colchester Volumes 1 to 3
Roberts, M.J. 1996 Collins Field Guide Spiders of Britain and Northern Europe. Harper Collins, London
From: Emerton, J.H. 1902, The Common Spiders of the United States. Via Wikimedia Commons.
