The preparation of spiders for determination includes mounting the epigynes on microscopic slides for detailed obersvation of the structure of its inner genital parts (e.g. vulva). This is generally a two-step process: 1. using a clearing agent to mazerate the soft tissue obscuring the view on the characteristic sclerotized parts (e. g. insemination ducts, receptaculae seminae) and 2. mounting the mazerated epigyne on a slide in a mounting medium and covering it with a cover slip before putting it under a microscope.

Hoyer’s mounting medium clears soft tissue in any preparation and improves visibility of objects because of its high refraction index. Compare object in ethanol solution (left) with object in Hoyer’s solution (right). Epigyne of Pireneitega spasskyi (Amaurobiidae).

Hoyer’s mounting medium is a clear, amber-colored solution

One commonly used mounting medium serves both of these purposes in one single fluid: Hoyer’s mounting medium. One just carefully tears the epigyne from the spider body (see how) and puts it into a drop of Hoyer’s solution, covers it with the cover slip and waits 15-30 minutes (or sometimes longer) before making microscopic observations.

In order to make 150 g of Hoyers solution you need the following ingredients:

• 25 ml Aqua dest. (ω = 17 %)
• 15 g Gum arabic (ω = 10 %)
• 100 g chloral hydrate (ω = 67 %) – yes, this is a lot, but it will work!
• 10 g glycerol (ω = 7 %)

Now carefully mix all ingredients until you obtain a clear, weakly amber-colored viscous liquid. Done. The destilled water, Gum arabic and glycerol cause the viscosity of the resulting fluid whereas the chloral hydrate is the actual clearing agent, mazerating the soft tissue while sparing the sclerotized, chitinous parts of interest to the observer.

Caution:

• 

  Gum arabic should be pure, as in crystals (left) or a solution (right).

  Be very careful with chloral hydrate: it is a poisonous and in some countries a controlled substance (see Wikipedia page for more information, including Hoyer’s solution)

• larger body parts can more effectively be cleared in potassium-/ or sodium hydroxide solution (ω = 10 %)
• In order to make such prepared temporary preparations permanent, the mounting needs to be sealed because Hoyer’s mounting medium tends to rehydrate and therefore deteriorates after some time, rendering the thus prepared object useless
• use only pure Gum arabic without dirt or you might get an unusable, hazy solution as a result. My suggestion: buy only clear amber-like crystals of Gummi arabicum or viscous solutions of it. Most sources recommend not to buy pulverized Gum arabic, as it is often impure.

Sources:

• Anderson, L.E. (1954). Hoyer’s Solution as a rapid permanent mounting medium for bryophytes. Bryologist, 57:242–244. [PDF]
• Kraus, O. (1984): Hoyers Gemisch statt Polyvinyl-Lactophenol. Mikrokosmos 73(2): 54-55. [PDF]
• Wikipedia contributors, ‘chloral hydrate’, Wikipedia, The Free Encyclopedia, <http://en.wikipedia.org/wiki/Chloral_hydrate> [accessed 16 February 2015]