As with mounting specimens on cards there are only two techniques which should be considered for slide mounting chalcids, namely by mounting in Hoyer's, a temporary water-soluble medium, and by mounting in balsam, a permanent non-water-soluble medium.
It may be found convenient to attach labels for data, etc. to the slide before commencing mounting. Labels can be attached at each end of the slide and if made of 4-ply card will allow slides to be stacked without the risk of the upper ones crushing the specimens beneath.
Mounting in Hoyer's medium
Hoyer's medium should be used in preference to balsam as a mountant only for a minority portion of each series of a species available for study. Its use should be limited because it is water soluble and subject to drying out. This often causes the coverslip to be pulled down hard onto the specimen leaving it badly crushed so that remounting may become totally impossible. This may happen even if the coverslip has been ringed (see below). Hoyer's should never be used where type-material (especially primary types) or potential type-material is involved.
A critical advantage of using Hoyer's medium over balsam is that its refractive index is much lower than that of chitin and thus cuticular structures are more clearly visible. However, this advantage is very much reduced if material is examined using a phase contrast microscope. Another advantage of Hoyer's over balsam is that it is relatively quick and requires relatively little expertise to manufacture reasonably good slide preparations. It is also advantageous when used for mounting whole specimens. However, whole mounted specimens are usually not very desirable for study because characters of taxonomic value may be obscured or even positioned such a way that they cannot be accurately measured. Hoyer's is not a an easy medium in which to mount dissected specimens such as those described below in the section covering the use of balsam.
Hoyer's medium is prepared by dissolving 12 g of gum Arabic in 20 g of warm distilled water, then adding 20 g of glycerine and 80 g of chloral hydrate. For best results the resultant liquid should be filtered with the aid of a suction pump.
Of the various methods used for mounting in Hoyer's, Rosen and DeBach (1979) found that the following consisitently gave the best results.
(1) Ensure that specimens are thoroughly dry. If material to be mounted is in alcohol then dry in an oven at about 60ºC before proceeding.
(2) Soak specimens in a 7:5 mixture of glacial acetic acid (or lactic acid) and lactophenol (or chloral phenol) at room temperature for 24-72 h, after which time the specimens should be cleared and the body shape returned to normal.
This treatment has the advantage that the specimen eventually becomes beautifully relaxed so that antennae, wings and legs can be positioned with ease.
(3) Place a small drop of Hoyer's in the centre of the microscope slide and gently spread this out until it fills the area to be covered by the coverslip (it may help to draw a ring the size of a coverslip on a piece of card which can then be placed beneath the slide to act as a guideline).
The size of the drip will depend on the size of the specimen, but for small, flattened species, eg Aphytis, it should generally be estimated as the volume required to just cover a coverslip to the depth of about 0.3-0.4 mm.
(4) Gently transfer the specimen from the acetic acid mixture to the Hoyer's and orientate with dorsal surface uppermost.
(5) Arrange antennae, wings and legs so that they are splayed out flat and in the same plane as the surface of the slide (Fig. 18.104.22.168.4)
If step (2) has been omitted, then it may be found that the wings and legs spring back to their original position.
(6) Gently lower the coverslip onto the specimen. If possible try to place the coverslip as flat as possible onto the Hoyer's without disturbing the specimen. This will require a certain amount of practice. Subsequent attempts to level off the coverslip could cause movement of the specimen and perhaps the displacement or loss of important setae from the dorsum of the thorax or folding of the wings.
(7) Allow the slide to dry a room temperature for about 2 weeks or in an oven at 40ºC for 5-7 days.
(8) Ring the coverslip with a suitable sealing compound to prevent further drying and thus possible damage to the specimen. Nail varnish is suitable for this, although Rosen and DeBach recommend double ringing with Zut.
Rosen and DeBach (1979) state that specimens on slides which have dried out completely (by not being ringed or because the ringing compound has become dislodged) may be reconstituted by removing the ringing material and placing the slide in a humidity chamber until the Hoyer's has again become quite liquid. The specimen can be removed on the point of a pin and remounted in the same manner as above. However, where the slide has dried out completely the specimen is often irreversibly damaged. There is also a very serious risk of damaging the specimen every time it is transferred to a new slide.
Last updated 19-Aug-2003 Dr B R Pitkin