

Separating material

Whilst it is relatively easy to sort specimens that have been reared or collected using a net and aspirator, it can be very time consuming sorting material from collections made using the "mass" sampling techniques such sweep-net catches that have been transferred to alcohol, or Malaise or yellow pan trap catches. In such cases is it often easier to separate the collection into two fractions, a small fraction that will pass through a 0.4mm mesh and a larger fraction that will not. To achieve this transfer the collection into a 4mm wire mesh cage (about 16x12cm and 7sm deep) and then gently agitated in suitable container of 70% alcohol, eg large plastic box. If the cage is held slightly off the bottom the smaller insects pass through and collect on the bottom. Do not to put too much material in the cage at once. With a little practice the smaller fraction can be separated fairly quickly. The larger fraction can be washed into another plastic box and from there into a suitable receptacle. The smaller fraction can be passed through a fine net to filter out the insects and then transferred to a suitable receptacle. In some cases where there are a lot of moth scales present it is difficult to pass the smaller fraction through a fine mesh because it gets clogged with scales. If this happens pour the whole into a large jar and leave to settle for a few hours. In that time the supernatant liquid becomes quite clear. Next gently pour off most the liquid to leave a little alcohol, and the insects and scales behind. Transfer this to a suitable container.

The larger fraction can be separated by naked eye, but the smaller fraction needs to be sorted at low power under a stereo microscope.

Chalcidoids or other microhymenoptera, can be separated from these small fractions using a plastic petri dish with parallel lines scored on the underside about 8-11mm apart. The lines make it easier to scan the collection. These can be made more distinct by filling the scored lines with Indian ink using a drawing pen. Similar to this is a commercially manufactured sorting dish that consists of a shallow, flat, white plastic tray approximately 8cm square suitably routed to leave a series of low, parallel ridges, the tops of which are separated by about 8-11mm (available from Rose Engineering, 17344 Eucalyptus St., Unit B3, Hesperia, California 92345, USA). When using these sorting dishes it is important not to overcrowd them with material. It is best to introduce the equivalent of only about 1/2 tsp of "small fraction" for each subsample to be sorted and to add a little alcohol to facilitate this. If very small specimens are required (eg aphelinids or trichogrammatids) it may be necessary to go through each subsample several times to ensure that all target groups have been removed. It such cases it may help to swirl the contents of the dish around and gently pour off the liquid to leave behind a residue of plant material, etc. If the supernatant liquid is searched it is often possible to find even the smallest specimens that have been overlooked, eg Megaphragma. Once sorting has been completed it may be advantageous to put the whole sample into a 250ml measuring cylinder (not more than will fill the lowest 70ml), fill to the top with 70% alcohol and seal with clingfilm or parafilm® and tie off with a rubber band. Invert the whole thing for 30 minutes or so and then invert again for just over a minute. If the supernatant liquid is then rapidly poured off this should separate off most, if not all, of the really small specimens from the remaining debris. Leave the supernatant to stand for several hours (24 hours is best) and pour off the clear liquid until a small amount, plus the "micro fraction" remains on the bottom of the receptacle. If this is scanned it is often surprising how many really small chalcidoids will be found.

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Last updated 19-Aug-2003 Dr B R Pitkin