17.1.1. Active collecting
Active collecting may involve physically picking individuals from the habitat, using a wet finger, fine-hair brush, forceps, or an aspirator (also known in Britain as a pooter). Such techniques are useful for relatively slow-moving insects, such as immature stages and sedentary adults that may be incapable of flying or reluctant to fly. Insects revealed by searching particular habitats, as in turning over stones, removing tree bark, or observed at rest by night, are all amenable to direct picking in this manner. Night-flying insects can be selectively picked from a light sheet — a piece of white cloth with an ultraviolet light suspended above it (but be careful to protect eyes and skin from exposure to ultraviolet light).
Netting has long been a popular technique for capturing active insects. The vignette for this chapter depicts the naturalist and biogeographer Alfred Russel Wallace attempting to net the rare butterfly, Graphium androcles, in Ternate in 1858. Most insect nets have a handle about 50 cm long and a bag held open by a hoop of 35 cm diameter. For fast-flying, mobile insects such as butterflies and flies, a net with a longer handle and a wider mouth is appropriate, whereas a net with a narrower mouth and a shorter handle is sufficient for small and/or less agile insects. The net bag should always be deeper than the diameter so that the insects caught may be trapped in the bag when the net is twisted over. Nets can be used to capture insects whilst on the wing, or by using sweeping movements over the substrate to capture insects as they take wing on being disturbed, as for example from flower heads or other vegetation. Techniques of beating (sweeping) the vegetation require a stouter net than those used to intercept flight. Some insects when disturbed drop to the ground: this is especially true of beetles. The technique of beating vegetation whilst a net or tray is held beneath allows the capture of insects with this defensive behavior. Indeed, it is recommended that even when seeking to pick individuals from exposed positions, that a net or tray be placed beneath for the inevitable specimen that will evade capture by drop- ping to the ground (where it may be impossible to locate). Nets should be emptied frequently to prevent damage to the more fragile contents by more massive objects. Emptying depends upon the methods to be used for preservation. Selected individuals can be removed by picking or aspiration, or the complete contents can be emptied into a container, or onto a white tray from which targeted taxa can be removed (but beware of fast fliers departing).
The above netting techniques can be used in aquatic habitats, though specialist nets tend to be of different materials from those used for terrestrial insects, and of smaller size (resistance to dragging a net through water is much greater than through air). Choice of mesh size is an important consideration — the finer mesh net required to capture a small aquatic larva compared with an adult beetle provides more resistance to being dragged through the water. Aquatic nets are usually shallower and triangular in shape, rather than the circular shape used for trapping active aerial insects. This allows for more effective use in aquatic environments.