A classification of functional pitcher types in Nepenthes (Nepenthaceae).

Abstract

We classify Nepenthes species into 12 functional pitcher types, based on combinations of traits that appear to comprise different syndromes for capturing nutrients, usually from animals. For nine of these types the trapping syndromes are already documented, six targeting live animals (hence carnivorous), and three targeting other nutrient sources (non-carnivorous). Yet, for three pitcher types here is no previous documentation of the syndrome and we do not yet know what sources of nutrients are being targeted. Mapping all these pitcher types on the latest, near comprehensive species-level phylogenomic tree of Nepenthes (Murphy et al. 2019) shows that apart from the ancestral pitcher type 1, most of the remaining pitcher types have evolved independently, in different parts of the phylogenetic tree, usually in several different places. Each of the 12 pitcher types is characterised morphologically and illustrated, its trapping syndrome discussed, and example species are given. An identification key to the 12 pitcher types is presented. The possibility of additional pitcher types being present is discussed. O_FIG O_LINKSMALLFIG WIDTH=96 HEIGHT=200 SRC="FIGDIR/small/852137v1_fig1.gif" ALT="Figure 1"> View larger version (16K): org.highwire.dtl.DTLVardef@bbfa57org.highwire.dtl.DTLVardef@13084e7org.highwire.dtl.DTLVardef@658770org.highwire.dtl.DTLVardef@19eca44_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOType 1 (Fig. 1).C_FLOATNO Ancestral (N. mirabilis) These species have upper pitchers which are more-or-less ovoid-cylindric in shape. The basal, ovoid part, often but not always a little wider than the upper part, is usually separated from the upper, usually cylindrical part by a "hip" or raised ridge that girdles the circumference of the pitcher on the exterior (also present in the six species of pitcher types 7, 8 and 10). Examination of the inner surface shows that the "hip" divides the glossy, broad, basal, fluid-containing, digestive, part (referred to as the "detentive" zone by Macfarlane (1908) because prey are detained here) from the upper opaque, dull, off-white to slightly purple, waxy or "pruinose" zone. This waxy zone is referred to as the "conductive" zone by Macfarlane because it helps conducts the insects downwards to the detentive zone with the digestive fluid. Such type 1 pitchers are shown in Fig. 1, exemplified by the upper pitchers of Nepenthes mirabilis (Lour.) Druce. The waxy zone is so-called because its surface is made up of numerous minute platelets of wax, each platelet like a roof-tile and held on a fragile stalk from the underlying surface. Juniper et al. (1989) documented how the waxy zone functions to prevent animals, usually insects, from leaving the pitchers once they have fallen in. Pressure from a foot placed on a wax "tile" will result in the stalk breaking, so that the insect loses its foothold and may fall back into the pitcher. C_FIG O_FIG O_LINKSMALLFIG WIDTH=111 HEIGHT=200 SRC="FIGDIR/small/852137v1_fig12.gif" ALT="Figure 12"> View larger version (39K): org.highwire.dtl.DTLVardef@1f13b4borg.highwire.dtl.DTLVardef@6bc534org.highwire.dtl.DTLVardef@1600959org.highwire.dtl.DTLVardef@5ee42_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOPitcher type 12. (Fig. 12).C_FLOATNO Flat lip (N. jacquelineae Nepenthes jacquelineae (Sumatra, Sect. Montanae) and N. platychila Ch.C.Lee (Borneo, Sect C.Clarke et al.).. Regiae, Lee 2002) both have all the features of type 3 pitchers: widely funnel-shaped to cup-shaped, non-waxy upper parts of pitchers draining into a more slender cylindrical lower part with viscous liquid. Both species however differ from all type 3 species in their very large, flat, dark red peristomes which can be up to 3.5 cm wide in N. jacquelineae. These peristomes have been speculated to act as a landing platform for large flying insects such as blattid cockroaches and moths, which might act with the contrasting lighter, green pitcher body as a light trap for such prey. It is also possible that such prey are lured into a precarious place above the mouth by the copious nectar produced from the lower surface of the lid from large nectar glands 1.5 mm diam. (Clarke 2001). The morphological convergence between the pitchers of these two species is remarkable. Further investigation of the prey trapped in the wild would be desirable for both species. Nepenthes echinostoma Hook.f. is a species closely related to N. mirabilis but with a remarkably flat and extended peristome which resembles type 3, although it has a different pitcher shape and inner surface C_FIG