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C. J. Potgieter et al.: Pollination in Plectranthus 109
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Fig. 7. Comparisons of floral tube length with proboscis length of visiting dipterans for two species of Plectranthus. a P.
hilliardiae above with Stenobasipteron sp. below, b P. zuluensis above with Psilodera sp. A below; note the close correlation
between proboscis and floral tube length. Bars: 5 mm
visiting bees and flies in most cases correspond well
with that of the corolla-tube, style and filaments of the
Plectranthus species in question (Fig. 4). Insects with
shorter mouthparts tend to concentrate on pollen
collection.
Pollen loads. Voucher specimens of anthophorid
bees and nemestrinid and other flies that yielded a
high percentage of Plectranthus pollen show that
although pollen transfer is achieved for Plectranthus,
these vectors are not exclusive to the genus. A few
observations of vectors moving between Plectranthus
flowers and those of other genera, e.g. Lobelia spp.
(Campanulaceae) and IsogIossa spp. (Acanthaceae),
have been made and this is supported by the
placement of foreign pollen in patches inaccessible
to Plectranthus stigmas on some insect specimens. It
is difficult to distinguish between pollen grains from
different species of Plectranthus, but it is seldom that
more than two species grow in close proximity.
Differences in filament and style lengths place pollen
in different areas on the insect body, thus isolating
pollen from different Plectranthus species. The lack of
pollen on the voucher specimens of Lepidoptera
suggests that butterflies are not important polinators
of P. madagascariensis. The Pieridae were present of
a limited period of the flowering season and were
seldom encountered.
Nectar robbing. The proboscis of Allodape pernix
is too short to reach nectar legitimately, hence the
behaviour of robbing nectar through a hole pierced at
the base of the corolla tube. This corresponds to
Chapter 2/ 25
primary nectar robbing according to the system
suggested by Inouye (1980). Glandular trichomes are
abundant on the calyces of Plectranthus and in other
members of the Lamiaceae have been shown to
secrete essential oils that have insecticidal properties
(see Levin 1973 and Konstantopoulou et al. 1992).
Individuals of A. pernix do not appear to be deterred
by these oils, as it is the corolla tissue, containing
fewer terpenoid-containing trichomes, that is pierced
above the calyx and nectary.
The generally small body size, short proboscis
length and pollen-collecting habit of the insects listed
in Table 5 make them unlikely primary pollinators of
Plectranthus. In his account on floral larceny Inouye
(1980) refers to pollen theft which is the collection of
pollen directly from the anthers by small bees, without
contacting the stigma. This is probably the case for
these species and that of the nectar robber A. pernix,
but the possiblility that some contribution is made to
intra-specific pollen transfer cannot be discounted.
Comparison of tube length with proboscis
length. This study records several new groups of
visitors to Plectranthus: flies of the families Nemes-
trinidae, Acroceridae and Tabanidae, and lepidopter-
ans of the families Lycaenidae and Pieridae. The
occurrence of pollination by long-tongued flies
(primarly of the family Nemestrinidae, but also one
species of Tabanidae) is confirmed for both longer-
and shorter-tubed species of Plectranthus. Species
with intermediate tube length (P. ecklonii and P.
zuluensis) allow for visits by shorter-tongued insects -