Biology Reference
In-Depth Information
pollen (Brncic, 1966). In South Africa, larvae of
D. flavohirta
live bathed in
nectar in the flower cups of
Eucalyptus
sp. and are thought to have an adverse
effect on honey production (Tribe, 1991; Nicolson, 1994). For laboratory-reared
larvae of the green lacewing
Chrysoperla carnea
(Neuroptera: Chrysopidae),
Drosophila
larvae are a suboptimal food, but the inclusion of pollen and su-
crose in their diet greatly enhances growth (Patt et al., 2003). This is another
example of a predator that supplements its diet with nectar.
Of the four holometabolous orders mentioned above, the Coleoptera are
least important as nectar feeders, although according to Armstrong (1979), a
quarter of beetle families and half of fly families in Australia are anthophi-
lous (flower-frequenting). In the Diptera, Lepidoptera, and Hymenoptera,
the evolution of a crop has been a key factor in carbohydrate feeding
(Stoffolano, 1995). All three of these advanced orders depend on liquid carbo-
hydrate resources to provide immediate energy for the flight between flowers,
which enables cross-pollination. They have independently evolved an expand-
able and impermeable crop—diverticular in structure in the Diptera and
Lepidoptera, and linear in the Hymenoptera—located in the abdomen. The
crop is crucial because of the unpredictability of nectar resources: it is used
for storage, transport back to the nests of social insects, and in addition, it
prevents the osmotic shock that would result from sudden exposure of the
haemolymph to high sugar concentrations. Nectar is released from the crop
to the midgut for digestion and absorption as required, or transferred to the
nestmates of social insects by trophallaxis. All nectar-feeding insects show
highly efficient assimilation of the common nectar sugars, regardless of the
concentration ingested (Hainsworth et al., 1990). For a review of the impli-
cations of a nectar diet for the energy and water balance of insects, see
Nicolson (1998).
In the following sections, I discuss the major nectar-consuming insect
orders. However, Corbet (2006) has recently provided a broad functional
classification of flower types and their insect visitors (Table 2) that cuts across
the insect orders. This classification emphasizes that gradients of nectar
quantity and accessibility in flowers are matched by gradients of body size,
tongue length, and endothermy in their insect visitors. This contrasts with the
concept of pollination syndromes, where plants are supposedly pollinated by
a single taxonomic group of pollinators. The importance of nectar accessibil-
ity was also highlighted by a multivariate analysis of classical pollination
syndromes (Ollerton & Watts, 2000), which showed that beetle, fly, and
wasp syndromes clustered together—all characterized by exposed nectar
presentation.
