NECTAR CONSUMERS - Nectaries and Nectar - page 300

dominated by hexose sugars, especially fructose, in spite of its high concen-
tration.

In laboratory studies of carbohydrate feeding in Diptera, feeding behaviour
varies greatly between insects fed ad libitum and those which are deprived of
food and then offered single meals, as demonstrated by Edgecomb et al.
(1994) for adult Drosophila melanogaster feeding on sucrose-agar diets. In
general, the volumes of sugar solution ingested are positively correlated with
concentration in previously starved individuals offered single meals, but in-
sects feeding ad libitum show compensatory feeding and the volume
imbibed is then negatively correlated with concentration. At the lower and
upper extremes of diet concentration, flies ingesting very dilute fluid may
use “bubbling behaviour” to evaporate excess water (Hendrichs et al., 1992),
or salivary secretions may be used to dissolve solid nectar sugars. In
blowflies, crop emptying rates are faster after ingestion of higher volumes or
lower concentrations, and sucrose solutions are processed at half the rate of
equimolar fructose solutions with half the energy content (Hainsworth et al.,
1990; also see the review of Stoffolano, 1995). The utilization of different
sugars by flies, especially parasitoids, is discussed by Chen and Fadamiro
(2006), who found poor survival of a parasitic fly, Pseudacteon tricuspis
(Phoridae) on the trisaccharide melezitose, which is common in aphid hon-
eydew. Where amino acids in nectar are concerned, six essential amino acids
elicit feeding responses in flies by stimulating the chemoreceptor that re-
sponds to sugars (Shiraishi & Kuwabara, 1970). Specialized fly flowers,
such as those pollinated by carrion flies, have high levels of amino acids in
nectar, if nectar is present (Baker & Baker, 1982), and flesh flies deprived of
a protein source have been shown to select nectars containing mixtures of
amino acids (Rathman et al., 1990). The responses of insects to amino acids
in nectar are discussed further by Nicolson and Thornburg (2007, Chapter 5
in this volume).

3.3 Lepidoptera

Lepidoptera are equipped with an elongate proboscis and feed from narrow
tubular flowers, but can also utilize many other flowers. Moths are many
millions of years older than butterflies, far more speciose, and probably more
effective pollinators, but nectar-related research has been mainly restricted to
the conspicuous hawkmoths (Sphingidae) and butterflies of several families.
Their energy requirements differ dramatically, owing to the large body size,
endothermy, and hovering flight of hawkmoths. The volumes of nectar in
flowers visited by hawkmoths may exceed those in hummingbird flowers
belonging to the same genus, as in Ipomoea and Nicotiana (Galetto &