Biology Reference
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Comparisons of floral and extrafloral nectars of the same plant species
are few (Baker et al., 1978; Koptur, 1994; Blüthgen et al., 2004). It has been
suggested that both concentration and sugar composition may be more vari-
able in exposed extrafloral netars (Koptur, 1992). Recent workers have
analysed the sugar and amino acid composition of extrafloral nectars in some
detail (Heil et al., 2000; Blüthgen et al., 2004). The extrafloral nectar of cer-
tain Mexican Acacia species with obligate ant associations is kept free of
sucrose by postsecretory hydrolysis. This provides a biochemical mechanism
for the mutualism, reinforced by the loss of gut sucrase in two Pseudomyr-
mex species and their resulting preference for hexose nectars (Heil et al.,
2005). This example is an exception to the generalization that sugar types do
not matter to insect consumers.
In the course of detailed research on a nectar-feeding ant community in
an Australian rainforest, Blüthgen et al. (2004) compared the sugars and
amino acids in a spectrum of food sources with the crop contents of ants of
several species found at these sources. Ants were observed at all extrafloral
but not floral nectar sources. When choice tests were conducted with artifi-
cial nectars, using the same ant community in its natural environment
(Blüthgen & Fiedler, 2004b), competition between ants on the same bait was
found to affect their selectivity, with preferences for higher amino acid and
sugar concentrations being most distinct in the dominant ant species, Oeco-
phylla smaragdina (Blüthgen & Fiedler, 2004a). Although the trisaccharide
melezitose is the most common insect-synthesized sugar in honeydew and
may be an attractant to ants tending aphids (Völkl et al., 1999), it was found
to be less attractive than sucrose to the rainforest ants (Blüthgen & Fiedler,
2004b). This ecosystem approach to preference tests indicates broadly simi-
lar preferences across the ant community (Blüthgen & Fiedler, 2004a) and
confirms earlier tests showing that amino acids in extrafloral nectar contrib-
ute to the attraction of ants (e.g., Lanza, 1991).
Nectar feeding in the ant Camponotus mus was investigated by weighing
foragers as they crossed a small bridge between the colony and the foraging
arena (Josens et al., 1998). Crop load increased with increasing sucrose con-
centration to a maximum at about 1.5 M (43%), then diminished because of
viscosity effects. Workers carried up to 60% of their own weight in the crop,
but the loads were partial for either dilute or very concentrated solutions, when
the motivational state of the ants or the physical properties of the solution
played a role, respectively. Duncan and Lighton (1994) used the consump-
tion of sugar solution by honeypot ants ( Myrmecocystus ) as a convenient
way to measure the cost of load carriage, but found no cost savings com-
pared with the external load carriage, which is more common in ants. One
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