Fig. 2. Conditioned responses from six different groups of animals, defined on the basis of the odour concentration present in the 1.8 mol l^-1 sucrose solution they received during a single trophallactic interaction. (A) General percentage of proboscis extensions (%PE_G). Letters indicate statistical differences among the different odour concentrations (G-test, P<0.001; see Results for details): the results of two groups that do not differ significantly are denoted by the same letter. (B) Percentage of proboscis extensions (%PE) corresponding to the first (%PE₁, white bars), the second (%PE₂, grey bars) and the third test (%PE₃, black bars). Animals were tested 21, 27 and 46 h following trophallaxis in a cumulative fashion. Asterisks indicate statistical differences among tests (McNemar test, ^*P<0.05). The number of animals is given in parentheses. In B, differences in the sample size within a given odour concentration are due to differences in mortality and US responsiveness prior to the tests.