Most adult Lepidoptera depend on a proboscis for fluid uptake. Although the proboscis has been regarded as a sealed tube with fluid uptake restricted to the distal end, recent evidence indicates that it is permeable along its entire length in at least some species. We, therefore, tested the effectiveness of the seal during feeding in four species of butterflies. Feeding rates in monarchs (Danaus plexippus L.), painted ladies (Vanessa cardui L.), and tiger swallowtails (Papilio glaucus L.) did not differ significantly when the proboscises were straightened and fully, versus partially, submersed in 1 or 15% sucrose solutions. To explore these results, we tested fluid uptake along the nearly transparent proboscises of buckeye butterflies (Junonia coenia Hübner) by applying colored droplets of water to the legular seam between the paired galeae. Colored fluid appeared in the food canal of straightened and naturally flexed proboscises within 10 s, regardless of whether the chemosensilla were stimulated with sugar. Statistically significant entry of fluid, however, occurred ≈30 s after droplets were applied and only if the proboscis was naturally flexed and stimulated with sucrose. The results suggest that fluid uptake along the length of the proboscis is influenced by changes in legular spacing when the butterfly naturally bends the proboscis and on activation of the cibarial pump when chemosensilla are stimulated with sugar.
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