Figure 2.

Determination of limiting pore size used by monarch butterflies. (a) Monarch butterflies were fed 25% aqueous sucrose solution from paper towels suspended at different heights to determine the limiting pore radius from which they could feed. (b) Minimum radius r_m of open pores in the paper towel (filled circle solid curve) and towel-saturation level (grey bars) versus height H of the stage from which butterflies were fed. Pie charts represent the proportion of butterflies that drank (grey) or did not drink (dark grey) at each height. At heights of 5–25 cm, all butterflies fed. At 30 and 35 cm, five of 14 butterflies did not drink. At 38.5 cm, only two of 14 butterflies fed. The pore size at 38.5 cm is considered the critical minimum pore radius. The bar graph (with standard deviations) shows the liquid content at each height relative to the liquid content in the completely saturated towel (rlc) (see §5 for details). (c) Change of drop volume (V) as a function of time (t), demonstrating that the spaces between the overlapping dorsal legulae (dl) are permeable to water (with standard deviation bars). A drop was deposited 5 mm (dark grey) and 10 mm (grey) from the head. c1, cross-section of monarch proboscis at 261.75 µm from the apex, with radius of food canal (R) and effective porous surface boxed in grey. c2, dorsal view of effective porous surface of the proboscis showing slits between dorsal legulae. (d) Geometrical parameters used for calculation of absorption rate of the drop (H_d, height of drop; L, length of drop base; see §5). The liquid bridge belongs to a series of bridges moving towards the head (to the left). (e–i) Schematic of drop penetration through the slits between the dorsal legulae of the proboscs. Arrows indicate the direction of liquid flow. (Online version in colour.)