Analysis of proboscis permeability. (A) Schematic diagram of the experiment showing operation of the Cahn DCA-322. A capillary tube with an inserted proboscis is connected to the balance arm by a hook. An oil droplet closes the rectangular vessel, preventing water evaporation and allowing the tube to move freely through the hole. Water moves from the reservoir to the tube by capillary action. The weight of the wicking water column is measured by the balance arm as the meniscus moves up the bore of the capillary tube. L, meniscus position; r, radius of the capillary tube; θ, contact angle; L(t), change in the length of the liquid column; L_p, length of the proboscis; R₀, radius of opening; φ, taper angle; xy, the plane of proboscis placement; a–c, loops (see Materials and methods). (B) Closed vessel after 1 h of saturation with water vapor. The proboscis is suspended above the water. (C) The same vessel at the moment when the proboscis touches the water. (D) Example of measurement of Jurin height (Z_c); arrow indicates the meniscus.

Proboscis-in-a-tube model. Left: proboscis tip with one galea removed to expose the food canal of a monarch butterfly; scale bar, 1 mm. Right: schematic diagram of the food canal model. L_p is the proboscis length used in wicking experiments, H is the length of the drinking region, R_p is the radius of the straight section of the food canal, R₀ is the radius of the opening and φ is the taper angle.

The dimensionless meniscus position (L*) versus dimensionless time (t*) for a single proboscis of the monarch butterfly. Circles are experimental points and the solid line is the best fit with the proposed model.

Pressure required to provide flow in tapered and non-tapered theoretical models of butterfly proboscises. P_H is the pressure differential required in a tapered food canal at the drinking region (position H in Fig. 3); P_p is the pressure differential in the cibarial pump in a tapered food canal (position L_p in Fig. 3); P_t is the pressure differential in the cibarial pump in the drinking-straw (non-tapered) model when the radius (R_p) of the food canal does not change along the proboscis. Percentages indicate aqueous sucrose concentrations. The red horizontal dashed line at 1 atm indicates the maximum limit of pressure differential that could be produced by the cibarial pump. The dashed line below the x-axis indicates the measured flow rates for monarch butterflies.

Image sequences extracted from a video recording of the proboscis tip of a monarch butterfly during feeding. The proboscis must be flexible to contract and expand quickly when the butterfly feeds. (A) Galeal sliding. (B) Galeal pulsing; the arrowheads are plotted to show the incremental change of the proboscis size. In the left image the arrowheads point to the proboscis boundary, while in the right image the proboscis boundary expands, making the arrowheads indented into the proboscis. The degree of expansion varies along the proboscis.

Determination of droplet volume and sucrose concentrations fed from by monarch butterflies. (A) Monarch butterfly feeding from a droplet of sucrose as an example of an image used for estimation of droplet volume. (B) Dynamics of the change in droplet volume V with respect to initial volume V₀ as a function of time for three sucrose concentrations; standard errors are from eight different experiments for each sucrose concentration.