Cabbage white butterfly is also known as Pieris rapae. These species are flower visitors. They use tubular proboscis to acquire liquids. It consists of “two medially concave, elongated maxillary galeae joined dorsally and ventrally by cuticular projections, termed “legulae,” forming a food canal” (Lehnert et al. 167). Each legula has upper and lower branches. The organ is twisted into a spiral in a quiescent state. Butterfly’s head has a special pump that creates a pressure to move liquid through the food canal to the gut.
P. rapae’s average forewing length is about 49.6 mm, while its proboscis’s length is around 9.45 mm. Butterfly’s feeding organ consists of three zones with different sizes. Zone 1 is the longest – it takes up to 92.5% of the overall proboscis’s length. This is a hydrophobic “nondrinking region of the proboscis that extends from the junction of the proboscis with the head to where the upper branches of dorsal legulae begin to enlarge” (Lehnert et al. 172). The length of Zone 2 is around 0.7 mm or 7.5% of overall proboscis’s size. This is a hydrophilic drinking region that extends from the upper branches of dorsal legulae’s widening to the part where there are not any of these extensions. Zone 3 is the smallest part of the proboscis and not all butterflies have it. The region is present in P. rapae. This is a hydrophilic zone that works as a functional subset of Zone 2. The region ends up with the proboscis tip without dorsal legulae. Cabbage butterfly’s proboscis has two types of sensory organs: sensilla basiconica and sensilla styloconica.
Dorsal ligulae are spaced more widely on the upper part of the proboscis. This structure allows “liquid to enter what otherwise is putatively a fluid-tight tube sealed with glandular secretions” (Monaenkova et al. 2). Proboscis works like a straw, but butterflies need specific sizes of pores to create appropriate hydrodynamic conditions of the current. The choice depends up feeding opening’s size. It has the diameter around 60 µm in cabbage white butterflies. Experiments with Monarch butterflies showed that they will not “drink from filled pores with a radius, rm, less than rm < 35±5 µm about the average radius, R, of the food canal in the distal region of the proboscis” (Monaenkova et al. 2). When butterflies drink from pores, they withdraw fluid as a series of “sips” separated by air bubbles. The procedure did not include the formation of liquid bridge that “activates” food canal. Legulae’s structure helps to overcome capillary forces. Extinctions’ formation works like a sponge. Capillary powers can not stop liquid from release to the canal, when the structure became full. Proboscis inner negative curvature reduces the pressure and helps butterflies to drink from different surfaces. “Supported by the wetting forces acting over the contact line, the liquid continues to overflow the linkage, forming a wetting film inside the food canal” (Monaenkova et al. 4).
Scientists called butterflies feeding behavior a consequence of evolution. Proboscis’ structure helped species to withdraw liquids from different droplets, films and porous substrates. Butterflies developed an ability to take food from limited sources. This development gave butterflies an opportunity to exploit not only nectar floral tubes, but also from rotten fruits, which amount increased dramatically because of flowering plants’ diversification. Proboscis’s dual mechanism made it possible to feed “from damp soil, animal products and other sources” (Monaenkova et al. 5). Scientists believe capillary action could play a serious role in the diversification of butterflies’ feeding strategies.
Works Cited
Lehnert, Matthew S., et al. “Structure of the Lepidopteran Proboscis in Relation to Feeding Guild”. Journal of Morphology. 2016: 167–182. PDF.
Monaenkova, Daria. “Butterfly proboscis: combining a drinking straw with a nanosponge facilitated diversification of feeding habits”. Journal of the Royal Society. 2011: 1-7. PDF.
