The extremely long-tongued neotropical butterfly Eurybia lycisca (Riodinidae): proboscis morphology and flower handling

Abstract

Few species of true butterflies (Lepidoptera: Papilionoidea) have evolved a proboscis that greatly exceeds the length of the body. This study is the first to examine the morphology of an extremely long butterfly proboscis and to describe how it is used to obtain nectar from flowers with very deep corolla tubes. The proboscis of Eurybia lycisca (Riodinidae) is approximately twice as long as the body. It has a maximal length of 45.6 mm (mean length 36.5 mm ± 4.1 S.D., N = 20) and is extremely thin, measuring only about 0.26 mm at its maximum diameter. The proboscis has a unique arrangement of short sensilla at the tip, and its musculature arrangement is derived. The flower handling times on the preferred nectar plant, Calathea crotalifera (Marantaceae), were exceptionally long (mean 54.5 sec ± 28.5 S.D., N = 26). When feeding on the deep flowers remarkably few proboscis movements occur. The relationship between Eurybia lycisca and its preferred nectar plant and larval host plant, Calathea crotalifera, is not mutualistic since the butterfly exploits the flowers without contributing to their pollination. We hypothesize that the extraordinarily long proboscis of Eurybia lycisca is an adaptation for capitalizing on the pre-existing mutualistic interaction of the host plant with its pollinating long-tongued nectar feeding insects.

Fig. 1

A. Eurybia lycisca probing with its extraordinary long proboscis for the corolla opening on a Calathea crotalifera flower. B. Untriggered flower in which the style (sty) holds back the hooded staminode (sta). C. Flower in which the pollen release mechanism has been triggered by an insect that is capable of pollinating the flower.

Fig. 2

A. Head of Eurybia lycisca (SEM) with coiled proboscis. B. Proboscis tip with four sensilla of uncertain type (s) on the apex of each galea (SEM): Intake slits (is) for imbibing nectar are located on the dorsal side of the tip region. Three different kinds of sensilla are distributed on the proboscis: sensilla of uncertain type (s) are restricted to the tip region sensilla basiconica (sb) and sensilla trichodea (st) can be found along the whole proboscis. C. Semithin sections at various points along the proboscis. Lateral intrinsic muscles (lim) occur throughout the length of the proboscis, median intrinsic muscles (mim) are present only in the knee bend.