Ultraviolet reflection and its behavioral role in the courtship of the sulfur butterflies Colias eurytheme and C. philodice (Lepidoptera, Pieridae)

Abstract

1. The courtship behavior of the sulfur butterflies, Colias eurytheme and C. philodice (Lepidoptera, Pieridae), was studied in the field and in large outdoor flight cages. Experiments were performed to investigate the nature of releasers of male courtship behavior and to determine the basis of conspecific mate recognition by females. 2. The courtship of these species is extremely brief and does not involve the elaborate displays characteristic of many other butterflies. Mature females employ a mate-refusal posture (Fig. 10) to effect mate selection against, and insure reproductive isolation from, non-conspecific males. The same behavior is also employed by unreceptive females in response to conspecific males. 3. Males rely on visual cues to locate and identify females. They respond to paper dummies of the appropriate color (Figs. 15 and 16) and attempt to mate with them (Fig. 7B). No chemical stimuli are necessary to elicit this behavior. The most attractive dummies closely match the yellowish green, ultraviolet-absorbing color of the female ventral hind wing (Fig. 17). Ultraviolet reflection added to an otherwise attractive dummy (Fig. 18A) strongly inhibits male approaches (Fig. 18B). One function of the ultraviolet reflection of male C. eurytheme (Fig. 1) is its use as an inhibitory signal directed to other males. This signal is displayed by individual males to communicate their sex to others, as well as by males in copulo to inhibit courtship attempts of intruders (Fig. 11). 4. Visible color differences between C. eurytheme (orange) and C. philodice (yellow) males play no role in mate selection by females of either species (Table 8). The ultraviolet reflection difference between male C. eurytheme (reflecting) and male C. philodice (absorbing) (Figs. 1 and 8) is used as a mate-recognition signal by females of C. eurytheme and in this way functions as a component of the system of isolating mechanisms in this species. C. philodice females do not respond to ultraviolet reflection patterns in any way and appear not to use any visual stimuli in mate selection (Table 8). 5. Females of both species rely on chemical cues for conspecific mate recognition. The chemical signals are not associated with so-called ‘androconial’ scales (Fig. 19) located in the black borders of the male wings. These scales are not necessary for successful courtship (Table 7) and no sexual function can presently be ascribed to them. For these and other reasons, we suggest that the term ‘androconia’ not be used for the black border scales of male Colias. No species-recognition pheromones are associated with the dorsal discal region of the male's wings (Table 6), but olfactory signals may be associated with their ventral surfaces, the anterior or posterior margins of their dorsal surfaces, the legs, or the body. 6. All male F1 hybrids are ultraviolet-absorbing. F1 hybrid females, derived from crosses between female C. eurytheme and male C. philodice, display the same mate-selection behavior as is characteristic for butterflies of their paternal parents' species (Table 4).