Structural Differences in the Tracheal Tapetum of Diurnal Butterflies

The three-dimensional structure of the tracheal tapetum lucidum, its reflection properties and the resulting eye glow hue were studied in members of the diurnal butterfly families Pieridae, Nymphalidae, Satyridae and Lycaenidae. Two main groups of tapeta can be distinguished by structural and physiological differences. Whereas in pierids the main tracheal trunk at the bottom of the rhabdom bifurcates into two side branches before bifurcating again more distally, the tracheal trunk in the members of the family Nymphalidae, Satyridae and Lycaenidae investigated first divides into four side brandies. A second bifurcation shortly after the first results in eight subbranches which are regularly arranged between adjoining receptor cells. The broad banded reflection colour from incidently illuminated tapetal structures (at the level of the first bifurca­ tion) varies between and within families but does not change significantly within the same eye. Whereas in nymphalids, satyrids and lycaenids the eye glow hue corresponds with the colour of the tapetal reflection, in pierids it is dominated by the coloured receptor screening pigment.