A standardized protocol for ruler-based measurement of wing length in monarch butterflies, Danaus plexippus L. (Nymphalidae, Danainae).

Abstract

Standardized measurements using well-defined landmarks are the most effective means to reduce measurement error. We describe such a protocol for monarch forewings based on single measurements with a ruler to the nearest 1.0 mm. Analysis of this protocol showed that it provides excellent intra-observer repeatability, excellent to substantial inter-observer repeatability, and similar wing length estimates as those of calipers at 0.1 mm, as long as sample sizes are > 30. In addition, our study showed that males and females differ in wing length; different observers differ in their measurements and in their measurement error; and wings shrink slightly when dried. We make these recommendations for study of monarch wing lengths: 1) males and females should be analyzed separately; 2) live butterflies should be measured after cooling and dead butterflies should be measured before they are dried; 3) measurements should be restricted to the right forewing; 4) the standard protocol should be practiced and calibrated until measurements are repeatable within and among measurers; 5) the samples should be mixed among all observers when possible to mitigate relative biases; and 6) names, handedness, measurement error, and archived raw data should be reported. Widespread adoption of this protocol will increase the comparability of wing length data from various investigators. Similarly based standardization of measurement would benefit wing measurement of all Lepidoptera.