Rapid photogrammetry of morphological traits of free‐ranging moths

Abstract

Photogrammetric studies of free‐ranging animals are limited to mammals and birds. Recent advances in insect photogrammetry, including 3D imaging, are entirely associated with museum specimens. We present a rapid, simple, accurate, and inexpensive morphometric method targeting thousands of free‐ranging insects attracted to light screens using images taken without collecting a specimen or even constraining the individual in any manner. A reference grid printed on the screen is used to calibrate the images for shape and size without prior knowledge of the camera‐subject configuration. The method requires only inexpensive, off‐the‐shelf, consumer equipment, and freely available programming (R statistical language) and image processing (ImageMagick) tools. We demonstrate the efficacy of the method using a dataset of 3675 images of free‐ranging hawkmoths (Lepidoptera:Sphingidae) imaged in natural repose on a screen. We show that this method introduces no bias and has a high degree of correspondence with traditional morphometry using collected specimens. We also propose error metrics, which quantify the calibration quality and identify images with poor data. Although this method is particularly suited for the hyperdiverse moth community, which dominates the dynamics of many terrestrial ecosystems, it can be used for other phototropic taxa identifiable on an image to (morpho)‐species. It will help in accumulating reliable trait data from hundreds of thousands of individual insects without any expenditure on specimen collection. It is particularly suited for studies which require multi‐epoch, multi‐locate sampling like investigations into ecosystem stability, climate change, and community assembly.

Intraspecific trait variability and community assembly in hawkmoths (Lepidoptera: Sphingidae) across an elevational gradient in the eastern Himalayas, India

“…We set up portable UV screens (Mungee & Athreya, 2020) between 7 p.m. and midnight during the 7 days before and 3 days after the new moon, when the moon was below the visible horizon during those 5 hr. We sampled at 2–5 elevations simultaneously to achieve some degree of uniformity of weather conditions (which can change drastically from day to day) across the elevational gradient.…”

“…Supplementary section B provides a brief description of the trait measurement procedure. More details may be obtained from Mungee and Athreya (2020).…”

“…Point sampling was carried out at UV-illuminated screens on no-moon nights along a vehicle track characterized by roadside scrub in close proximity to primary forest (5-20 m away). The sampling was carried out in a single compact transect to reduce the impact of variation in gamma diversity while sampling across widely separated transects (McCain, 2007) We set up portable UV screens (Mungee & Athreya, 2020) between 7 p.m. and midnight during the 7 days before and 3 days after the new moon, when the moon was below the visible horizon during those 5 hr. We sampled at 2-5 elevations simultaneously to achieve some degree of uniformity of weather conditions (which can change drastically from day to day) across the elevational gradient.…”

“…We measured body length, thorax width, wing custom length, and wing breadth from field images after calibration and distortion corrections (Mungee & Athreya, 2020). We derived from these primary measurements the three functional traits of body mass, wing loading, and wing aspect ratio.…”

“…We selected the three traits of body mass, wing loading, and wing aspect ratio because (a) they impact multiple aspects of a moth's life history such as thermoregulation (Dillon et al., 2006; Dudley, 2002; Heinrich, 1996), dispersal (Athreya & Singh, 1990; Azevedo et al., 1998; Frazier et al., 2008; Gilchrist & Huey, 2004; Lentink et al., 2007; Rohner et al., 2018), reproduction (Moretti et al., 2017; Suding & Goldstein, 2008), and starvation resistance (Cushman et al., 1993; Lindsey, 1966) and therefore should be functional response traits, and (b) we were able to measure these traits from images of free‐flying moths without even momentarily constraining them, let alone collecting specimens (Mungee & Athreya, 2020).…”