Insect morphometry is reproducible under average investigation standards

Abstract

Morphometric research is being applied to a growing number and variety of organisms. Discoveries achieved via morphometric approaches are often considered highly transferable, in contrast to the tacit and idiosyncratic interpretation of discrete character states. The reliability of morphometric workflows in insect systematics has never been a subject of focused research, but such studies are sorely needed. In this paper, we assess the reproducibility of morphometric studies of ants where the mode of data collection is a shared routine.We compared datasets generated by eleven independent gaugers, that is, collaborators, who measured 21 continuous morphometric traits on the same pool of individuals according to the same protocol. The gaugers possessed a wide range of morphometric skills, had varying expertise among insect groups, and differed in their facility with measuring equipment. We used intraclass correlation coefficients (ICC) to calculate repeatability and reproducibility values (i.e., intra- and intergauger agreements), and we performed a multivariate permutational multivariate analysis of variance (PERMANOVA) using the Morosita index of dissimilarity with 9,999 iterations.The calculated average measure of intraclass correlation coefficients of different gaugers ranged from R = 0.784 to R = 0.9897 and a significant correlation was found between the repeatability and the morphometric skills of gaugers (p = 0.016). There was no significant association with the magnification of the equipment in the case of these rather small ants. The intergauger agreement, that is the reproducibility, varied between R = 0.872 and R = 0.471 (mean R = 0.690), but all gaugers arrived at the same two-species conclusion. A PERMANOVA test revealed no significant gauger effect on species identity (R ² = 0.69, p = 0.58).Our findings show that morphometric studies are reproducible when observers follow the standard protocol; hence, morphometric findings are widely transferable and will remain a valuable data source for alpha taxonomy.

Keywords: entomology; measurement error; morphology; repeatability; species delimitation; taxonomy.

FIGURE 1

Precision versus accuracy. The bullseye represents the value of the measurand. Accuracy is indicated by closeness to the bullseye—measurements closer to the bullseye are more accurate. Precise measurements are tightly clustered. Accurate and precise measurements are tightly clustered in the bullseye. Graphics produced and used with permission from Dr. Bethan Davies (antarcticglaciers.org)

FIGURE 2

Illustrations for morphometric characters. Head in dorsal view (a) with measurement lines for CL: Head capsule length, CW: Width of head including eyes, CWb: Width of head capsule, PoOC: Postocular distance and SL: Scape length; frontal region of the head dorsum (b) with measurement lines for FRS: Frontal carinae width (red accessory lines and arrows identify the torular lamella); lateral view of mesosoma (c) with measurement line for ML: Mesosoma length; lateral view of propodeum, petiole, and postpetiole (d) with measurement lines for STPL: Propodeal spine tip erection, NOH: Maximum height of the petiolar node, NOL: Length of the petiolar node, PPL: Postpetiole length, and SPST: Spine length; dorsal view of mesosoma (e) with measurement lines for MW: mesosoma width; lateral view of propodeum, petiole, and postpetiole (f) with measurement lines for PEH: Maximum petiole height, PEL: Petiolar length, and PPH: Postpetiole height; dorsal view of propodeum, petiole, and postpetiole (g) with measurement lines for SPBA: Spine base width, SPTI: Propodeal spine tip distance, PEW: Petiole width, and PPW: Postpetiole width. Detailed verbatim trait definitions for characters are given in Table 1

FIGURE 3

Ordination biplot for principal component analysis based on (a) distribution of observations by species identity and (b) distribution of observations by gaugers. Black and red dots represent repeated observations on the same objects, while black dots represent Nesomyrmex devius, and red dots represent Nesomyrmex hirtellus. Convex hulls for spatial distribution of observations within morphospace represent (a) species and (b) gaugers. Descriptions for abbreviations of morphometric characters (red letters) are as follows: CL: Head capsule length, CW: Width of head including eyes, CWb: Width of head capsule, FRS: Frontal carinae width ML: Mesosoma length; MW: mesosoma width; NOH: Maximum height of the petiolar node, NOL: Length of the petiolar node, PEH: Maximum petiole height, PEL: Petiolar length, PEW: Petiole width, PoOC: Postocular distance, PPH: Postpetiole height; PPL: Postpetiole length, PPW: Postpetiole width, SL: Scape length, SPBA: Spine base width, SPST: Spine length, SPTI: Propodeal spine tip distance, STPL: Propodeal spine tip erection. Gauger alphabet codes (B) in triad format: A: MYRM_9000_100x, B: DIPT_0_100x, C: MYRM_5000_288x, D: MYRM_60000_360x, E: MYRM_500_50x, F: MYRM_500_50x, G: MYRM_450_50x, H: WASP_1000_230x, I: WASP_0_230x, J: MYRM_300_100x, K: MYRM_300_100x. Compositional differences between treatments expressed as the results of the PERMANOVA (coefficient of determination, F and p values, details in the text)