Symmetry systems on the wings of Dichromodes Guenée (Lepidoptera: Geometridae) are unconstrained by venation

The potential importance of wing venation

This is the second in a series of three studies examining whether the relationship between wing pattern and venation in Geometridae is consistent with the NGP (Figs. 1 and 2B), with previous observations of Macroheterocera in the family Noctuidae (Fig. 2D), or with neither. The first study in this series focused on very simple wing patterns that consist of dark and light bands (Schachat, 2019). The present study focuses on wing patterns that appear to conform to the nymphalid groundplan in that they contain at least one symmetry system. The last study in this series will focus on “higher-level” symmetry systems, which occur in geometrids but not in butterflies (Süffert, 1929).

The relationship between wing pattern and venation has been classified with two models originally based on microlepidoptera. The first, the “alternating wing-margin model” (Baixeras, 2002; Brown & Powell, 1991; Schachat & Brown, 2015; Schachat & Brown, 2016), predicts that alternating light and dark bands of color straddle each wing vein along the costa (Fig. 2A). All available evidence indicates that this model represents the ancestral state for Lepidoptera (Schachat & Brown, 2016). The second, the “uniform wing-margin model” (Schachat & Brown, 2016; Schachat & Brown, 2018; Schachat, 2017b; Schachat, 2017a), predicts that pattern elements of the same color straddle every wing vein along the costa (Fig. 2C). These models are a focus of the present contribution because the former was validated when it was used to predict the existence and the precise location of an ancestral wing vein in moths (Schachat & Gibbs, 2016) and the latter holds in nearly all lepidopteran lineages examined for which the former does not (Schachat, 2017a).

Both of these models are relevant to the NGP. In nymphalid butterflies the NGP conforms to the alternating wing-margin model (Schachat & Brown, 2016), and in acronictine noctuids the NGP conforms to the uniform wing-margin model (Schachat & Goldstein, 2018). The NGP can be plotted onto a hypothetical geometrid wing according to either of these models, providing testable hypotheses.

If the NGP develops in geometrids according to the predictions of the alternating wing-margin model (Fig. 2A), as it does in nymphalid butterflies, then pattern elements located distal to the central symmetry system (CSS) will straddle alternating veins along the costa (Fig. 2B). If the NGP develops in geometrids according to the predictions of the uniform wing-margin model (Fig. 2C), as it does in acronictine moths, then pattern elements located distal to the CSS will occur on or between all adjacent veins that terminate along the costa and termen (Fig. 2D).

The influence of ancestral wing venation provides an additional opportunity to distinguish between the two models. Wing patterns that develop according to the alternating wing-margin model are strongly influenced by ancestral lepidopteran veins that may not be present in the adult wing (Baixeras, 2002; Brown & Powell, 1991; Schachat & Brown, 2015; Schachat & Brown, 2016; Schachat & Gibbs, 2016). However, this constraint does not appear to hold for wing patterns that develop according to the uniform wing-margin model: in areas where ancestral veins are not present on the adult wing, it is possible to observe more (Schachat, 2017b) or fewer (Schachat & Brown, 2016; Schachat, 2017a; Schachat & Brown, 2018) pattern elements than would be expected based on the configuration of those veins. Of note, the predictions of these two hypotheses overlap somewhat: each predicts that the distal edge of the central symmetry system reaches the costa proximal to the distalmost branch of the Subcostal vein (Figs. 1, 2B and 2D), a prediction that dates to the inception of the NGP (Schwanwitsch, 1924; Süffert, 1927) and is supported by recent studies (Nijhout, 1991; Otaki, 2012; Penz & Mohammadi, 2013).

The genus Dichromodes Guenée, 1858 (Scoble, 1999) was chosen for this study because it contains many species with easily identifiable central symmetry systems, but does not contain the higher-level symmetry systems observed in other geometrids (Süffert, 1929), thus facilitating comparisons with butterfly wing patterns as epitomized by the nymphalid groundplan (Fig. 3). Because geometrids in the subfamily Larentiinae are vastly overrepresented in morphological studies of wing pattern (Henke, 1928; Süffert, 1929; Henke, 1933; Schwanwitsch, 1956; Schachat, 2019), Dichromodes expands the phylogenetic scope of this body of knowledge. The primary aim of this contribution is to determine whether the most prominent symmetry system on the wings of Dichromodes conforms to the alternating wing-margin model, the uniform wing-margin model, or neither model. The wing patterns of Dichromodes are also compared to a historical classification scheme based primarily on moths in its taxonomic family, Geometridae.

Illustrations

One specimen was examined per sex/species. The more intact of the two forewings was chosen; only one forewing could be examined per specimen because the application of Histolene made the entire specimen greasy, thus rendering the other forewing unusable. All photographs were taken with a Leica DFC500 camera through a Leica M205C microscope, with Leica Application Suite software version 4.4. First, the forewing was photographed in its original condition, without the clearing agent. Then, without moving the specimen, the clearing agent was applied, the specimen was illuminated from below to help see the veins, and the forewing was photographed again. A vector drawing of the wing venation was created in Affinity Designer graphics software version 1.5.5 by tracing the veins from the second photograph. The vector drawing of the veins was then superimposed on the first photograph to visualize the natural wing pattern and venation simultaneously.

Because fringe extends beyond the wing margin in Dichromodes, the wing veins do not reach the apparent edge of the wing as seen in photographs. Dashed lines were used for all inferred veins that could not be observed in the specimen. Color coding was used for the veins that reach the costa: Sc is in bright red, R is in cyan, Rs₁ is in purple, Rs₂ is in dark blue, and Rs₃ is in pale red. In all illustrations, an arrow indicates the distal edge of the CSS along the costa.

Terminology and classification

“Costa” refers to the costal, or anterior, margin of the wing; “termen” to the distal margin of the wing; and “posterior margin” to the anal margin of the wing, which is referred to by some authors as the “dorsum” (Carter & Kristensen, 1998). The terminology used for venation follows conventions that are widely accepted in studies of all winged insect orders (Wootton, 1979). Although the Radius and Radial sector veins are often conflated as branches of “R” in the Macroheterocera literature, Wootton’s terminology, which has been used in many larger-scale studies of Lepidoptera (Carter & Kristensen, 1998), is used here. According to this terminological scheme, geometrids have an unbranched “R” vein followed by four branches of the “Rs” vein: Rs₁, Rs₂, Rs₃, and Rs₄ (Fig. 2).

In the NGP, “background color” is the color that surrounds darker pattern elements such as parafocal elements and symmetry systems. Recent studies have found that color is not a reliable indicator of homology, and that dark and light colors can “flip” between pattern elements (Schachat & Brown, 2018); this is observable even in the Micropterigidae, a mandibulate family that includes some species with NGP-like wing patterns (Schachat & Brown, 2016). The term “ground color,” typically used in taxonomic descriptions of moth wing patterns, can therefore be misleading in the context of homology. Here, the term “background” is used simply because of its inclusion as a component of the NGP (Otaki, 2012).

The most salient feature of the NGP is the central symmetry system (CSS). The terminology used here for the CSS and other elements of the NGP comes from Otaki (2012) and is illustrated in Fig. 1. The CSS surrounds the discal spot (DS) and is bounded by the proximal band of the central symmetry system (pBC) and the distal band of the central symmetry system (dBC).

In addition to the background color, various elements of the NGP occur proximal to the CSS. Most proximal, at the base on the wing, is the wing root band (WRB), consisting of a single color. Between the WRB and the CSS is the basal symmetry system (BSS). The bands within the BSS develop so that they are self-symmetrical in color, as is also the case for the CSS, but the BSS more often contains only two colors, and lacks additional features such as the discal spot.

Two additional elements of the NGP occur distal to the CSS. Closest to the CSS is the border symmetry system (BoSS), which often develops into eyespots in nymphalids. Most distal is the marginal band system (MBS), typically in a single color. The MBS includes the pattern elements that reach the termen. Spots that reach the termen on butterfly wings, between adjacent veins, are called “parafocal elements” (Nijhout, 1990). Here, this term is used for pattern elements that resemble the parafocal elements identified by Nijhout (1990) on the wings of Nymphalidae; as is the case with all other elements of the NGP, such as the symmetry systems, this terminology is used in a strictly descriptive sense and is not intended to imply homology with butterflies or any other lineage of Lepidoptera. Parafocal elements on the wings of Macroheterocera (Schachat & Goldstein, 2018) often resemble the parafocal elements 86, 87, 88, 89, 97, and 98 in Nijhout’s classification system developed for butterflies (Nijhout, 1990).

An early classification scheme, based primarily on Geometridae, categorizes symmetry systems according to their complexity (Henke, 1933). This scheme includes six categories (Fig. 4) ranging from a single, incomplete symmetry system (Fig. 4A) to multiple symmetry systems that may become confluent in the interior of the wing (Fig. 4F). In the first category (Fig. 4A), the entire proximal portion of the wing is covered by a pattern element that resembles the distal half of a symmetry system—with no discal spot or any other features—and the entire distal portion of the wing has nothing but the background color. The second category (Fig. 4B) contains a single, highly rudimentary symmetry system. The third category (Fig. 4C) contains a complete CSS, plus a BSS that extends to the base of the wing with its proximal portion missing. The fourth category (Fig. 4D) contains the features of the third, plus a BoSS that extends to the termen with its distal portion missing. The fifth category (Fig. 4E) contains a complete BSS and CSS, with no BoSS. The sixth and final category (Fig. 4F) contains eight symmetry systems, some of which become confluent and do not extend entirely from the costa to the posterior margin. The wing patterns of Dichromodes were evaluated according to this scheme.

Identification of the central symmetry system

Two criteria were used to identify the CSS, both consistent with Schwanwitsch’s identification of the CSS in the geometrid Eulithis destinata Packard, 1873 in that the CSS is a transverse pattern element that reaches the costa and encompasses the discal spot (Schwanwitsch, 1956).

The first criterion identifies the CSS as the pattern element that surrounds the discal spot and whose distal edge is adjacent to a line or color field in the lightest shade on the wing. This criterion is taken from the description of the CSS for butterflies, according to which the proximal and distal bands at either edge of the CSS are the darkest, the color field between these two bands—comprising the interior of the CSS—is of an intermediate shade, and the wing surface surrounding the CSS is of a lighter shade (Nijhout, 1991; Otaki, 2017). This definition of the CSS has been used for Geometridae beginning with the first extensive application of the NGP to this family (Henke, 1928), in which the CSS was called the Zentralkern and was defined as the area bounded by the two Zentralbinden, which represent the darkest color on the wing. These Zentralbinden correspond to the pBC and dBC of the NGP.

The second criterion identifies the CSS as the largest possible self-symmetrical pattern element that surrounds the discal spot, does not extend to the base of the wing, and does not abut the closest pattern element that occurs distally on the wing. Accordingly, the CSS can include the lightest color on the wing even if this color does not occur in the central color field between the pBC and dBC. This criterion was used in recognition of the recent finding that color can be misleading in the context of wing pattern homology (Schachat & Brown, 2016; Schachat & Brown, 2018). This criterion is also supported by Penz & Mohammadi (2013)’s recent study of nymphalid butterflies: on the forewing of Caligopsis seleucida Hewitson, 1877, the lightest color on the wing occurs within the CSS, and on the forewing of Eryphanis bubocula Butler, 1872, the same phenomenon occurs and the lightest color on the wing does not even border the CSS distally.

The second criterion is less straightforward because of the frequent difficulty in differentiating the adjacent, distal pattern element from the background color. Therefore, the photographs, Table 1, and the Kolmogorov–Smirnov test are all organized according to the first criterion.

In each photograph, an arrow indicates the point where the distal edge of the CSS reaches the costa. If the distal edge of the CSS varies according to the criterion used, two arrows are used. The proximal edge of the CSS is not indicated in the figures because it cannot be used to evaluate concordance with either predictive model.

Kolmogorov–Smirnov tests

The points at which the distal edge of the CSS reaches the costa in different specimens form a discrete, ordered distribution. This distribution was visualized with a histogram produced in the R package ggplot2 version 2.2.1 (Wickham, 2009). Kolmogorov–Smirnov tests were used to test whether this distribution is consistent with either a binomial or a uniform distribution.

First, a uniform and a binomial distribution were simulated in R version 3.4.2 (R Development Core Team, 2017). The binomial distribution was chosen because it offers a discrete approximation of a normal distribution and was simulated using the base-R function dbinom. Both distributions were simulated with 1,000 points. Kolmogorov–Smirnov tests were then performed, using the base-R function ks.test, to compare the observed distribution to the uniform and binomial distributions. A variant of the Kolmogorov–Smirnov test created for discrete data was also performed, using the ks.boot function in the R package Matching version 4.9-2 (Sekhon, 2011). Because the p-values returned from the ks.test and ks.boot functions are identical, only one p-value is reported for each distribution.

The central symmetry system

With the sole exception of Dichromodes semicanescens Prout, 1913 (Fig. 5A) all specimens clearly violate both hypotheses presented here—based on the alternating wing-margin model and the uniform wing-margin model (Fig. 2)—in that the distal edge of the CSS reaches the costa distal to the point where Sc terminates. In different Dichromodes specimens, the CSS terminates at Sc (Fig. 5B, Fig. S1), between Sc and R (Fig. 5C, Figs. S2, S3, S4), at R (Fig. 5D, Fig. S5), between R and Rs₁ (Fig. 5E, Fig. S6A), or at Rs₁ (Fig. 5F, Fig. S6B, S6C, S6D, S7).

Both a male and female specimen were examined for 12 of the 28 species included in this study. The location of the CSS is consistent between the sexes in nine out of the twelve, or 75%, of these species (Table 1). The three exceptions are as follows. In Dichromodes atrosignata Walker, 1861, the CSS reaches the costa between R and Rs₁ on the wing of the female specimen (Fig. S6A) and at Rs₁ on the wing of the male specimen (Fig. 5F). In Dichromodes explanata Walker, 1861, the CSS reaches the costa between Sc and R on the wing of the female specimen (Fig. S2E) and at R on the wing of the male specimen (Fig. S5C). In Dichromodes indicataria Walker, 1866, the CSS reaches the costa between R and Rs₁ on the wing of the female specimen (Fig. 5E) and between Sc and R on the wing of the male specimen (Fig. S3C).

The color of the interior of the CSS, surrounding the discal spot, often matches the background color of the wing. Typically, the CSS is a contiguous pattern element that spans the wing in a transverse direction from the costa to the posterior margin. In the male representative of Dichromodes anelictis Meyrick, 1890 (Fig. S1B) the CSS consists of two separate pattern elements, one reaching the costa and one reaching the posterior margin; the female belonging to this same species has a typical, contiguous CSS (Fig. S1A).

The pBC and dBC are not always continuous along the entirety of the anterior-posterior axis of the wing but can always be identified along at least part of the margin of the CSS. In some specimens, the dBC in particular is faintest toward the costa (Fig. 5F) or cannot be distinguished at all along the costa (Fig. 5C, Fig. S6A). In other specimens, the pBC and dBC are most easily distinguished near the costa (Figs. S1E, S5A, S5B, S5F). This is the case for Dichromodes orthotis Meyrick, 1890 (Fig. S3D), a species whose wing pattern is unique in that the pBC and dBC also form triangular shapes straddling the veins that cross the CSS. The pBC is nearly, but not entirely, complete in the female representive of D. indicataria (Fig. 5E). It is difficult to discern in Dichromodes rimosa Prout, 1910 (Fig. S4A), Dichromodes scothima Prout, 1910 (Fig. S4C), the male representative of Dichromodes estigmaria (Fig. S5B), and in Dichromodes ornata Walker, 1861 (Figs. S7A, S7B).

In the specimens examined here, the discal spot is outlined in the color of the pBC and dBC; the interior of the CSS, excepting the discal spot, is the same color or is of a lighter color. The only specimen for which the discal spot cannot be distinguished is D. scothima, which has a CSS that is uniformly dark in color (Fig. S4C); however, the consistent relationship between venation and the discal spot allows its location to be inferred.

Quantitative analyses

When the location of the distal edge of the CSS in each specimen is visualized with a histogram, the distribution is multimodal and does not resemble any canonical distributions such as normal, uniform, lognormal, etc. (Fig. 6). In 15 of the specimens examined here, representing 37.5% of the total, the distal edge of the CSS reaches the costa between Sc and R. In eight specimens, 20% of the total, the distal edge of the CSS reaches the costa at Rs₁. In seven specimens, 17.5% of the total, the distal edge of the CSS reaches the costa at Sc, and in another seven specimens the distal edge of the CSS reaches the costa at R. The distal edge of the CSS reaches the costa between R and Rs₁ in only two specimens, and it reaches the costa before Sc in only one specimen. When the “at Rs₁” category is excluded, the distribution appears to be normal (Fig. 6). However, this category includes 20% of all specimens.

The Kolmogorov–Smirnov test confirms that the distribution of the location of the distal edge of the CSS is significantly different from a binomial distribution (p = 0.005**) and from a uniform distribution (p = 0.005**). A significant result is also recovered if Dichromodes semicanescens is removed from the dataset (p = 0.013* for both distributions).

Other elements of the NGP

With two exceptions, the wing patterns of Dichromodes do not conform to any of Henke’s categories (Fig. 4). The male representative of Dichromodes molybdaria Guenée, 1858 (Fig. S5E) has very faint markings outside of its CSS and therefore conforms to Henke’s second category (Fig. 4B). The wing pattern of D. atrosignata (Fig. 5F) appears to be consistent with Henke’s fourth category (Fig. 4D). Beyond the CSS, the wing patterns of Dichromodes are more complex toward the termen than toward the base of the wing and so these fall outside the scope of Henke’s categories. As noted above, the CSS of Dichromodes ornata is asymmetrical in color and resembles Henke’s simplest category (Figs. S7A, S7B, Fig. 4A). Wing pattern in this species does not contain any elements proximal to the CSS, such as the wing root band or basal symmetry system. However, Dichromodes ornata does not correspond precisely to Henke’s first category because it has multiple pattern elements that occur distal to the CSS: medium-to-dark brown markings in the location of the BoSS, and dark parafocal elements.

All other species of Dichromodes examined here have a symmetrical CSS, but there is considerable variation in the additional elements of the NGP. The two elements of the NGP that occur proximally to the CSS—the wing root band and basal symmetry system—typically cannot be identified. This area of the wing often has mottled coloration. Many specimens lack any pattern elements in this area and simply have a few dark speckles formed by isolated dark scales (Fig. 5E, Figs. S2A, S2B, S3C, S3D, S4B, S4C, S5A, S5B, S5D, S5E, S5F, S6A, S6C, S6D, S7A, S7B). These specimens often have darker scales along the costa (Fig. 5E, Figs. S3D, S5F, S6A, S6C, S6D, S7A, S7B). A few specimens have spots at the base of the wing (Fig. 5C) or near it (Fig. 5D, Fig. S1E), and these spots can be quite faint (Fig. 5B, Fig. S1F). Some specimens have variation in scale color in this area without any discernible boundaries to permit the identification of discrete pattern elements (Figs. 5A, 5F, Figs. S1C, S2E, S2F). When distinguishable pattern elements do appear in this area they typically do not resemble the wing root band or the basal symmetry system (Figs. S1B, S2C, S2D, S3A, S3B, S4A, S4D, S5C, S6B, S7C, S7D). Only two specimens, the male representatives of D. anelictis and Dichromodes denticulata Turner, 1930, have pattern elements in this area that appear to be consistent with the NGP (Fig. S1A, S1D).

Distal to the CSS, where the border symmetry system occurs in the NGP, the Dichromodes specimens typically have a pattern element that is darker than the background color but is simpler than a symmetry system. This pattern element is usually of variable width (Fig. 5E), does not always reach the posterior margin (Fig. 5F), and is not always contiguous (Fig. S1E).

The parafocal elements occur along the termen only, violating the prediction from noctuid moths (Fig. 2D). Certain specimens have a pattern element between Rs₂ and Rs₃, near the apex of the wing, that is identical in color to the parafocal elements but is so severely flattened that it appears as a line (Figs. 5A, 5B, 5E, Figs. S1B, S1F, S2D, S3A, S3B, S3D, S5A, S5B, S5E, S6C, S6D, S7A, S7B). However, other specimens have this same pattern element in a lighter color than the parafocal elements (Figs. S1C, S1D, S1E, S4B), suggesting that this pattern element is not a parafocal element. The parafocal elements of Dichromodes are consistently triangular in shape, most closely resembling Nijhout’s parafocal element 86 (Nijhout, 1990). The triangular parafocal elements vary in concavity, in whether they extend to the wing veins, and in whether their distal edge lies entirely along the termen. Some specimens have parafocal elements whose shape is more ovoidal (Fig. S7B) or rectangular (Fig. S6D).