Dispatch from the field II: the mystery of the red and blue Opadometa male (Araneae, Tetragnathidae, Opadometa sarawakensis)

Abstract Background Males of Opadometa are difficult to associate with conspecific females, and sex-matching errors may persist in the taxonomic literature. Recommended best practices for definitive sex matching in this genus suggest finding a male in the web of a female, or better yet, mating pairs. New information A male Opadometa was observed hanging on a frame line of the web of a female Opadometa sarawakensis, a species for which the male was previously undescribed. This occurred during a tropical ecology field course held at the Danau Girang Field Centre in Sabah, Malaysia. A taxonomic description was completed as a course activity.


Introduction
The cover of Koh and Ming's 2014 field guide to the Spiders of Borneo was graced with a striking but at the time undescribed red and blue Opadometa species. Koh and Ming (2014) included a discussion on the "Mysteries of the Opadometa Males", in which they detailed the complexities of associating the rarely collected males of Opadometa with conspecific females. They included photographs of a male Opadometa collected in the vicinity of the red and blue female, but warned that it would be premature to conclude that these are conspecific. Males of Opadometa are rare in collections and notoriously difficult to associate with conspecific females, which are more than twice their length and much heavier. Confirmation of male-female conspecificity, they state, should be accepted, "…only if the males and females are found in the same web, or better still, are seen copulating." (p. 260). Dzulhelmi et al. (2015) published a formal description of what is almost certainly the same red and blue Opadometa species based on a female specimen from Sarawak, Malaysia. The male was not described, and Dzulhelime et al. echoed some of the sentiment expressed by Koh and Ming regarding the difficulties of associating male and female Opadometa.
In 2018, students participating in a two-week tropical ecology field course offered by the Naturalis Biodiversity Center and Leiden University and hosted by the Danau Girang Field Centre (DGFC) in Sabah, Malaysia, found a mature male at the margin of the web of a red and blue Opadometa. The female spider matched Dzulhelmi et al.'s (2015) description of Opadometa sarawakensis. A survey of orb web-building spiders near DGFC found no other Opadometa species. The students resolved, along with lecturers and field station scientific staff, to describe the male and provide additional data on the female, as well as data on the ecology and behavior of the species, and submit their results in the form of a manuscript before the end of the course. This is the second contribution to spider taxonomy and natural history to be produced in this way (Miller et al. 2014).

Males of Opadometa
Four species (plus eight subspecies) of Opadometa are currently cataloged (World Spider Catalog 2018): O. grata (Guérin, 1838), O. fastigata (Simon, 1877), O. kuchingensis Dzulhelmi & Suriyanti, 2015, and O. sarawakensis Dzulhelmi & Suriyanti, 2015(World Spider Catalog 2018. The type species is O. grata, described from New Guinea. Males have been described and illustrated multiple times under this name (Kulczyński 1911, Berland 1938, Archer 1951, Chrysanthus 1963, Wang 1991, Song et al. 1999, Zhu et al. 2003, Kuntner et al. 2008, Ono 2011Fig. 1). Opadometa fastigata was originally described from the Philippines. The male was described (as Callinethis elegans Thorell, 1895 from Burma, synonymized with fastigata by Pocock 1900) in a lengthy Latin text but has never been illustrated. The eight subspecies (seven of O. grata and one of O. fastigata) were established on the order of 100 years ago and have not been revisited (Strand 1911,  Dispatch from the field II: the mystery of the red and blue Opadometa male ... Strand 1915, Hogg 1919; their status seems dubious, but this issue is outside the scope of this contribution. The two species recently described in Dzulhelmi et al. (2015) were based on females only. Some authors have expressed skepticism that males and females of the two classical species are properly matched (e.g., Murphy andMurphy 2000, Koh andMing 2014) because males fitting descriptions of O. grata have been collected together with females of O. fastigata (e.g., in Singapore). Examination of illustrations of the male pedipalp in the legacy taxonomic literature reveals a clue: the Cymbial Basal Process (CBP) of specimens from New Guinea and New Hebrides extend more or less retrolaterally from the cymbium before curving distally ( Fig. 1a, b, c, d, g); males from elsewhere have the CBP extend nearly posteriorly before curving (Fig. 1e, f, h). So it could be that records of O. grata west of New Guinea are erroneous, and the widespread Southeast Asian and tropical East Asian species is O. fastigata.

Materials and methods
Trails around the Danau Girang Field Center were surveyed for Opadometa and other medium to large orb weaving spiders during day and night searches. Webs were assessed for several characteristics: diameter, angle, number of radii, number of spirals, and height of hub from the ground. Vegetation density was assessed by counting stems at breast height in a circle around the web hub with a radius of 3 meters. Percent canopy cover above the hub was estimated using a photograph processed with ImageJ (Rasband 2016).

Laboratory methods
Specimens were fixed and stored in 70% denatured ethanol. For imaging, specimens were positioned using cotton wool in a petri dish. Images were taken with an iPhone SE illuminated with an LED head light mounted on a gooseneck clip. The iPhone was mounted on a second gooseneck clip with a magnetic base and stabilized with newspaper, tape and a tongue depressor (Fig. 2). Some images were taken using a macro lens clip added to the iPhone camera. Versions of some images were taken with 5 mm graph paper as a guide for creating scale bars. Measurements are in millimeters unless otherwise specified. Color and levels were adjusted using Photoshop CS6. Anatomical abbreviations in text and figures follow Kuntner et al. (2008). All specimens have been deposited at the Universiti Malaysia Sabah's Institute for Tropical Biology and Conservation, Borneensis (BORN).

Legacy taxonomic literature
The taxonomic publication featuring the original treatment of Opadometa sarawakensis was semantically enhanced with XML markup using GoldenGATE Imagine (Sautter 2018) software and deposited at Plazi.org's TreatmentBank (see also Miller et al. 2015).

Description
Male: from Sabah, Malaysia (DGFCW2018022300). Prosoma uniform orange. Eight eyes in two rows, with the medians closer together than to the laterals; posterior median eyes oriented slightly toward the front; lateral eyes touching. Sternum dusky orange, darker posteriorly. Chelicerae orange, enlarged, diverging distally, armed in front and basolaterally with strong macrosetae; macrosetae absent from frontal-basal region (Fig. 3); with 2 promarginal and 2 smaller retromarginal teeth. Legs without macrosetae; femur of legs I and IV orange, distal segments darker; legs II and III overall dark (Fig. 4a); femur IV with row of very long prolateral trichobothria along entire length (Fig. 3a, b).  Abdomen gray dorsally with silvery patches and an anteriodorsal dark spot, black posteriorly and ventrally with two posteriolateral and one ventral orange spot, with a small anteriolateral black spot and a larger posteriolateral black spot, which joins with the black ventral marking.

Diagnosis
Cymbial basal process (CBP) of male palp projects initially posteriorly (Fig. 5c), distinguishing it from males illustrated from New Guinea and New Hebrides (presumably true O. grata; Fig. 1a, b, c, d, g) in which the CBP extends almost retrolaterally from the cymbium before curving distally; distinguished from males illustrated from further West in Southeast Asia (presumably true O. fastigata) by the length of the CBP, which is shorter and more gradually curved in O. sarawakensis (Fig.  5c) than in O. fastigata (Fig. 1e, f, h). Opadometa males further distinguished by the base of the chelicerae, which project forward with large distally oriented macrosetae in O. fastigata (Fig. 1j, k); absent in the male of O. sarawakensis (Fig. 3c, d), which is naked at the base of the chelicerae, and also apparently absent in O. grata, although the lateral view of male Opadometa from New Guinea and points East has never been illustrated (but see Fig. 1i). The male chelicerae of O. sarawakensis (Fig. 3d) and O. grata (Fig. 1i) are divergent distally, not divergent in O. fastigata. Males of O. sarawakensis may be further distinguished from those of O. fastigata by the orientation of the posterior median eyes (Fig. 3c), which are set further back in O. fastigata and are oriented slightly posteriorly in lateral view (Fig. 1k). The secondary conductor apophysis (SCA) in O. sarawakensis (Fig. 5a, b, d) appears to be shorter than in either O. grata (Fig. 1b, c, g) or O. fastigata (Fig. 1e, h)

Ecology
Trails around the Danau Girang Field Center were surveyed for Opadometa and other medium to large orb weaving spiders during day and night searches. Opadometa sarawakensis was the largest orb-weaver encountered after Nephila (2 juveniles in the 10-12 mm size range; no adult Nephila were encountered during the survey, although they have been seen at other times); no other Opadometa species were encountered.
Opadometa sarawakensis build open-hub webs with an inflection point so that the top half is more steeply inclined than the bottom half. The specific angles were quite different between the two webs measured (Fig. 8

Sexual size dimorphism
Sexual size dimorphism in Opadometa is extreme. The male-female pair found together (DGFCW2018022300) have a female/male size ratio of 2.2, meaning the female is more than twice the total length of the male. a b Table 1.
Characteristics of Opadometa sarawakensis webs and their habitats.

Discussion
Given the troubled history of matching sexes in Opadometa, Koh and Ming (2014) were justifiably conservative in not prematurely concluding that the male Opadometa they found in proximity to the red and blue Opadometa was conspecific. In light of the new data presented here, it appears that the male photographed in Koh and Ming (p. 260) is in fact O. sarawakensis.
The evidence for matching sexes in O. sarawakensis presented here is behavioral (male at margin of female web) and faunistic (only one Opadometa species found in survey of orbweaving spiders). Another possible line of evidence would be DNA barcode sequences (Hebert et al. 2003, Barrett andHebert 2005), which have been used to establish malefemale conspecificity in sexually dimorphic spiders (Barone et al. 2016, Magalhaes et al. 2017. For an unplanned field discovery such as this one, arranging for sequencing at a domestic institution requires time and resources beyond the scope of this study. Technologies for DNA barcoding in the field appear tantalizingly close to practicality and one of us (MS with Taxon Expeditions) is involved with field trials of the MinION, a portable DNA sequencer from Oxford Nanopore Technologies (Menegon et al. 2017). We hope to see more successful DNA sequencing under field conditions in the near future, but for now, we will have to be content with non-molecular (e.g., behavioral and faunistic) lines of evidence for sex matching in remote field studies.
More work clearly needs to be done to sort out the distributions of the known Opadometa species. If our analysis based on the legacy of male descriptions is correct, and O. grata is limited to New Guinea (and possibly points East) while O. fastigata is found further West, this should clear up some of the confusion regarding the distribution, sex matching, and anatomical features found in this genus.