A pictorial key for identification of the hoverflies (Diptera: Syrphidae) of the Madeira Archipelago

Abstract Background Syrphid flies are important ecological indicators and provide crucial ecosystem services, being important pollinators and biological control agents of insect pests. These charismatic insects are conspicuous and, due to their size and colourful patterns, are relatively easy to identify. However, the lack of user-friendly literature (e.g. photographic guides) for most areas may hamper its wider selection as a study group in biodiversity and ecological studies. The syrphid fauna of Madeira Archipelago comprises 26 species, including four endemics (Eumerushispidus Smit, Aguiar & Wakeham-Dawson, 2004; Melanostomawollastoni Wakeham-Dawson, Aguiar, Smit, McCullough & Wyatt, 2004; Myathropausta, Wollaston, 1858 and Xanthandrusbabyssa, Walker, 1849), but, despite the current good taxonomic knowledge on this group, information on species distribution, ecology and conservation is still lacking. Here, we provide a pictorial key to the adult hoverflies of Madeira Archipelago highlighting diagnostic characteristics and present photographs of both males and females (in dorsal and lateral views) in colour plates. The key and plates will help researchers to differentiate these species, thus encouraging the use of this insect group in future bioindication studies. In addition, this study also aims to engage a broader audience of non-experts in improving the knowledge on the distribution and ecology of Madeira syrphids. New information We provide a checklist for the hoverflies of Madeira Archipelago and a pictorial key to help on species identification.


Introduction
Syrphids, commonly known as hoverflies or flower flies, belong to a large family of flies (Diptera: Syrphidae) with over 6,000 known species (Rotheray 1993Evenhuis and Pape 2021). These flies are conspicuous and easy to distinguish from other insects due to their colour patterns, morphology and flying behaviour (Ball and Morris 2015). Adult hoverflies feed on honeydew, pollen and nectar and are amongst the most important flower visiting insects in many ecosystems (Wackers et al. 2008, Inouye et al. 2015, Doyle et al. 2020. Larvae, on the other hand, exploit a wide variety of food resources, including fungal fruiting bodies (mycophagous), dung, tree sap, nests of social insects, decaying vegetation and wood (saprophagous), whereas other larvae mine the leaves and stems of plants (phytophagous) or predate other insects (zoophagous), including leafhoppers, coccids and aphids (Rotheray 1993). For these reasons, hoverflies are considered important pollinators and biological control agents of insect pests, providing crucial ecosystem services (Ankersmit et al. 1986, Nelson et al. 2012, Dunn et al. 2020, Pekas et al. 2020. More recently, the role of hoverflies as ecological indicators has also been stressed in many studies since they are easy to sample and identify, their life cycle is well-known and the larvae from different species have distinct environmental requirements for their development (Sommaggio 1999, Sommaggio and Burgio 2014, Ball and Morris 2015, Dunn et al. 2020).
The syrphid fauna of Madeira has been studied since the mid-nineteenth century by several authors who contributed to a better understanding of species diversity and distribution in this Archipelago (Walker 1849, Wollaston 1858, Loew 1860, Schiner 1868, Thomson 1869, Bigot 1884, Osten-Sacken 1884, Becker 1908, Becker 1921, Frey 1939, Frey 1949. Frey (1949) updated the syrphid fauna checklist and reported 21 species for Madeira Archipelago. During this first century of reports on the Madeiran syrphid fauna, the number of recorded species increased considerably, showing a similar pattern to most of the other groups of terrestrial arthropods in the Archipelago ). More recently, other authors made significant contributions to the knowledge of this group of flies in Madeira Archipelago (Gomes and Baez 1990, Barkemeyer 1999, Pita and Gomes 2003, Aguiar et al. 2005, Pita et al. 2009). For instance, both Smit et al. (2004) and  described new endemic species, clarified the identity of ambiguous taxa and provided a thorough revision of Madeiran Syrphidae nomenclature. The current knowledge on the taxonomic diversity of Madeiran syrphids was updated by Smit (2008) and it was included in a comprehensive reference work on Madeira Archipelago biodiversity . Smit (2008) listed 26 species of hoverflies including four endemics (Eumerus hispidus Melanostoma wollastoni Wakeham-Dawson, Aguiar, Smit, McCullough & Wyatt, 2004;Myathropa usta, Wollaston, 1858 andXanthandrus babyssa, Walker, 1849) and provided general information on species distribution in the Archipelago. Despite the current good taxonomic knowledge on Madeiran syrphids, there is still a significant gap regarding species distribution, ecology and conservation.
During the last decades, there has been a growing interest in biodiversity conservation by the general public that has extended to several charismatic invertebrate groups, such as dragonflies and butterflies. In oceanic islands, like Madeira, invertebrate conservation needs to be fostered by engaging researchers, decision-makers and common citizens in knowing, valuing, protecting and making public the unique diversity of life forms of these ecosystems. This interplay is urgent since the biodiversity of oceanic islands worldwide is under threat due to various factors (e.g. land-use change, invasive species, climate change) and, jointly with significant declines in endemic species abundance, many humandriven extinctions have been documented in these unique ecosystems, including in Madeira Archipelago (Goodfriend et al. 1994, Gardiner 2003, Fontaine et al. 2007, Régnier et al. 2009, Rando et al. 2012, Terzopoulou et al. 2015. Halting biodiversity loss in oceanic islands is mandatory and, to accomplish this goal, a multidisciplinary strategy needs to be implemented. This strategy should include monitoring programmes targeting specific invertebrate groups and the use of expeditious, user-friendly and reliable techniques (Borges et al. 2018).
Here, we aim to provide a user-friendly pictorial key for the identification of Madeira's hoverflies, a charismatic bioindicator and ecologically-important insect group. The key was designed for use by non-experts and, altogether with the photos of male and female specimens of all known species occurring in the Archipelago, aims to engage a diverse audience in improving current knowledge on these conspicuous flies.

Study area
Madeira Archipelago is located in the Atlantic Ocean, nearly 600 km from the African coast (Morocco) and 450 km north from the Canary Islands, between latitudes 32°24′ and 33°07′N and longitudes 16°16′ and 17°16′W. The Archipelago is formed by three groups of volcanic islands and islets: Madeira, Porto Santo and the Desertas Islands (Boieiro et al. 2015). Madeira is the largest island of the group (~ 740 km ) and is characterised by a rugged topography with a steep coastline combined with deep ravines, high peaks and an altitudinal plateau in the central part of the Island (the highest mountaintop is Pico Ruivo at 1862 m). Madeira harbours various habitats, from coastal dry areas to humid laurel forests (Laurisilva) and heathland at higher altitudes, that support a diverse fauna and flora (Menezes et al. 2011, Boieiro et al. 2013, Boieiro et al. 2015. Porto Santo, the second largest island, lies ~ 40 km NE of Madeira, has several islets surrounding it, all included in a network of protected areas (Alves et al. 2015). Natural vegetation cover was severely destroyed by human activities following colonisation and, currently, is mostly composed of herbaceous plants which are scarce in some areas. Further, in some mountaintops, pine forests were planted to mitigate soil erosion effects. The Desertas Islands is a group of three islands (Ilhéu Chão, Deserta Grande and Bugio) that include a Nature Reserve. Both Bugio and Deserta Grande (the largest ones) are crest-like islands with steep slopes and have large areas deprived of vegetation or are covered by herbaceous plants while Ilhéu Chão, with a flat surface, has a well-preserved herbaceous vegetation cover .

Laboratory work
Specimens, both males and females, from all the known species reported to Madeira Archipelago were studied under a stereomicroscope. Most of the specimen's images were taken with a Leica M125 motorised stereomicroscope, equipped with a IC80 HD digital camera and LAS-Leica Application Suite 3.8 Software. For image stacking, we used the LAS Module "Multifocus" and post-processed the images in ®Adobe Photoshop CC. We also used a Canon 7D digital slr camera with a Canon EF 100 mm 2.8 L Is USM macro lens to capture the habitus of some specimens. The study specimens are deposited in the entomological collections of the Laboratório de Qualidade Agrícola (ICLAM) (Madeira, Portugal) and Naturalis Biodiversity Center, Natural (RMNH) (Leiden, the Netherlands).

Key to the hoverflies of the Madeira Archipelago
The taxonomic key to the adult stages of hoverfly species of the Madeira Archipelago relies on pictorial information to ease interpretation of characters and includes information on morphological differences between males and females. Additionally, photos of male and female specimens of all species (in dorsal and lateral views) are presented in colour plates. In the dichotomous key, the couplet leads present one or more morphological characteristics indicated with symbols (e.g. asterisks for one lead and Roman numerals for the other) which help to easily identify those characteristics in the associated figures. The terminology of morphological characters used in this key follows Thompson (1999). Abdominal tergites and sternites are abbreviated with a 't' or 's', respectively. 2 1 Face entirely yellow (i); scutellum always yellow, clearly lighter than scutum (ii) (Fig. 1) 2 -Face dark or yellow with median dark stripe, sometimes obscured by dense pollinosity (*); scutellum never yellow, sometimes orange-brown (**) (Fig. 1) 15 2 Very large species, over 18 mm, with hornet-like appearance; scutum with yellow markings on anterior half and orange-red colouration on posterior half; metafemur with a small tooth apicoventrally (i); wings with a yellow tinge along the costa (ii) (Fig. 2) Milesia crabroniformis -Smaller species, at most 15 mm, never with a hornet-like appearance; if yellow markings present on scutum, then restricted to lateral margins and scutum never with orange-red markings; metafemur never with a small tooth apicoventrally; wings without yellow tinge along costa Abdomen more or less parallel-sided, as broad as scutum (i) (Fig. 6) 7 -Abdomen clearly broadening and oval shaped, clearly broader than scutum (*) (Fig. 6) 8 7 Abdomen with 'double bands' on t3, t4 (i) (Fig. 7) Episyrphus balteatus -Abdomen without 'double bands', with oblique yellow spots (*), sometimes connected to form bands (Fig. 7) Meliscaeva auricollis 8 Wing vein R4+5 in basal half of cell r4+5 almost parallel to M, curving upwards in apical part (i); eyes pilose (ii); frons distinctly swollen, more obvious in males (iii); larger species: 10-15 mm (Fig. 8)

Syrphus vitripennis
13 Face in frontal view at least as broad as one eye (i); femora at least partially black at the base (ii) (Fig. 12) 14 -Face in frontal view clearly narrower than one eye (*); femora entirely yellow (**); abdomen normally with broad yellow maculae, sometimes connected to form bands (Fig. 12)

Analysis The hoverfly species of Madeira Archipelago
The syrphid fauna of Madeira Archipelago comprises 26 species, all considered to be native to these islands (Table 1). Four of them are endemics (Eumerus hispidus, Melanostoma wollastoni, Myathropa usta and Xanthandrus babyssa) which occur mostly in native laurel forests (Laurisilva) and altitudinal heathlands in Madeira Island (except for E. hispidus which occurs in coastal and drier habitats of different islands) , Smit 2008. There are significant differences on the syrphid fauna between island groups since 25 species were recorded in Madeira, 14 in Porto Santo and seven in Desertas Islands. Differences in island area, altitude and habitat heterogeneity are the main drivers of species richness differences between islands, with some species being restricted to native forest areas in Madeira Island. We believe that the syrphid species inventory of Madeira Archipelago is near complete, but additional sampling should be carried out in Porto Santo and Desertas since these smaller islands were less sampled than Madeira proper. Furthermore, taking in consideration the increase in trade and tourism to Madeira in recent decades, it is expected that new species will arrive at the Archipelago; therefore, it is important to implement a monitoring scheme for early detection of introduced species in the short term. Differences in leg colouration and body pollinosity.