Review of the flower-inhabiting water scavenger beetle genus Cycreon (Coleoptera, Hydrophilidae), with descriptions of new species and comments on its biology

The hydrophilid genus Cycreon Orchymont, 1919, previously known from two historical specimens only, is reviewed based on the numerous material collected recently from the inflorescences of various Araceae species in the Malay Peninsula and Borneo. Four species are recognized in the genus: C. sculpturatus Orchymont, 1919 from Sumatra, C. armandi Shatrovskiy, 2017 from Singapore, C. adolescens sp. n. from peninsular Malaysia, and C. floricola sp. n. with two subspecies, the nominotypical one from Peninsular Malaysia, and C. floricola borneanus subsp. n. from Borneo. All species are very similar, differing only by the pronotal punctation, shape of the clypeus and the mentum, and the form of the median lobe of the aedeagus. Specimens of C. floricola sp. n. and C. adolescens sp. n. were collected from inflorescences of various genera of the family Araceae. The field observations and analysis of mid gut contents indicates that they feed on organic material on internal organs of the inflorescences, including the pollen of the host plant. They were also observed to carry a large amount of pollen and are likely pollinators of their host species of Araceae. Key Words


Introduction
Among the water scavenger beetles (Polyphaga: Hydrophiloidea: Hydrophilidae), the members of the tribe Megasternini stand out in terms of species and morphological diversity, faster speciation rate and the wide array of microenvironments inhabited (Bloom et al. 2014;Fikáček et al. 2009Fikáček et al. , 2012. Most of the megasternine species are associated with various kinds of decaying organic matter, like mammal dung (e.g., Smetana 1978, Ryndevich 2008, Ryndevich et al. 2017, Arriaga-Varela et al. 2017, humid forest leaf-litter (e.g., Deler-Hernández et al. 2015;Fikáček et al. 2009;Fikáček and Short 2006) or rotten seaweed (e.g., Smetana 1978;Ryndevich 2001). In contrast to this general pattern, few genera are known to inhabit the interior of various inflorescences: the Neotropical Pelosoma Mulsant has been collected inside Heliconia flowers (Archangelsky 1997), and the Neotropical Nitidulodes Sharp and Oriental Cycreon Orchymont, 1919 were recently reported to be associated to Araceae flowers (Bloom et al. 2014;Low et al. 2016;Hoe and Wong 2016;Hoe et al. 2018). However, very little is known about the biology and the systematics of these genera.
Only two specimens of Cycreon are known so far in the literature, representing two different species. The genus was described by d' Orchymont (1919) with Cycreon sculpturatus d 'Orchymont, 1919 as the only species, based on a single female specimen collected in Palembang, Sumatra without any detailed collecting data. An additional male specimen from Singapore was later examined by d'Orchymont and labeled as 'Cycreon emarginatus sp. n., however, whether is it not the male of C. sculpturatus', but never published. Both specimens were moreover on loan from d'Orchymont collection when M. Hansen was preparing a generic review of the hydrophiloid beetles (Hansen 1991). Shatrovskiy (2017) examined both these specimens and described the second specimen, male from Singapore, as Cycreon armandi.
Extensive sampling of insects associated with inflorescences of Malayan aroid plants was performed recently by Low et al. (2014Low et al. ( , 2016, Hoe and Wong (2016) and Hoe et al. (2018) in order to study their pollination biology, and Takizawa (2010) in order to study the association of species of Chaloenus Westwood, 1861 (Chrysomelidae) with these inflorescences. As a result, a high number of Cycreon specimens from both Peninsular Malaysia and Borneo was accumulated. In this study, we use this material to redescribe and illustrate the genus Cycreon in detail, to revise the systematics of the genus, and to sum up the available data on the biology of the genus.

Material and methods
Examined specimens and depositories. A total of 1444 specimens of Cycreon were examined. Label data are reproduced verbatim; notes on the label data or additional information are written between square brackets []. List of examined specimens is available in DarwinCore-formatted spreadsheet file at Zenodo repository (https://doi. org/10.5281/zenodo.1258208). This file was also used to prepare the distribution map using QGIS software and freely available GLOBE altitude data and DIVA-GIS country borders data. The authors did not examine holotypes of C. sculpturatus and C. armandi as these are not accessible for the examination, and adopted the information about them from Shatrovskiy (2017). In addition, A. Shatrovskiy kindly provided a new photograph of ventral view of the head of C. sculpturatus used to illustrate the shape of the mentum of this species (Fig. 5P).

Diagnosis.
(1) antennal grooves on prosternum small and marked by a weak ridge close to the lateral margins of prosternum ( 7) aedeagus with the median lobe not fused to the bases of parameres, reaching into the phallobase; (8) median portion of male sternite 9 tongue-like ( Fig. 3D, H, M).
Note. Orchymont (1919) mentioned the absence of antennal grooves, which were supposed by Hansen (1991) who did not have the chance to study specimens of this genus. Shatrovskiy (2017) revealed that antennal grooves are present although very small, and weakly marked by a faint ridge.
Differential diagnosis. Cycreon is distinct among Megasternini in lacking the median carina of abdominal ventrite 1 (Fig. 2G); in this character it only corresponds to the Megasternini genera Pyretus Balfour-Browne and Acaryon Hebauer. From Pyretus, Cycreon is easily diagnosed by the narrowly laminate elevation (in contrast to widely pentagonal and widely contacting metaventrite in Pyretus), simply carinate median portion of prosternum (forming an elevated prosternal plate in Pyretus), and small antennal grooves (antennal grooves are large and reaching lateral pronotal margin in Pyretus). The genus Acaryon from Madagascar is similar to Cycreon in many characters, including the relatively large eyes, simply carinate prosternum, antennal grooves not reaching pronotal margin, narrowly carinate elevation on mesoventrite, metaventrite without additional ridges, and dorsal punctation (with semicircular to circular punctures in Cycreon, and circular setiferous punctures intermixed with usual punctation in Acaryon). However, Cycreon can be distinguished from Acaryon by the shape of the mentum (deeply emarginate anteromedially in Cycreon, weakly sinuate on anterior margin in Acycreon), presence of the grooves for reception of procoxae at sides of the mesoventral elevation (absent in Cycreon, present in Acaryon) and the dorsal colouration (unicoloured or bicoloured in Cycreon, unicoloured yellow with dark central pronotal spot in Acaryon).

Key to the species of Cycreon
Diagnosis. Cycreon sculpturatus can be distinguished from other species of the genus by the deeply incised clypeus, pronotum densely covered with complete ring-like impressions (punctures) and straight meso-and metatibiae.
Addition to description. Body 2.6 mm long; colouration light reddish-brown, with slightly darker head and elytra; clypeus about 2.5× as wide as long, with anterior margin conspicuously emarginate medially (Fig. 5O); frons and clypeus with punctation composed of complete circular impressions (punctures); mentum ( Fig. 5P) subtrapezoid, widest at posterior fifth, about 2.0× wider than long, anteromedian emargination reaching about 0.2× the mentum length; pronotum with dense and moderately deep punctation consisting of complete circular impressions with a small setiferous puncture in posterior part (Fig. 5Q), punctation of approximately same diameter and density all over pronotum; mesoand metatibiae straight (Fig. 5R); male genitalia unknown (because male remains unknown for this species).
Distribution. Only know by a single female specimen from the type locality (Indonesia, Sumatra, Palembang) (Fig. 7).
Remark. According to Shatrovskiy (2017) the proportions of the clypeus were given as 4× as long as wide, however, the picture in the paper (Shatrovskiy 2017: 591) shows that the ratio is about 2.5×. Type locality. Singapore.
Diagnosis. Cycreon armandi can be distinguished from other known species by the deeply incised clypeus, the semicircular impressions (punctures) on the pronotum, and the curved meso-and metatibae.
Distribution. Only known from the type locality in Singapore (Fig. 7).
Remarks. According to Shatrovskiy (2017) the proportions of the clypeus are given as 3× as long as wide, however, the picture in the paper (Shatrovskiy 2017: 591) shows that the ratio is closer to 2.0×. Diagnosis. This species is most similar to Cycreon floricola borneanus ssp. n. in the very weakly emarginate anterior margin of the clypeus and pronotal punctation consisting of circular punctures, and straight meso-and metatibiae. It differs from the latter in the structure of the male genitalia (relatively longer phallobase and widely pointed apex of the median lobe) and by the less transverse mentum (1.7× wider than long), with many ring-like impressions in posterior half.
Head. Clypeus about 2.4× as long as wide, with anterior margin of clypeus margin very weakly emarginate medially. Frons and clypeus with punctation composed of complete circular impressions (punctures) with a small setiferous puncture on anterior margin (Fig. 4A). Interocular distance about 5.5× the width of one eye in dorsal view. Mentum (Figs 4J, 5B) subtrapezoid, widest at posterior fifth, about 1.7× wider than long, with a moderately pronounced emargination reaching about the anterior fifth of mentum length; lateral angles weakly marked; surface with few sparse, moderately long setae in anterolateral angles, posterior half glabrous, punctures moderately large and deep, vanishing mesally, 11-13 punctures close to posterolateral angles with ring-like impressions (punctures).
Prothorax. Pronotum transverse, widest at base 2.2× wider than long; 1.6× wider at base than between anterior angles, 1.7× wider than head including eyes. Punctation dense and shallow, consisting of circular impressions with one small setiferous puncture on posterior margin (Fig. 4B), punctation of approximately same diameter and density all over pronotum.
Etymology. The species name reflects the teneral condition of the holotype (from Latin adolescens = growing up, maturing).
Distribution. The species is only known two localities in Pahang province, Malaysia.
Biology. No details about collecting circumstrances are available for type specimens; additional specimens from Tanah Rata were collected from inflorescences of Araceae (H. Takizawa, pers. comm. 2018).  at basal fifth of elytra; weakly convex, 3.1-3.4× as long as high (height of holotype: 0.9 mm). Colouration. Light brown with darker elytra (Figs 1A-C).
Head. Clypeus about 2.5× as wide as long, with anterior margin of clypeus margin very weakly emarginate medially. Frons and clypeus with punctation composed of complete circular impressions (punctures) with one small setiferous puncture on anterior margin (Figs 4G, D). Interocular distance about 4.9-5.2× width of one eye in dorsal view.
Mentum (Figs 1J, 2B, 4K, 5E,H) subtrapezoid, widest at posterior fifth, about 2.1× as wide as long, with deep emargination reaching beyond anterior third of mentum length; surface with sparse, moderately long setae in anterior half, posterior half glabrous, punctures moderately large and deep, becoming smaller mesally, with 2-3 punctures with ring-like impressions close to posterolateral angles.
Prothorax. Pronotum transverse, widest at base, 2.2× as wide as long; 1.5-1.6× wider at base than at anterior angles, 1.6× as wide as head including eyes. Punctation dense and shallow, consisting of semicircular to complete ring-like impressions with a small setiferous puncture on posterior margin (Figs 4E, H, 5F, I), punctation approximately same in size and density all over pronotum.
Legs. Metatibiae wide and flattened, very weakly curved, 0.35× as long as elytra, 4.8× as long as wide.
Etymology. The species name reflects the association of this species with flowers, it consists of flori-(from Latin flos, floris = flower) and -cola (from Latin incola = inhabitant).
Comment. This species is composed of two phenotypically distinguishable forms which are geographically exclusive and are here descibed as subspecies. Morphological differences are mainly restricted to punctation on the pro- Diagnosis. This subspecies is very similar to Cycreon floricola borneanus ssp. n. with which it shares most of the external characters including genital morphology. Cycreon floricola floricola can be distinguished by the pronotal punctation consisting exclusively of the incomplete ring-like punctures. The dorsal coloration of the is slightly more contrasting in most specimens of C. floricola floricola than in representatives of C. f. borneanus ssp. n., with the pronotum darker compared to the elytra.
Pronotum 2.2× wider than long; 1.6× wider at base than between anterior angles, with punctation dense and shallow, consisting of semicircular impressions (punctures) with a small setiferous puncture on posterior margin. Punctation approximately same in size and density all over pronotum.
Elytra widest at anterior fifth, 1.1-1.2× as long as wide, 2.9-3.0× as long as pronotum, 1.1× as wide as pronotum. Punctation on intervals composed of semicircular impressions with one setiferous puncture on posterior margin.
Distribution. The subspecies is known from two regions in Peninsular Malaysia, in provinces of Johor and Kelantan.
Biology. Many specimens of C. floricola floricola were collected inside of inflorescences of Schismatoglottis species and Kiewia ridleyi (Low et al. 2018) (both Araceae).  Diagnosis. This species is very similar to Cycreon floricola floricola with which it shares most external characters and genital morphology. Cycreon floricola borneanus ssp. n. can be distinguished from C. floricola floricola by the shape of the pronotal punctation consisting only or largely of complete ring-like punctures. The dorsal coloration of C. floricola borneanus sp. n. is usually more uniform, with elytral colouration not much darker than pronotal one.
Pronotum 2.2× wider than long; 1.5× wider at base than between anterior angles, 1.6× wider than head in-cluding eyes. Punctation dense and shallow, consisting of complete ring-like impressions with one small setiferous puncture on posterior margin (Figs 4E, 5F), punctation of approximately same in size and density all over pronotum.
Variation. In most specimens examined from Borneo, the pronotal punctation consists exclusively of completely ring-like punctures. However, in few localities on the north-western coast of Borneo the punctation of 5-20% of specimens shows a mixture of complete and incomplete rings. In the absence of genetic data, we are unable to analyze this variation in detail, and hence temporarily treat even these specimens as C. floricola borneanus.
Etymology. The name of this species is derived from Borneo, the historical name of island where all the known specimens of this subspecies were collected.
Distribution. The species seems to be widespread in Borneo, it is recorded from Indonesia: Kalimantan Barat and Kalimantan Selantan and Malaysia: Sabah, Sarawak (Fig. 7) Biology. This subspecies has been collected in inflorescences of a number of plant species belonging to the Araceae family. It was collected in high numbers in flowers of the genus Schismatoglottis: S. calyptrata, S. colocasioideae, S. erumpens complex, S. giamensis (Fig.  6G), S. mayoana, S. modesta, S. muluensis, S. multinervia, S. pellucida, S. petradoxa and S. serratodentata. It has been also collected in numbers close to one hundred specimens in an inflorescence of Ooia glans (Figs 6H-I) and O. havilandii. Other known records of host plants include Alocasia longiloba complex (Fig. 6F), Gamogyne loi, several species of Homalomena (this paper) Phymatarum borneense and Schottarum sarikeense ( Fig.  6A-B) (Low et al. 2016). According to H. Takizawa (pers. comm. 2017), C. borneanus can be found in aggregations in a wide range of aroid inflorescences from lowlands to montane areas (ca. between 100-1500 m a.s.l), mainly in flowers on small open places like trail sides in or near well-preserved primary or secondary forests, or along small streams.

Biology of Cycreon
All specimens of both subspecies of C. floricola have been collected in inflorescences of various Araceae genera, often in high numbers, indicating their tight association with this microhabitat. The fact that there were only two specimens of the genus known up to now likely correspond with the biology of Cycreon beetles, since no study of flower-inhabiting beetles associated with Araceae in the Malaysian Peninsula and Sunda islands was performed previously. Low et al. (2016) studied the biology of insects associated with inflorescences of Araceae in Borneo and demonstrated that Cycreon floricola borneanus specimens are only present in the upper part of the inflorescences of Phymatarum borneense and Schottarum sarikeense, never in the pistillate zone (= bottom), and that they are not attracted by the smell of the inflorescences, unlike the co-occurring Chaloenus beetles (Chrysomelidae) and Colocasiomyia flies (Drosophilidae). Observations of few specimens covered by pollen grains were reported, but only Colocasiomia flies were considered as pollinators. Subsequent investigations of the pollination biology of the Schismatoglottis calyptrata complex (Hoe et al. 2018) revealed that Cycreon floricola borneanus visited all the investigated species except Schismatoglottis calyptrata and S. laxipistillata, but their abundance differed based on host plant species: they were very abundant in S. giamensis, S. caesia and S. muluensis, but present in single or few specimens only in S. pseudoniahensis, S. pantiensis, S. adducta, and S. roh. Cycreon beetles were observed to feed on the exudations from the interpistillar staminodes, mated on the pistillate zone and remained inside the lower spathe chamber. They were also revealed as the most effective pollen carriers, carrying 6-15 times more pollen than Colocasiomyia flies and hence considered as secondary pollinators (Colocasiomyia flies were 4-6 times more abundant and are hence considered as main pollinators). No hydrophilid larvae were found in the inflorescences, indicating that they may live in different microhabitat.
Mouthparts of Cycreon (Fig. 1F-K) are unusual when compared to other members of the tribe Megasternini examined so far, differing from them in three aspects: (1) structure of the mandible with two large teeth on the apex (Fig. 1F-G), (2) excision of the clypeus (varying from weak to very deep, depending of the species; Fig. 5), and (3) shape of the mentum, with deep anterior excision (Figs 1J,2B,5). The mandibles of other Megasternini examined so far bear simple apex (e.g., Fikáček 2010, Arriaga-Varela et al. 2018, while those of Cycreon have the apex deeply bifid and with two large teeth. When the mandible is examined in mesal view (Fig. 1H), both apical teeth are situated on sides of straight edge, and they become strongly abraded in some specimens (Fig. 1G). This may indicate that mandibular apex is used for processing of some hard/solid material, possibly for scraping organic material from internal parts of the aracean inflorescences, in agreement with the above observations by Hoe et al. (2018). The inspection of the gut contents of Cycreon floricola borneanus (Fig. 6C-E) revealed that the midgut contains two components, both stained by safranine dye: (1) pollen grains of the respective plant species and (2) the heterogeneous organic matter which cannot be further identified; it does not seem to be just remains of crushed pollen grains, as no partially crushed pollen grains or remains of their exine were found (safranine stains various organic compounds including cellulose, lignine, glucosamines and cell nuclei, and does not allow detailed identification of this component -it may represent organic detritus scraped by the beetles from interior of the inflorescence). The presence of this unspecified organic matter in the intestines indicates that Cycreon beetles are not specialized pollen feeders, but the presence of pollen grains in the midgut content confirms that pollen is part of the diet and may be possibly digested. When compared to specialized pollen-feeding New Zealand genus Rygmodus (Hydrophilidae: Cyclominae) analyzed by Minoshima et al. (2018), important differences in mandible morphology can be found, confirming that Cycreon is not specialized pollen feeder: (1) mandibular apex is simple and spoon-like in Rygmodus, whereas strongly bifid in Cycreon, (2) mola is simply tuberculate in Rygmodus (a supposed adaptation to disrupt the pollen exina by grinding), but bears poriferous lamellae in Cycreon and all other hydrophilids examined. Unlike in Cycreon, the midgut of Rygmodus contains nearly exclusively pollen grains, with very little fine organic matter (see Minoshima et al. 2018 : fig 4).
The excised clypeus and anterior margin of mentum is unusual in the Megasternini. Excised mentum is only known in the Central American genus Nitidulodes, which is also associated with aroid inflorescences (Hansen 1991, Bloom et al. 2015. Excised clypeus is only present in Cycreon. Moreover, the strong variation of mentum and clypeus shape between different species of Cycreon is also unusual within Megasternini. Usually these characters are very similar in congeneric species and differ at most between genera. We suppose that this interspecific variation in Cycreon may indicate species-specific food and hostplant preferences. The data by Hoe et al. (2018) indicate some kind of specificity in host plants, which would be in agreement with this assumption. For example, various genera and species of Schismatoglottideae were sampled in Mulu National Park (Malaysia, Sarawak) from December 2017 to February 2018, but Cycreon beetles were only found in sampled species of Schismatoglottis (beetles were present in all sampled plants), whereas not a single specimen was found in inflorescences of Anadendrum, Aglaonema, Alocasia, Bucephalandra and Lasia. Additionally, the material collected from various inflorescences of the single aroid tribe Schismatoglottideae by Low et al. (2014Low et al. ( , 2016, Hoe & Wong (2016) and Hoe et al. (2018) includes two very closely related subspecies only (Cycreon floricola floricola and C. floricola borneanus), despite consisting of more than 1000 specimens; not a single specimen of another Cycreon species was collected even in peninsular Malaysia, despite the sampled localities were close to those of two other species (C. armandi and C. adolescens). We suppose this may be caused by the focus on a single narrow group of host plants, and is congruent with the expected species-specific host preferences in Cycreon beetles.

Discussion
Interactions of Cycreon with aroid inflorescences can be interpreted as an initial stage of development of cantharophyly, i.e. interrelations between beetles and plant reproductive organs, in which only adult are associated with flowers (e.g., Kirejtshuk 1994Kirejtshuk , 1997. This type of inter-relations is particularly possible in cases when blossoming period ends by decaying of part of flowers with fungal and microbial infection (Teichert et al. 2012). On the other hand, species of Chaloenus beetles (Chrysomelidae: Alticini) co-occurring with Cycreon may represents another type of interrelations with plants, connected with a secondary transition of adults from leaf-feeding to flower-feeding. Besides, one sample collected from Leucocasia giganteum in peninsular Malaysia included representatives of Aethina Erichson (Coleoptera: Nitidulidae: Nitidulinae); some species of the genus are known to be associated with fungi, others to feed on inflorescences (Kirejtshuk 1986). Few examined samples also included few staphylinids, possibly as occasional visitors of decaying aroid inflorescences.