Review of Psenulus species (Hymenoptera, Psenidae) in the Hong Kong SAR, with description of three new species

The wasp genus Psenulus is the most diverse genus of the family Psenidae in the superfamily Apoidea, with its diversity peaking in the Oriental realm. Six species of the genus are here recorded for the first time from the Hong Kong SAR. Three of these, Psenulus ephippius sp. nov., Psenulus gibbus sp. nov. and Psenulus pallens sp. nov. are described as new to science. An identification key, figures for all taxa recorded in Hong Kong and phenology of five of the six species are also provided.


Introduction
Psenulus Kohl, 1897 is a diverse genus of small apoid wasps, including over 160 species (Pulawski 2019). The genus is distributed on all continents with the exception of Antarctica. The greatest diversity is found in the Oriental realm where 110 nominotypical Lith 1951;Benno 1957;Tsuneki 1970Tsuneki , 1973. Brood cells are provisioned with nymphs or adults of Hemiptera (Sternorrhyncha and Auchenorrhyncha) (Bohart and Menke 1976;Matthews 2001). They are unusual among wasps in lining the cells of their nests with silk which the females produce from glands opening on S4 and S5 (Melo 1997).
A recent entomological survey of mangrove remnants in Hong Kong SAR provided numerous new records of arthropods and species new to science (e.g. Grootaert et al. 2019). Among these, Psenulus was found to be one of the most abundant wasp genera, with large numbers of specimens occasionally recovered from a single Malaise trap. Other sampling efforts within forested or more anthropogenic habitats have, over the past two decades, also provided numerous records which are summarized here. In total, six species were identified among the material collected by the authors from both intertidal (mangroves) and inland habitats (gardens and riparian bands). Three species are here described as new to science, Psenulus ephippius sp. nov., Psenulus gibbus sp. nov. and P. pallens sp. nov. Whereas P. carinifrons rohweri van Lith, 1962, has previously been recorded from Taiwan, the remaining two species are newly recorded for China. We also provide descriptions for the hitherto unknown males of P. continentis and P. maculatus. Specimens collected in Hong Kong also improve our understanding of variation within the last three species and full descriptions are provided for all species. A dichotomous key for the Psenulus species recorded from Hong Kong is provided. In addition, distribution maps of all species and insights into the phenology patterns of five of the six taxa recorded are provided.

Materials and methods
The material used for this study was mostly obtained using Malaise traps set in various locations in the Hong Kong SAR. Between 2017 and 2018, members of the Insect Biodiversity and Biogeography Laboratory of the Hong Kong University (IBBL) conducted a survey of insect diversity in remnant mangrove forests in the Hong Kong SAR. Additionally, material recovered from traps placed in three inland locations and one mangrove forest from 2014 to 2020 by CB (Fig. 1B) and a suite of specimens of P. gibbus sp. nov. collected by CB by hand net in April of 2020 completed the specimens examined here.
The trapping efforts for the mangrove sites sampled by IBBL and the four sites sampled by CB are noticeably different. The seventeen sites sampled by IBBL (sites 1 to 17 of Fig. 1B) that yielded Psenulus spp., had each up to five traps, during two periods from October to December 2017 and May to July 2018 for a total sampling effort of 1414 trap days. The four sites sampled by CB (sites 7 and 18-20 of  days but also temporal continuity) between the IBBL and CB sites renders abundance analysis problematic. Despite this heterogenous sampling, the data generated by these surveys, essentially presence/absence, allows us to propose activity periods of the six species of Psenulus recorded to date from Hong Kong and, the results are represented on figure   present (Sep 2020) at three CB sites (Pak Sha O, Ping Shan Chai and Mang Kung Wo) have been consistent and standardised throughout this period and allows us to provide some insights into the abundance of five of the six species recorded in Hong Kong, P. carinifrons rohweri being excluded as it was never collected in inland sites. This subset of data includes 115 recorded occurrences; or about 29% of the records generated in this study, and the results are plotted on figs 2B and 2C along a 52-week horizonal axis, weeks noted as W1, W2, etc. on these figures and in the text.
The habitat structure for the trapping sites was as follows: sites 1-17 were characterised by intertidal habitats dominated by mangroves (Kandelia obovata Sheue, Liu & Yong, Avicennia marina (Forssk.) Vierh., Bruguiera gymnorhiza (L.) Savigny, etc.), these habitats are generally highly fragmented and surrounded by recent development (infrastructural or urban). Site 18 was an upland basin situated within a riparian band, its habitat composed of forest pioneers (herbs and bushes such as Euphorbiaceae, Vitex negundo, Miscanthus sp., etc.) and secondary evergreen forest (40+ years). Site 19 was in a garden reclaimed from an abandoned orchard; it had an early successional structure and was adjacent to a secondary evergreen forest (30+ years) to the south and a low-land marshland to the north. Site 20 was in a garden in a very low-density residential area, had an early successional structure and was adjacent to secondary evergreen forest. The anthropogenic disturbance of inland sites was highest at site 20 and lowest at site 18.
Identification of species were obtained using keys provided by van Lith (1962van Lith ( , 1969 and for comparison, descriptions provided by the same author were also used ( van Lith 1972van Lith , 1973van Lith , 1976van Lith , 1978. All examined material is deposited at the Biological Sciences Museum of The University of Hong Kong and in CBs' private collection, while holotypes and some paratypes will be deposited at the California Academy of Sciences as indicated. The terminology used mostly follows the Hymenoptera Glossary (HG). However, the location of two features characteristic of Psenulus but not recorded in HG, the interantennal carina and the episternal sulcus ("anterior oblique suture of the mesopleuron" of van Lith, 1962) are indicated on figs 3A and 3C, respectively and used throughout this study; see van Lith (1962) for further discussion of these features. We use the abbreviations T1, T 2, T3, etc., to denote the first, second, third, etc., metasomal terga, while S1, S2, S3, etc., denote the first, second, third, etc., metasomal sterna, and F1, F2, F3, etc., denote the antennal flagellomeres. In the Some morphological characters referred to in this work are illustrated in figures 3A-3F. We have used a suite of standard measurements to produce ratios/indices for the specimens examined. Measurement values (except for specimen length) are relative values as measured in the stereomicroscope's reticule at various magnifications as indicated below in "[ ]". These measurements are referred to in figures 3A-G and detailed here and, the number of specimens measured is represented by the value "n" in the text: Length of specimen in millimetres (mm), this is obtained by adding two measurements in lateral view, the first from frons to posterior apex of propodeum and the second from the petiole attachment to apical part of T1. This because (a) the petiole is often at awkward angles to the mesosoma in preserved specimens and, (b) metasomal segments after T1 tend to be either expanded or retracted in preserved specimens.
These dimensions are used to define the following eight ratios: These measurements were taken from a random sample of ten specimens for those species represented by large series, i.e. P. carinifrons rohweri and P. continentis. We measured all material available for the other four species, except for one male specimen of P. maculatus maculatus that was lost before measurements and one female specimen of P. ephippius that was destroyed at mounting before measurements. A total 83 specimens were measured.

CR
We have felt compelled to add descriptions of all species listed in this work, because past descriptions (mainly by van Lith 1962van Lith , 1969van Lith , 1972van Lith , 1973van Lith , 1976van Lith , 1978 have been scattered and inconsistent in their presentation, primarily diagnostic rather than descriptive in nature and we have noted greater colour variability than what van Lith (1962van Lith ( , 1969van Lith ( , 1972van Lith ( , 1973van Lith ( , 1976van Lith ( , 1978 proposed, mainly for P. continentis and P maculatus. In addition, we provide the first descriptions of the males of the latter two species. Following the descriptive sections of each species we also provide notes with taxonomic and distributional discussion, as well as a brief account on ethology. The distributional map (Fig. 1A) is based on records from van Lith's descriptions, our own Hong Kong records and additional distributional records as noted by Pulawski (2019) in his species file and covers only those species that have been recorded from the Hong Kong SAR. In the distribution section for the taxa previously described, "*" refers to a new record. The acronyms of collections where material is deposited are as follows:   Head black with appressed silvery pubescence (Figs 7A, 9A); antenna with scape yellow, remainder of antenna black except yellowish on venter of pedicel and very base of flagellum; mandible yellow with black tip. Mesosoma black with tegula yellowish brown (Fig. 11A), following yellow: pronotal collar, pronotal lobe, scutellum except scutellar groove, metanotum. Fore and mid legs with coxae black, remainder yellow except ends of tarsi brownish; hind leg black except basal half to two-thirds of tibia white, tarsus brown. Metasoma black except T7 brown.
Head black with appressed silvery pubescence (Figs 7A, 9A); antenna with scape yellow, remainder of antenna black except yellowish on venter of pedicel and very base of flagellum; mandible yellow with black tip. Mesosoma black with tegula yellowish brown (Fig. 11A), following yellow: pronotal collar, pronotal lobe, scutellum except scutellar groove, metanotum. Fore and mid legs with coxae black, remainder yellow except ends of tarsi brownish; hind leg black except basal half to two-thirds of tibia white, tarsus brown. Metasoma black with T2 and S2 black or with more or less well demarcated, large, lateral red spots.
Distribution. China (*Hong Kong, Taiwan); Singapore; Indonesia (Sumatra, Java, Sumba, Pulau Buru, Kangean Islands, East Kalimantan), Malaysia (Sabah), Philippines (Luzon). ( van Lith 1962van Lith , 1969van Lith , 1976. Notes. Psenulus carinifrons rohweri is readily distinguished from others Psenulus in the Hong Kong SAR by its mostly black coloration. The Hong Kong specimens are here assigned to P. carinifrons rohweri which they resemble in sculpture of the propodeum and mesoscutum, as well as the entirely yellow fore and mid trochanters and femora ( van Lith 1969). However, they do differ from previous descriptions of this subspecies in the occasional presence of a pair of large red spots on the T2 of the female, a character that appears variable in the Hong Kong population.
Psenulus carinifrons has an extensive distribution extending between India, Japan and north-eastern Australia ( van Lith 1962van Lith , 1969. It may be notable that this species has to date only been collected in the Hong Kong SAR among mangroves; apart from one single "older" (2006) record in a terrestrial habitat at Ping Shan Chai. Unfortunately, there appears to be no record of its habitat preferences in other parts of its range, although all records attributed to van Lith are from littoral localities; save for one in Bogor, which are commonly colonised by mangroves in Southeast Asia. It has a long activity period spanning from March (W13) to end of November (W47). van Lith, 1962 Figs 4B-17B, 18C, D, 19C, D, 20B, 21B Psenulus continentis van Lith 1962: 97-98   . Males, head in dorsal view 9A P. carinifrons rohweri 9B P. continentis 9C P. ephippius sp. nov., paratype 9D P. gibbus sp. nov.; paratype 9E P. maculatus maculatus 9F P. pallens sp. nov., paratype. narrow, not broadened dorsally. Mesosoma mostly polished, mesoscutum moderately punctate with intermixed larger and smaller punctures mostly separated by at least their own diameter (Fig. 11B); prescutal sutures short, not extending beyond anterior level of tegula; episternal sulcus broadly foveolate (Fig. 13B); propodeal enclosure with several pairs of longitudinal carinae; propodeum laterally and posteriorly coarsely reticulate (Fig. 15B). Petiole subcylindrical, T1 relatively low (Fig. 5B). Fore wing ( Fig.  19D) with first recurrent vein interstitial or reaching second submarginal cell near base, second recurrent vein reaching third submarginal cell near the base.

Psenulus continentis
Head black with appressed silvery pubescence (Figs 7B, 9B); antenna with scape, venter of pedicel and base of venter of flagellum yellow, dorsum of pedicel and remainder of flagellum blackish; mandible yellow with black tip. Mesosoma black with following areas yellow (Figs 5B, 11B, 13B): pronotal collar, pronotal lobe, mesoscutum with two broad yellow submedial longitudinal stripes reaching almost to posterior margin ( Fig. 11B), as well as broad marginal stripe laterally; scutellum except scutellar groove and small triangular median spot posteriorly; metanotum; mesopleuron anterior to anterior oblique suture, as well as small spot at top of hypoepimeral area and larger patch behind junction of anterior oblique suture and epicnemial crest; large paired patches laterally and posteriorly on propodeum. Legs mostly yellow, hind trochanter and base of hind coxa dark brown, hind femur, tibia and tarsus reddish. Petiole basally yellow, distally black; remainder of metasoma mostly reddish, S7 dark brown (Fig. 5B).
Female: Clypeus ( Fig. 21B) with two blunt ventral teeth separated by a shallow rounded emargination, remainder of clypeal margin largely straight. Subantennal carina absent (Fig. 6B); interantennal carina narrow, not broadened dorsally. Mesosoma mostly polished, mesoscutum moderately punctate with intermixed larger and smaller punctures mostly separated by at least their own diameter (Fig. 11B); prescutal sutures short, not extending beyond anterior level of tegula; episternal sulcus narrowly foveolate (Fig. 12B); propodeal enclosure with several pairs of longitudinal carinae; propodeum laterally and posteriorly coarsely reticulate (Fig. 15B). Hind tibia with cluster of small dark denticles dorsobasally (Fig. 18C) and transverse row of three short spines dorsoapically. T1 swollen, nearly twice as high above dorsum of petiole as maximum height of petiole (Fig. 4B). Fore wing (Fig. 19D) with first recurrent vein interstitial or reaching second submarginal cell near base, second recurrent vein reaching third submarginal cell near the base.
Head black with appressed silvery pubescence (Figs 7B, 9B). Antenna paler than male, pedicel and flagellum mostly brown except dorsum of flagellum blackish. Mesosoma yellow with following black (Figs 4B, 10B, 12B): lower part of pronotum, three longitudinal spots anteriorly on mesoscutum, extending about two-thirds of length, low triangular patch on posterior margin of mesoscutum; scutellar groove and lateral pocket of scutellum; epicnemial area, episternal sulcus and around posterior end of mesopleural scrobe; mesoscutum with spot roundly extending onto base of mesopleuron; metapleuron dorsal enclosed area and median posterior groove of propodeum. Legs yellow except hind femur to tarsus reddish. Petiole basally yellow, distally black; remainder of metasoma mostly reddish, S7 dark brown (Fig. 5B).
Distribution. *China (Hong Kong); Malaysia (Penang); Singapore ( van Lith 1962van Lith , 1976. Notes. Psenulus continentis was described by van Lith (1962) from a single female specimen and the male had not previously been described. Males here assigned to P. continentis are identified on the basis of their association with females of the species as well as their close similarity to males of the related species P. interstitialis. van Lith (1962) did not specify his reasons for regarding P. continentis as a distinct species from P. interstitialis but comparison of the descriptions of the two species indicates that P. continentis has the mesopleural suture foveolate (vs. simple in P. interstitialis) and numerous longitudinal carinae in the propodeal enclosure (vs 'only a few').
The discovery of P. continentis in Hong Kong represents a new taxon for the territory and China and a significant extension of the known range of this species previously recorded only from the Malaysian Peninsula ( van Lith 1962van Lith , 1976. It is a common taxon, found at 12 of the 20 sites surveyed and it is abundant throughout these sites. It has one of the longest activity period of all Hong Kong Psenulus, spanning from February (W7) to end of November (W47).  (Figs 16C, 17C), this species comes closest to the Psenulus crabroniformis species group as defined by van Lith (1962). However, the species assigned to that group, P. crabroniformis (Smith, 1858) of western Indonesia and P. philippinensis (Rohwer, 1921) of the Philippines, have a well-developed subantennal carina in the female (in contrast to the notably weak carina in the current species), and the first recurrent vein is received by the first submarginal cell as opposed to being interstitial to first and second submarginal cell. Psenulus crabroniformis further differs in having the antennal scape yellow whereas P. philippinensis possesses a pair of well-developed frontal tubercles ( van Lith 1962), both absent in P. ephippius sp. nov. The new species also resembles P. decipiens van Lith, 1976 in overall coloration and lack of antennal tyloids in the male but the latter species has longer prescutal sutures, as long as the scutum, and the position of the first recurrent vein is as in P. crabroniformis and P. philippinensis.  Description. Male: Antenna without tyloids. Clypeus (Fig. 21C) with two blunt ventral teeth separated by a rounded emargination, remainder of clypeal margin more or less straight on the first two-fifths and terminated by a flattened lobe on the last Figure 10. Females, mesosoma in dorsal view 10A P. carinifrons rohweri 10B P. continentis 10C P. ephippius sp. nov., holotype 10D P. gibbus sp. nov.; holotype 10E P. maculatus maculatus 10F P. pallens sp. nov., holotype.
Head entirely black with appressed silvery pubescence (Figs 7C, 9C); antenna with scape dark brown, venter of pedicel and venter of flagellum of a lighter brown, dorsum of pedicel and remainder of flagellum dark brown; mandible black on the apical third and basal sixth, the remainder yellow. Mesosoma entirely black (Fig. 5C). Femurs, trochanters and coxae black, tibiae and tarsi light brown/yellow. Petiole mostly black, distally red/orange; remainder of gaster mostly reddish but T2-T5 and S3-S7 with large dark brown/black areas (Fig. 5C).
Female: Clypeus ventrally (Fig. 20C) with two blunt/rounded teeth, separated by a deep rounded emargination, the remainder of the clypeal margin forming a shallow concavity extending across half of margin and terminated on last third with a blunt lobe intersecting eye margin; dorsal margin flattened on central third of transverse length of clypeus, the remainder reaching ventral margin along a shallow convex line. Subantennal carina weak, extending about one-third of distance between interantennal carina and eye; interantennal carina narrow, not broadened dorsally. Mesoscutum moderately punctate with small punctures separated by at least their own diameter (Fig. 11C); prescutal sutures short, not extending beyond anterior level of tegula; episternal sulcus and mesopleural suture narrowly foveolate along entire length (Fig. 12C). Propodeum laterally and posteriorly weakly reticulate, dorsal half polished with very small and largely interspaced punctures (Fig. 14C). Dorsal part of hind tibia, normal, no polished area and no small denticles dorsobasally (Fig. 18E). Petiole subcylindrical; tergum I low (Fig. 5C). Pygidial plate with very faint medial carina extending about half length of T6 (Fig. 16C). Fore wing (Fig. 19F) with first recurrent vein interstitial or reaching second submarginal cell near base, second recurrent vein reaching third submarginal cell at about one-sixth (0.17×) of cell length from base.
Head entirely black with appressed silvery pubescence (Figs 7C, 9C); antenna entirely dark brown/black. Mandible yellow on basal half and then gradually darkening to become black at the apex. Mesosoma entirely black (Fig. 5C). Femurs, trochanters and coxae black, tibiae and tarsi light brown/yellow except hind tibia dark brown/ black. Petiole mostly black, distally red/orange; remainder of metasoma mostly reddish but T2-T5 and S3-S7 with large dark brown/black areas (Fig. 5C).
Etymology. Species named as a toponomy in reference to the mountain facing the type locality, Ma On Shan or Horse Saddle Mountain in Chinese (ephippium = saddle in Latin).
Notes. This species has only been recorded so far from one site in the SAR, at Mang Kung Wo which presents the highest anthropogenic disturbance of all our sampled sites. It remains uncommon, having only been recorded from five specimens since the beginning of continuous sampling at this site in March 2018. This species has apparently the shortest activity period of all Hong Kong Psenulus and is recorded from end of March (W13) to September (W37).
Etymology. Specific name derived from the prominent hump [= gibbus in Latin] formed by the interantennal ridge, extending below the antennal sockets and ending at middle of toruli.
Notes. By its marked differences with the other known Psenulus of similar coloration as detailed in the diagnosis section, we have concluded that this is in fact a new species to science.
The first collection of this species occurred in 2019 and thus far the distribution of this species is limited to a single location in Mang Kung Wo, which is the sampled site with the highest anthropogenic disturbance. In March 2020 a series of seven specimens (four males and three females) were collected by hand net. Nonetheless, this species is one of the least common Psenulus encountered in Hong Kong. With the limitations presented above, knowledge about its activity period is limited to March (W11-W13) and April (W15-W17). van Lith, 1962 Figs 4E-17E, 18I, J, 19I, J, 20E, 21E Psenulus maculatus maculatus van Lith 1962: 61-62, figs 69-72   Description. Male: Antenna with small tyloids present on F7 to F11. Clypeus (Fig.  21E) with two acute ventral teeth separated by rounded emargination, remainder of clypeal margin evenly rounded. Subantennal carina well developed (Fig. 7E); interantennal carina narrow, not broadened dorsally, Mesosoma mostly polished, mesoscutum moderately punctate with punctures separated by about their own diameter (Fig. 11E), remainder of mesosoma more sparsely punctate; prescutal sutures short, ending about anterior level of tegula; episternal sulcus broadly foveolate (Fig. 13E); propodeal enclosure with few pairs of longitudinal carinae; remainder of propodeum coarsely reticulate Figure 15. Males, propodeum in dorsal view 15A P. carinifrons rohweri 15B P. continentis 15C P. ephippius sp. nov., para type 15D P. gibbus sp. nov.; paratype 15E P. maculatus maculatus 15F P. pallens sp. nov., paratype.
Head black with appressed silvery pubescence (Figs 7E, 9E); antenna with most of scape, venter of pedicel and venter of flagellum basally yellow, dorsoapical spot on scape, dorsum of pedicel and remainder of flagellum blackish; mandible yellow with black tip. Mesosoma mostly black, the following yellow (Figs 5E,11E,13E): pronotal collar, pronotal lobe; mesoscutum with two longitudinal submedian stripes inside prescutal sutures, broadening posteriorly, ending prior to posterior margin of mesoscutum, often narrowed or absent in anterior half of mesoscutum, long posterolateral spot also present on mesoscutum; tegula; large spot on axilla; broad anteromedian spot on scutellum, as well as narrow stripe along posterior margin laterally behind wing insertion; metanotum; mesopleuron with two spots dorsally anterior and posterior to oblique mesopleural suture; propodeum with paired spots posteriodorsally and laterally, larger than in male, may be merged into single posterior patch. Legs mostly yellow, coxae black basally; fore and mid femora posteriorly brown; hind trochanter and dorsum of hind femur brown; hind tibia and tarsus reddish. S1 yellow at very base of petiole, otherwise blackish; T1 dark brown mediobasally; remainder of metasoma mostly reddish except apex of metasoma dark brown.
Notes. The coloration of the mesoscutum has previously been described as having two submedian triangular marks in the posterior half but specimens from Hong Kong vary from as described to having longitudinal yellow stripes extending most of the length of the mesoscutum. Males of this species can readily be distinguished from other Psenulus in the Hong Kong SAR by the presence of tyloids on the underside of the distal antennal segments; females can be distinguished by their prominent subantennal carina and an array of long red spines dorsobasally on the hind femur. Psenulus maculatus also differs from other Hong Kong Psenulus in having the fore and mid femora partially brown instead of entirely yellow.
Psenulus maculatus maculatus was previously known from the Malaysian Peninsula with other subspecies known from Java and Sri Lanka ( van Lith 1962van Lith , 1978. The discovery of this species in Hong Kong is a new record for China and also extends notably the known distribution of the taxon. It is the least common of all species and its occurrence confirmed at only three of the 20 sites sampled and never very abundant at any of these sites. Its activity period is the second shortest (after P. ephippius) and spans from March (W11) to September (W38).

Psenulus pallens sp. nov.
http://zoobank.org/08453112-8D76-481C-B958-02AEC71AD76E Figs 4F-17F, 18K, L, 19K, L, 20F, 21F, 22A-D Diagnosis. Psenulus pallens sp. nov. is readily distinguishable from most other species of its genus by its remarkably light coloration with black patches restricted as described below (Figs 4F,5F,. From other similarly light-coloured species, it differs from P. lamprus van Lith, 1972 of Sulawesi by its less reticulated propodeum with pol-ished area dorsally (Figs 14F, 15F); in the female of the latter (the male is unknown), the back and most of the sides of the propodeum are densely and coarsely reticulate. The female of P. pallens also differs from P. lamprus in its cluster of denticles on the apex of hind tibia (Fig. 18K) as opposed to three reddish spines in the latter. It shows also considerable difference in the shape of the ventral clypeal margin (Figs 20F,21F) with that of similarly coloured P. lamprus and P. interstitialis, the latter two with small teeth, reduced to denticles and no emargination in-between. Psenulus impressus van Lith, 1976 of Luzon has the episternal sulcus foveolate. Psenulus singularis van Lith, 1962 also from Luzon has a distinctly shaped clypeus with the median section of the ventral margin bluntly protruding. Description. Male: Antenna without tyloids. Clypeus (Fig. 21F) with two welldeveloped ventral teeth separated by triangular emargination, remainder of clypeal margin straight. Subantennal carina well developed; interantennal carina narrow, not broadened dorsally (Fig. 7F). Mesosoma mostly polished, mesoscutum lightly and minutely punctate (Fig. 11F), punctures very sparse elsewhere on mesosoma; prescutal sutures short, extending shortly beyond anterior level of tegula; episternal sulcus simple (Fig. 13F); propodeal enclosure with several pairs of longitudinal carinae; rear of propodeum (Fig. 15F) dorsally smooth, ventrally and laterally coarsely reticulate with reticulations encroaching on posterior part of side of propodeum. Petiole subcylindrical, T1 relatively low (Fig. 5F). Fore wing (Fig. 19L) with first recurrent vein more or less interstitial with first and second submarginal cells, second recurrent vein meeting third submarginal cell at about one-eighth of cell length from base.
Head black with silvery appressed pubescence (Figs 7F, 9F); antenna with scape, pedicel and base of flagellum ventrally yellow, dorsum and terminal segments of flagellum black; mandible yellow with black tip. Mesosoma and legs mostly yellow, hind legs reddish from trochanter to tarsus, the following black: three small spots on mesoscutum, along posterior margin of mesoscutum and scutellar groove; mesosternum, encroaching onto base of mesopleuron; propodeal enclosure posteriorly. Petiole basally yellow, becoming reddish apically; remainder of metasoma reddish.
Head black with silvery appressed pubescence (Figs 7F, 9F); antenna with scape, pedicel and base of flagellum ventrally yellow, dorsum and terminal segments of flagellum black; mandible yellow with black tip. Mesosoma and legs mostly yellow, hind legs reddish from trochanter to tarsus, amount of black coloration on mesosoma varying from almost entirely absent with only black marks on scutellar groove and mesosternum (including base of mesopleuron) remaining to slightly more extensive than in male with black marks also present anteroventrally on epicnemium, on lower half of metapleuron, and over entire propodeal enclosure. Petiole basally yellow, becoming reddish apically; remainder of metasoma reddish.
Etymology. From the Latin for 'pale', in reference to this species' distinctive coloration. Notes. Because of its marked differences with other known lightly coloured Psenulus we have concluded that this is a new species. We have collected females that show a great deal of discoloration and are even paler than the holotype, with hardly any black at all, save for a reduced band at intersection of the mesoscutum and scutum (Fig. 22B) and the head (Fig. 22A, B). This species has been consistently collected at Mang Kung, the site with the highest anthropogenic disturbance, and was also found in mangroves (13.6% occurrence). It has the second longest (after P. continentis) recorded activity period, spanning from March (W10) to December (W50). Van Lith (1962) regarded Psenulus as rare overall, noting the low number of specimens available for most Oriental species. In contrast, Psenulus species (at least P. carinifrons and P. continentis) were both commonly and abundantly collected in the Hong Kong SAR during the present study. Psenulus are apparently more cryptic and rarely seen rather than truly rare and may show a preference for particular habitats. Unfortunately, past descriptions of Oriental Psenulus species have not recorded the circumstances in which specimens were collected other than location, making it difficult to evaluate this possibility across the realm as a whole at this time. Nevertheless, it is notable that nests of Psenulus interstitialis studied by Matthews (2001) were found in bamboo stems in a coastal location and we also found Psenulus to be abundant in coastal mangroves. These habitats often comprise reeds and other pithy plants that might; along with the pithy core of some mangrove species, be expected to provide abundant nesting sites of the type favoured by Psenulus (Bohart and Menke 1976).

Discussion and conclusion
The phenology patterns extracted from our collection data ( Fig. 2A) suggest that all species in Hong Kong have; generally speaking, a long activity period of six months or more, indicating that both nesting sites and larval food resources are available throughout the period. On the other hand, it could be hypothesised that the apparent shorter activity periods of P. ephippius sp. nov., and to a lesser extent of P. maculatus maculatus, is the result of either nesting site or larval food resource shortage/depletion.
In terms of abundance; as shown on figures 2B and 2C, we observe that P. continentis is the most abundant (66 occurrences) species at the inland sites sampled and was recorded for a long continuous period of time ( Fig. 2A, B), with a peak of abundance observed between April and June. On the other hand, P. ephippius presents the second lowest abundance (six occurrences) (Fig. 2B) and had a rather discontinuous activity period ( Fig. 2A). This taxon has only been collected recently from Mang Kung Wo and this discontinuity might be the result of interannual variations. P. pallens, although recorded only recently (Sep. 2018), shows the second highest abundance (23 occurrences) with a peak of abundance early in the year (March) (Fig. 2C) while having a long activity period extending from March to December ( Fig. 2A). P. maculatus was recorded more or less continuously throughout its activity period ( Fig. 2A) and was relatively abundant (17 occurrences) (Fig. 2C). The pattern observed for P. gibbus is less clear due to the recent collection of this species (since March 2019) and the relative low abundance recorded (three occurrences) (Fig. 2B) but is the likely result of interannual variations. Given the known development time of many species in the "Crabronidae" (sensu Pulawski 2020; Krombein 1967;Tsuneki 1970Tsuneki , 1973Barthélémy 2012), we can hypothesise that all Psenulus species in Hong Kong are at minimum bivoltine, some likely multivoltine with 3-4 generations per year. A study focused on Psenulus nesting habits would represent a valuable contribution to answer these questions regarding activity patterns and voltinism.
Biogeographically, Psenulus carinifrons rohweri was the most commonly collected species at mangrove sites, being present in 15 out of 17 sites sampled, and for seven sites was the only Psenulus species collected. In contrast, this taxon was never caught in any of the inland sites, save for one occurrence at site 18 in 2006, suggesting that this species may be specialised to use mangrove or coastal habitats. Further work in other regions of the Oriental realm could confirm this rather specialized ecology, underlying the importance for the conservation of coastal habitats.
While sampling effort needs to be accounted for, it should be noted that the maximum richness of Psenulus was recorded at the site with the highest anthropogenic disturbance (site 20: Mang Kung Wo) with five species recorded (all known species except P. carinifrons rohweri). On the other hand, the lowest richness was recorded at the site with the lowest apparent anthropogenic disturbance (site 18: Ping Chan Chai) with only two of the most common species, P. continentis and P. c. rohweri, collected. Although anecdotic, this might indicate that some Psenulus species could be favoured by disturbance (to a certain extent); and if confirmed, the underlining ecological processes sustaining this should be investigated further. In mangroves sites, a total of four species (sites 3 and 8 with the highest richness of three species per site) have been recorded; indicating the importance of these habitats in Hong Kong for Psenulus species, as previously shown for other insect families (e.g. Grootaert et al. 2019). It should be noted, however, that an important variability in the species richness observed among sites could be observed, with a single species (either P. c. rohweri or P. continentis) collected in 47% of the mangrove sites sampled (8 out of 17).
Interestingly, both P. continentis and P. maculatus represent species for which knowledge of distribution range is greatly expanded in this study. Overall, biogeographic information on Oriental Psenulus species is limited and here the geographic disjunction observed in both species is clearly illustrated (Fig. 1). Two hypotheses can be proposed to explain this remarkable biogeographic pattern. Firstly, one may consider this disjunction as a result of the poor collecting records and the paucity of works on the genus in the region, and it should be noted that the Hong Kong fauna contains essentially tropical taxa reaching their northern limit, with larger distribution known in the south either in Indonesia (P. carinifrons) or Singapore (P. continentis and P. maculatus). Furthermore, in view of the large distributional range of P. carinifrons rohweri (from Flores to Taiwan), some of these taxa present naturally a widespread range associated to good dispersal abilities. Secondly, and, alternatively, it could also be hypothesized that these two species have been introduced within Hong Kong. Indeed, Hong Kong is known as a major gateway for exotic species (Lu et al. 2018) with several examples illustrated in other Hymenoptera families (Tang et al. 2019). Interestingly, limited geographical overlap has been observed in the species described in the late 1960's and 1970's by van Lith (1969van Lith ( , 1972van Lith ( , 1976 from Vietnam, Laos and Thailand. Indeed, out of the 26 species reported from these countries, nearly 60% (57.7%) are unique to either country, potentially reflecting a high level of endemism in the genus, whereas none of these Indochinese species have been recorded from Hong Kong or China.
Future studies on the distribution of Psenulus species within Asia could thus provide valuable information about the native or potential introduced range of these species and elucidate these seemingly opposite hypotheses regarding the origin of these populations. Ideally, population genetic studies could provide important support but the apparent rarity and/or cryptic nature of some of these species in the regions in which they have been described might limit such studies.