A new species of Corethrella Coquillett (Diptera: Corethrellidae) from Death Valley, California, United States of America

Abstract

A new species of frog-biting midge in the genus Corethrella Coquillett from Death Valley National Park, California, United States of America, is described, based on a single female. The species belongs to the rotunda species group, which are otherwise known to have hyporheic larvae. The species is placed phylogenetically within the group, and a modified portion of a Nearctic key is presented. The rotunda species group now contains 13 species, three of which are in the Nearctic, and the remaining 10 of which are in the Neotropical region.

Introduction

The immatures of many groups of holometabolous insects are incompletely known but often with the assumption that those not yet discovered will be in similar habitats to those that have been already described. The Culicoidea includes the families Dixidae, Corethrellidae, Chaoboridae, and Culicidae, with a total of more than 3800 species (Borkent 2012). Their immatures are all aquatic and generally restricted to lentic habitats, where the strong majority of larvae use their siphons to periodically breath at the water surface (the highly modified Chaoborus Lichtenstein is the only taxon with apneustic species). Some Dixidae and Culicidae are found in the backwaters or otherwise calm microhabitats of streams. Within Chaoboridae, some genera have become planktonic (e.g., Mochlonyx Loew, Chaoborus), the only insects to have done so. Of the 125 known species of Corethrellidae (Borkent 2014), all within the single, nearly worldwide genus Corethrella Coquillett, 40 are known as larvae or pupae (sometimes not the same species), and virtually all of these occur in ground pools and various phytotelmata or rarely in the hyporheic zone of streams (the region of the streambed below the freely running water above). Of the four families of Culicoidea, most have been collected by researchers working on mosquitoes, largely because the four families were considered at one time to belong to only Culicidae (see Belkin publications in Borkent 2008). North America has been particularly intensely sampled.

Of the eight Nearctic Corethrella species (one unnamed) revised by Borkent (2008), the larvae and pupae were known for six species, with some confusion distinguishing C. brakeleyi (Coquillett) and C. condita Borkent and an unnamed species from the Florida Keys (Florida, United States of America) recorded only as a single female. Boulton et al. (1992) reported the first remarkable records of Corethrella larvae (of an unnamed species) collected in the hyporheic zone of small streams in Arizona, United States of America. Additional specimens from the hyporheic zone were subsequently collected from Barton Creek, Travis Co., Texas, United States of America, in 1998 by A. Leopold (Borkent 2008). This was a habitat previously unknown for the Culicoidea, but these larvae were neither described nor reared. Borkent (2008) suggested that at least one larval form was that of C. kerrvillensis (Stone), known only from Texas and in the rotunda species group. Dorff et al. (2022) described a new species, C. kipferi Dorff et al., from the hyporheic zone in Missouri, United States of America, based on larvae and adult females and provided evidence that further species were yet present in North America. This species also belongs to the rotunda species group of Corethrella (Borkent 2008), and it now appears that the group is more diverse than previously suspected. Here, I describe a new species from Death Valley, California, United States of America, based on an adult female and whose larvae is not known but also likely from the hyporheic zone.

Materials and methods

Only a single female adult was collected, with a sweep net, from the type locality. The specimen was first examined and photographed in ethanol and subsequently slide-mounted, as described in Borkent and Spinelli (2007). Terms for structures follow those used in Borkent (2017). The specimen was examined, studied, measured, and photographed using a Wild M3 dissecting microscope (Leica, Wetzlar, Germany) and a Zeiss Jenaval compound microscope (Zeiss, Oberkochen, Germany). Stacked images were generated using Zerene Stacker, version 1.04 (Zerene Systems; https://zerenesystems.com/cms/stacker).

The acronym and details for the museum housing the holotype are as follows:

CNCI – Canadian National Collection of Insects, Agriculture and Agri-Food Canada, K.W. Neatby Building, Ottawa, Ontario, K1A 0C6, Canada.

Results

Corethrella (Corethrella) galba Borkent, new species

(Figs. 1A–B, 2A–F)

Figure 1. A, Habitus of female C. galba in lateral view; B, antennal flagellomeres 1–8.

Figure 2. Structures of female C. galba: A, head capsule in anterior view; B, right maxillary palpus in anterior view; C, foreleg; D, midleg; E, hind leg; F, wing.

ZooBank Registration number: urn:lsid:zoobank.org:act:B435F33D-63CF-4108-A5D2-45A2DC20C763

Type material

Holotype, female adult on microscope slide, labelled “HOLOTYPE Corethrella galba Borkent ♀,” “Wildrose Cmpgrd, Death Valley NP, CA, 36° 15' 56.31" N 117° 11' 37.69" W, 10-IV-2022, A. Borkent CD2968” (CNCI).

Diagnosis

Adult male: Unknown. Adult female: Only extant species of Corethrella in the New World with nearly uniformly light yellowish-brown pigmentation (only some slight darkening near wing base, slight dark patch on katepisternum). Immatures: unknown.

Description

Adult male: unknown.

Adult female (n = 1): Overall pigmentation uniform light yellow (Fig. 1A). Head: Outline in anterior view nearly circular (Fig. 2A). Coronal suture extending to the dorsal margin of the innermost ommatidia (Fig. 2A). One large seta on each side of frons between ventromedial area of ommatidia (Fig. 2A). Antennal pedicel with slightly developed more elongate, stout, dorsal, or dorsolateral setae; flagellomeres 1–8 (Fig. 1B; others missing), sensilla coeloconica not visible due to damage. Clypeus (Fig. 2A) squarish. Mandible with small, pointed teeth. Palpus segment 3 (Fig. 2B) swollen apically, 3.7 times longer than wide. Thorax (Fig. 1A): Posterior portion of dorsocentral row with single elongate setae situated somewhat lateral to one another. Prescutal suture elongate. Anterior anepisternum divided diagonally by sinuous suture, dorsal portion about equal to ventral portion. Ventral portion of posterior anepisternum not differentiated from dorsal portion, anterior margin slightly more darkly pigmented than posterior margin. Without (?, visibility poor) posterior anepisternal setae, 5 anepimeral setae. Wing (Fig. 2F): length = 1.37 mm. Ratio of R₁/wing length = 0.65. Apex of R₂ basal to apex of M₁. Plain, without pattern of pigmented veins and/or scales; veins (other than wing margin) with only setae. Halter slightly lighter than scutellum. Legs (Figs. 2C–E): uniformly pigmented, other than base of hind femur slightly lighter. With only slender setae, lacking scales (except for some in patch of whip-like setae on posterior portion of hind tibia). Apices of fore- and mid-leg fifth tarsomeres undivided, with claws slightly subapical to apical. Claws of each leg equal to those of others; equal on each leg, simple (without inner teeth). Empodia elongate, of intermediate thickness, with five branches along length, apical portion with number of bifurcations (similar to Amaral et al. 2023, Fig. 12C). Abdomen (Fig. 1A): Uniformly yellowish brown.

Immatures: Unknown.

Distribution

Corethrella galba is known only from the type locality at Wildrose Campground in Death Valley National Park, California, at an altitude of 1228 m. Specifically, the specimen was swept from shrubs in riparian habitat bordering a small stream originating just southwest of the junction of Emigrant Canyon and Wildrose roads, just west of Wildrose Campground.

Taxonomic discussion

Adult female Corethrella galba key to couplet 32 in Borkent’s (2008) key to New World species, where it can be distinguished from others in that couplet by its very lightly and nearly uniformly pigmented katepisternum (Fig. 1A). It keys out to couplet 37 in Amaral et al.’s (2023) key to New World species, where its nearly uniformly pigmented thorax distinguishes it from subsequent taxa. Within the Nearctic, the adult female (and likely the unknown male) of C. galba will key out to couplet 2 in Borkent’s (2008) Nearctic key, which can be replaced by the following, also incorporating for the first time the recently described C. kipferi (Dorff et al. 2022).

1. 1. Palpal segment three with a uniform diameter (see Borkent 2008, Fig. 33V); thorax nearly uniformly dark brown (see Borkent 2008, Fig. 42D)…species near peruviana (see Borkent 2008) (Key Largo, Florida)

2. –. Palpal segment 3 apically swollen (Fig. 2B); thorax nearly uniformly pigmented or with some notable pigmentation)…2

3. 2. Thorax generally uniformly yellowish brown throughout with a few faint markings (Fig. 1A)……C. galba (California)

4. –. Thorax with notable dark pigmentation, at least on the scutum and katepisternum …… 3

5. 3. Thorax generally pale with patches of darker pigmentation on the scutum (anterior and lateral) and the dorsal portion of katepisternum (see Borkent 2008, Fig. 40D)… C. kerrvillensis (Texas)

6. –. Thorax generally dark with pale sclerites at the base of the wing (see Dorff et al. 2022, Fig. 4)…C. kipferi (Missouri)

The phylogenetic relationships of the species in the rotunda species group were presented by Borkent (2008) and confirmed by Baranov et al. (2016), who organised the synapomorphies provided by Borkent (2008) into a matrix and analysed these with maximum parsimony and Bayesian approaches. The character numbers that follow are those presented and discussed in Borkent (2008; Fig. 4).

Four species have been described in the rotunda group since Borkent (2008), and these are generally congruent with the phylogeny presented by Borkent (2008). The only possible exception is C. yucuman Caldart and Pinho, described by Caldart et al. (2016) but not placed phylogenetically and partially redescribed by Amaral et al. (2023). Based on Caldart et al.’s (2016) description, this species has synapomorphies 25 and 31–34 and the plesiomorphic condition for 35–38, and therefore placing it as part of a polytomy with C. grandipalpis Borkent, C. kipferi, and (C. kerrvillensis, (C. remiantennalis Borkent + C. blandafemur Borkent) and (C. brevivena Borkent + C. globosa Borkent)). However, Caldart et al. (2016) reported that the male of the species had a wing with R₂ shorter than the stem of R₂₊₃. In addition, the wing of the male of C. yucuman appears to have CuP not extending to the wing margin, although this is questionable (Caldart et al. 2016, Fig. 3G). These two features were previously considered to be autapomorphies of C. brevivena (Borkent 2008; CuP previously considered to be A₁). In C. brevivena, it is both the male and female that exhibit these derived features; this suggests that the short R₂ evolved first in the male was shared thereafter by C. brevivena and C. yucuman and then subsequently was acquired by the female of just C. brevivena. Although Caldart et al. (2016) described the wings as having very narrow scales, Amaral et al. (2023) reinterpreted this character as setae present on the wing veins (character 35). Corethrella yucuman and C. brevivena are the only known species with a katepisternal seta (Amaral et al. 2023), previously considered an autapomorphy of C. brevivena (Borkent 2008) and hence a synapomorphy here. Along with the particularly short R₂, this character indicates C. yucuman and C. brevivena are sister species, which is how C. yucuman is placed here. If accurately placed, C. yucuman is missing the two female synapomorphies 37 and 38, interpreted here as reversals (with the possibility of the sexes being misassociated).

Figure 3. Type locality at Wildrose Campground in Death Valley National Park, California, United States of America: A, overview of riparian habitat; B, area with flowing water.

Corethrella xokleng Amaral et al. was described by Amaral et al. (2019), who suggested it may be the sister to C. remiantennalis, C. blandafemur, C. brevivena, and C. globosa, but their evidence showed it to form part of a polytomy, including C. kerrvillensis, C. galba, (C. remiantennalis + C. blandafemur), and (C. brevivena + C. globosa).

Dorff et al. (2022) placed their new species, C. kipferi, in this phylogeny but, partially because the male was unknown, could provide no further resolution other than to place it as one lineage of a polytomy with each of C. kerrvillensis, (C. remiantennalis + C. blandafemur), and (C. brevivena + C. globosa). However, C. kipferi has the plesiomorphic state of character 35 (in Borkent 2008) and actually forms a polytomy with C. grandipalpis, C. yucuman, and (C. kerrvillensis, (C. remiantennalis + C. blandafemur) and (C. brevivena + C. globosa)), the original misplacement being my error in that paper.

Most recently, Corethrella pindorama Amaral and Pinho was described by Amaral et al. (2023) and placed as part of a polytomy with C. rotunda Borkent and a group of nine members of the rotunda species group (including C. yucuman and C. xokleng; and now also including C. galba).

Corethrella galba is known only as a female and therefore is missing the interpretation of the male features of synapomorphies 32 and 36. The presence of synapomorphies 25, 31, and 33–35 and the plesiomorphic condition for characters 37 and 38 results in C. galba being placed as one lineage of a polytomy with each of C. kerrvillensis, C. xokleng, (C. remiantennalis + C. blandafemur), and (C. brevivena + C. globosa). Of these species, C. kerrvillensis is known from the Nearctic and the others are from the Neotropical region.

Because there are no conflicting characters other than the possible loss of character 36 in C. yucuman, a phylogeny presents only the relationships and synapomorphies in Figure 4.

Figure 4. Phylogeny of the species of the rotunda species group. Numbers refer to those character states presented by Borkent (2008), here listed in their apomorphic state: 25 – wing without patterned pigmentation; 31 – thorax with ventral portion of the posterior anepisternum continuous with dorsal portion and with ventral portion with anterior margin more darkly pigmented than posterior portion; 32 – male antennal flagellomere 8 shorter than both flagellomeres 7 and 9; 33 apex of R₂ more basal than apex of M₁; 34 – female coronal suture short, not extending ventrally between ommatidia; 35 – wing veins with setae; 36 – male antennal flagellum with one ventral seta in basal whorl of trichoid sensilla more stout and apically expanded; 37 – female antennal flagellomeres 1–3 each very short, with flagellomere 4 moderately short; 38 – female palpal segment 3 length/ width < 2.3. The asterisk indicates two synapomorphies: male wing with R₂ shorter than the stem of R₂₊₃; katepisternal seta present.

The holotype is only in moderate condition, partially because it was slightly dry by the time it was removed from the aspirator and preserved in ethanol. Further to this, specimens with pale pigmentation, as is true for other species with similar colouration, are challenging to clear and slide mount, and some collapsing of the antennae, palps, and thorax took place during preparation. Although it is true that specimens left in ethanol over time may become pale, this is not the situation for this specimen, which was seen in the field shortly after preservation and prepared shortly after on a microscope slide.

Bionomics

Because only a single female adult was collected, little can be suggested regarding its bionomics. However, the species belongs to the rotunda species group and, considering that at least two species in that group have hyporheic larvae, it is likely that this species has the same. The only aquatic habitat present at the type locality was a small, barely flowing stream (Fig. 3A, B). The remaining 11 species in this group are unknown as immatures, having never been collected by mosquito researchers, who have extensively scoured all conceivable lentic habitats. This furthers the likelihood that all members of this species group have hyporheic larvae.

The female has serrate mandibles, indicating that it is a blood feeder and, as is known in all other Corethrella species to date, likely feeds on male frogs, being attracted by their calls. Six frog and toad species (Anura) are known from Death Valley National Park, but only two are common: the red-spotted toad (Bufo punctatus Baird and Girard) (Bufonidae) and the Baja California treefrog (Pseudacris hypochondriaca Hallowell) (Hyladae), with the others restricted to very specific localities in the park more distant from the type locality (Persons and Nowak 2006, who recorded the latter as the Pacific treefrog). It is unknown which might occur specifically at the type locality, but one or more of these anuran species is the likely host of females of C. galba.

Etymology

The specific epithet, galba (yellow), refers to the overall colour of the female of this species.

Discussion

Of the 13 species now known in the rotunda species group, only C. kipferi is known in the larval stage with certainty (Dorff et al. 2022), although hyporheic larvae collected in Texas were tentatively associated with C. kerrvillensis (Borkent 2008). As such, the larvae of the other species in the rotunda species group likely will also be hyporheic. Dorff et al. (2022) described morphological differences distinguishing the larvae that were known from Arizona (Boulton et al. 1992; Borkent 2008), and these likely represent a further unnamed species. It appears quite probable that the hyporheic zone of streams will produce more undescribed species in the rotunda species group and very likely the immatures of the previously described species that are currently unknown as larvae or pupae.

The species of the rotunda species group are restricted to the New World and represent a relatively early lineage within the genus (Borkent 2008). As Dorff et al. (2022) note, a number of species in other early lineages are known from Borneo (Borkent and Grafe 2012), where the author and Annette Borkent spent many hours searching surface pools and phytotelmata without discovering any larvae other than C. calathicola Edwards, a previously known inhabitant of Nepenthes monkey-cups (Nepenthaceae) that is related to a relatively derived group of Neotropical taxa in phytotelmata. Furthermore, mosquito workers in Borneo have never found Corethrella larvae there. A single larva of C. marksae from Australia, another early lineage, was found in “a small, pebbly back-water of a flowing river.” Immatures of the six named Afrotropical species and an additional six undescribed species from Madagascar, with a number belonging to early lineages (Borkent 2008, 2017; Robert and Borkent 2014), also remain unknown, despite extensive collecting by mosquito workers, who collect only from surface waters on that continent. Of the 11 species known from the Palaearctic and Oriental regions (one further named species is unidentifiable), five are known as larvae, but four of these are of relatively derived lineages (found in surface waters or phytotelmata; Borkent 2008). The only exception is C. solomonis Belkin, from the Solomon Islands, a relatively early lineage that is found in the phytotelmata of leaf axils. Overall, this suggests that many of those species unknown as immatures in early lineages, such as the rotunda species group and a number of others in the Old World, may be present in hyporheic habitats, and this collecting opportunity should be pursued in the future. Furthermore, the presence of early hyporheic lineages in both Corethrellidae and Simuliidae may be an indication that the hyporheic trait is plesiotypic in these two families, even though they are more distantly related to each other within the Culicomorpha (Borkent 2012). It will be interesting to interpret the plesiotypic larval habitat of Chironomidae once their phylogenetic relationships become more firmly established, based on outgroup comparisons. Although the genera Archaeochlus Brundin and Austrochlus Cranston are presently considered as relatively derived lineages (Cranston et al. 2012), the presence of female adult serrate mandibles and functional posterior spiracles of the larvae, plesiomorphic features but unique in the family, seems striking. Larvae of these two genera are associated with seeps and episodic streams, or for one species in Namibia, the “immature stages were found in remnant waters in river drainage channels after atypical above-ground flow” (Cranston et al. 2002).

This is the second species of Corethrella known from Death Valley, with C. aridicola Borkent recorded from Saratoga Springs (also known from Guadalupe Hot Springs in Baja California, Mexico). These records from Death Valley are the only known localities for Corethrella in California.

Considering that the single female was collected with a sweep net and that, generally, females of many species of Corethrella are attracted by frog-call traps, attempting this collecting method may obtain more specimens of this life stage. Six of the current 13 species of the rotunda species group have been now collected with a frog-call trap (Borkent 2008; Caldart et al. 2016; Dorff et al. 2022; Amaral et al. 2023). Collecting the larvae (see Dorff et al. 2022) would potentially allow for rearing the pupal and adult stages – also excellent possibilities to expand our knowledge of this species.