The Unique Cauda-Liked Structure Represents a New Subfamily of Cunaxidae: Description of New Taxa and Discussion on Functional Morphology

Simple Summary In this study, a cauda-like structure found in Cunaxidae is defined, and with it the new taxa, Cunaxicaudinae Chen & Jin subfam. nov., and its two new genera, Cunaxicaudus Chen & Jin gen. nov. (type genus) and Brevicaudus Chen & Jin gen. nov., are erected. It is proposed that the specialized cauda may be the result of the evolution of the sperm transfer mode. Abstract A cauda-like structure was found, firstly in Cunaxidae, and with it the new taxa Cunaxicaudinae Chen & Jin subfam. nov., and its two new genera, Cunaxicaudus Chen & Jin gen. nov. (type genus) and Brevicaudus Chen & Jin gen. nov., were erected. Cunaxicaudinae Chen & Jin subfam. nov. differs from the known members of the family Cunaxidae by the unique conspicuous cauda derived from the posterior end of the hysterosoma. The generic features of Cunaxicaudus Chen & Jin gen. nov. are as follows: the posterior of the hysterosoma elongated as a much longer cauda; palp between genu and tibiotarsus without apophysis; e1 closer to d1 than f1; and e1 closer to mid-line than c1 and d1. The generic features of Brevicaudus Chen & Jin gen. nov. are as follows: the posterior of hysterosoma elongated as a short cauda; palp between genu and tibiotarsus with one apophysis; distance between setae e1 and d1 approximately equal to e1; and f1, e1 as close to mid-line as c1 and d1 to mid-line. It is proposed that the specialized cauda may be the result of the evolution of the sperm transfer mode.

Description (Figures 1-8); male (n = 17). The idiosoma length was 324 (305-364) from the base of subcapitulum to the posterior edge of median shield, and the width was 188 (170-219); the posterior end of the hysterosoma was elongated as a very long cauda (Figures 1, 2 and 4).
Gnathosoma. Palp (Figures 6 and 7A). Five-segmented, 136 (117-144) long, with granulated papillae and terminated with a bifid claw. Palp chaetotaxy was as follows: trochanter without setae; basifemur with one dorsal simple seta; telofemur with one dorsal stout seta and one short and tapering apophysis; genu with two stout setae and two simple setae; and tibiotarsus with one stout seta, three simple setae and one acute solenidion.
Description (Figures 9-13); male (n = 1). The idiosoma length was 276 from the base of the subcapitulum to the posterior edge of the median shield and the width was 189; the posterior end of the hysterosoma was elongated, forming a long cauda (Figures 9 and 10).
Dorsum ( Figure 9). The propodosomal and hysterosomal shields were entirely complemented by reticulations, surrounding integument striate. The propodosomal shield was 81 long, 130 wide, sclerotized and with a reticulated pattern, and had one pair of anterior (at) and one pair of posterior (pt) setose trichobothria and two pairs of tactile setae (lps and mps); at was shorter than pt, and lps was near the pt base; the area anterior to at was covered with papillae. Lengths of the setae and distances between the bases of setae were at 150, pt 182, lps 7, mps 6; at-at 23, pt-pt 103, lps-lps 98, mps-mps 45, lps-mps 34, at-lps 56, pt-mps 28, pt-lps 22, at-mps 65 and at-pt 79.
Dorsum (Figure 9). The propodosomal and hysterosomal shields were entirely complemented by reticulations, surrounding integument striate. The propodosomal shield was 81 long, 130 wide, sclerotized and with a reticulated pattern, and had one pair of anterior (at) and one pair of posterior (pt) setose trichobothria and two pairs of tactile setae Cauda dorsum (Figure 9). From the dorsal view, the posterior end of the idiodoma elongated significantly as a clearly defined long cauda, and gradually narrowed. It consisted of a caudal base with light striae, a caudal petiole with broad transverse plicated striae on the edge, and a smooth caudal xiphoid. The cauda was 226 long, measured from the posterior edge of the median shield to the end: the caudal base was 101, the caudal petiole was 74, and the caudal xiphoid was 51 in length. The anal region was dorsally located on the caudal base with fine dotted papillae, and the dorsal area of the genital region with g3-g4 also had dotted papillae; the anal region had two pairs of pseudanal setae (ps1-ps2), 6 and 10 long, respectively; the length of the setae were h2 7, and lyrifissures (ih) were present.
Venter (Figures 10 and 11). Coxae I-IV were fused, resulting in a clearly whole ventral shield completely covered with dotted papillae; coxae III-IV had an obviously reticulated pattern. One pair of propodogastral setae (ppgs) was close to coxae II, there were two pairs of hysterogastral setae (hgs1-hgs2), 6 and 14 in length, and there was one pair of clear foveolae medially located between the coxae III groups. The area between hysterogastral setae hgs2 had transverse striation. The setal formula of coxal plates I-IV was 3-1-3-3 sts.
Cauda venter (Figures 10 and 11). The cauda was 224 long: the caudal base was 99, including the genital area; the caudal petiole was broad with a plicated striae, 75 long; the caudal xiphoid was smooth and 50 long. The cauda was clearly defined and gradually narrowed, as in its dorsal view. The caudal base had horizontal striation except for the genital region (genital shield), with dotted papillae; the genital region had four pairs of genital setae (g1-g4), of which g3-g4 were dorsally located (Figure 9), and two pairs of visible genital suckers. The lengths of setae g1-g4 were 13, 14, 15 and 14, respectively.
Gnathosoma. Palp ( Figure 12A) were five-segmented, 112 long, with granulated papillae and terminating with a bifid claw. Palp chaetotaxy was as follows: trochanter without setae; basifemur with one dorsal simple seta; telofemur with one dorsal stout seta and one short and tapering apophysis; genu with two stout setae and two simple setae; and tibiotarsus with one stout seta, three simple setae and one acute solenidion.
Legs ( Figure 13). Leg IV was the longest and leg II was the shortest, with the tarsal lobes fairly well-developed; the dorsum of all leg segments had finely granulated papillae. Etymology. The new specific epithet was formed by adding neo-(meaning new) to the name C. mohanensis Chen & Jin sp.nov., indicating its similarity to the latter species.

Brevicaudus Chen & Jin gen. nov.
Type species: Brevicaudus trapezoides Chen & Jin sp. nov. Generic features (male): posterior of hysterosoma elongated with short conspicuous cauda, caudal petiole and caudal xiphoid short (as opposed to long in Cunaxicaudus Chen & Jin gen. nov.); palp between genu and tibiotarsus had one apophysis; the distance between setae e1 and d1 was approximately equal to that between e1 and f1; e1, c1 and d1 were in a longitudinal line; lyrifissures (im) was situated lateral to e1 and f1.
Description (Figures 14-21); male (n = 4).      The idiosoma length was 284 (283-315) from the base of the subcapitulum to the posterior edge of the hysterosomal shield, the width was 195 (190-220), and the posterior end of the hysterosoma was elongated as a short cauda (Figures 14, 15 and 16C).

Discussion
According to Simely [5] and Skvarla et al. [10], the number of segments and length of palp, absence or presence of multi-branched seta on palp III (telofemur), cheliceral seta absent or present and T absent or present on tibiae IV were used as the main diagnostic features for the subfamilies in Cunaxidae. For example: palp with three segments = Counaxoidinae; palp with four segments = Scirulinae; palp with five segments and multi-branched seta present on palp III (telofemur) = Bonzinae; palp with five segments and multi-branched seta absent on palp III (telofemur), and palp extending beyond the subcapitulum by at least the distal half of palp IV(telofemur) = Cunaxinae; palp with five segments, multi-branched seta absent on palp III (telofemur), and palp not extending beyond the subcapitulum by more than the distal half of palp IV (genua), T present on tibiae IV, cheliceral seta usually present = Coleoscirinae; palp with five segments, multi-branched seta absent on palp III (telofemur), and palp not extending beyond the subcapitulum by more than the distal half of palp IV(genua), T absent on tibiae IV, and cheliceral seta absent, seta hg1 geniculate = Orangescirulinae.
Our specimens of the three proposed new species have the following characteristics: palp with five segments, multi-branched seta absent on palp III (telofemur), and palp extending beyond the subcapitulum by at least the distal half of the palp IV (genua), cheliceral seta present and T present on tibiae IV. Moreover, we think that the feature of posterior configuration of hysterosoma can also be used as a main diagnostic feature for the subfamilies in the family Cunaxidae. The posterior end of the idiosoma elongated remarkably to form a cauda-like structure, which is an unusual new trait unknown in the family to date.
There was a tendency for the posterior area to the hysterosomal shield, including the genital region and anal plate (region) in the ventral view, to have the idiosoma elongated rearward in some members of Cunaxidae, in which case the anal pore was located on the end of the idiosoma and the anal plate (region) covered both ventral and dorsal around the pore in both males and females. What is more significant than that is that the end of the idiosoma obviously extended with the exposed aedeagus in males of some species, such as Dactyloscirus humuli (illustrated and mentioned) [26] and Armascirus hastus (illustrated but not mentioned) [27]. However, the completed and fully equipped cauda-like device is primarily defined in the present study.
A similar structure, with cauda and petiole as mating apparatuses [28], presents in some males of the water mite family Arrenuridae, especially those of the subfamily Arrenurinae, which makes the arrenurid mites sexual dimorphic [29]. The Arrenurid cauda is formed by an elongation of the hysterosoma posteriorly to a variable extent, and the petiole, either simple or complicated, and is a device derived from the integument of the idiosoma either caudally or directly from the posterior edge of dorsal shield [30]. The genital field or anal area may be or not on the cauda. Jin et al. [31] hypothesized that the formation and evolution of the arrenurid cauda and petiole was driven by the behavioral evolution of the reproductive mechanism from non-mating (indirect sperm transfer) to mating (direct sperm transfer). The cauda of Cunaxicaudinae Chen & Jin subfam. nov. is highly homologous, but remarkably different from that of the arrenurid mites. In cunaxicaud mites, both the genital area and the anal area were located on the male cauda, which implies that the traits (idiosoma posteriorly elongated with the accessory devices of cunaxicaud and arrenurid) evolved independently in two different branches. Interestingly, the aedeagus was not observed internally in all specimens of Cunaxicaudinae Chen & Jin subfam. nov., and therefore we propose that the caudal xiphoid might be the evaginated aedeagus with the idiosoma extending. Such an aedeagus was also observed in Dactyloscirus condyles [32].
Unfortunately, the females of Cunaxicaudinae Chen & Jin subfam. nov. were not collected. According to the literature, there are certain sexual differences, although they are not regarded as typical sexual dimorphism in the known members of Cunaxidae. These include the number of dorsal shields (females only have one, while males have two) [5,27,33,34], the setal formula of coxal plates (the setal formula of coxal plates I-IV was 3-2-3-3 sts in females, while 3-1-3-3 sts in males) [10], leg chaetotaxy (generally, males have longer bsl than females, especially on genua I-IV, tibia I and tarsi I-II) [35] and the number of genital setae (g) and eugenital setae (eu) (female genital setae were more than male, females had a pair of eugenital setae present, while in males this was absent) [13,16].
Therefore, we could put forward the hypothesis that the cauda of the new taxa may be the result of the evolution of the reproductive mode from indirect sperm transfer to direct sperm transfer; the new taxa may have sexual dimorphism, just like arrenurid mites, with the males having an exceptional body consisting of the mating device and females having a common oval body.

Conclusions
In the present work, a new subfamily, Cunaxicaudinae Chen & Jin subfam. nov., was described and illustrated based on a unique cauda-like structure in male Cunaxicaudinae. Additionally, we defined and discussed its morphology from the functional perspective. The findings highlight the species diversity and morphological evolution trends of the Cunaxidae.