1 Introduction

In Coleoptera, the aedeagus consists externally of the tegmen and the median lobe; and the tegmen comprises of the phallobase (basal piece) and the parameres (lateral lobes) (Sharp and Muir 1912; Lawrence and Britton 1994). Since the phallobase is almost or completely missing in most species of Staphylinidae, the aedeagus consists of the median lobe and paired parameres (Crowson 1981); and in the median lobe, there is internal endophallus. The external and internal structures of aedeagi are highly diverse morphologically, and they are considered to be very useful both for classifying the species and also for classifying the species groups of genera in Staphylinidae; and thus, taxonomists have paid special attention to them when attempting to do the revisional studies of a group in question.

In Staphylinidae, the endophallus comprises of a copulatory tube, sclerites, sclerotized bands, etc.; and the copulatory tube is located inside a membranous reversible internal sac, and the latter connects the rim of the apical foramen (i.e., the ostium) with the base of the copulatory tube. The simple form of the endophallus is made up of a simple reversible internal sac. However, the endophallus very often includes the following three components: (a) paired expulsion hooks that have a function of triggering the copulatory tube, (b) paired longitudinal bands that have a function of reinforcing the membrane of the internal sac, and (c) a copulatory tube. These relatively complex endophalli are found in such genera as Stenus (Naomi 2006a, b; Naomi et al. 2017) and Sepedophilus (Naomi and Maruyama 1997). In some genera, e.g., Scopaeus (Frisch 1997, 1998, 1999), Fenderia (Puthz 2003a), and Aploderus (Shimada 2002), the endophallus is highly complex, because in addition to all or two of those three components, it is also armed with a variety of sclerites, tufts, and other structures. Crowson (1981:70) suggested that in some groups of Coleoptera, the sclerotizations of the endophallus may play a role in shaping a spermatophore, but the function of the endophallic sclerites in Staphylinidae still remains unclear in most cases.

Out of the various structures of the endophallus in Staphylinidae, the copulatory tube can be regarded as the structure that plays an important role in copulation, because it serves as a tube for transmitting the sperm or spermatophore to the vagina or spermatheca. Thus, it is worthy of studying from the morphological point of view. After Sharp and Muir (1912) first studied in detail the basic structures of the aedeagus in Coleoptera (including the Staphylinidae), Blackwelder (1936) and Naomi (1990) each studied the aedeagus of Staphylinidae in a comparative perspective. However, the endophallic copulatory tubes in Staphylinidae have not been so far studied in detail probably due to the three reasons described below.

First, not all staphylinids possess the copulatory tube as a component of the aedeagus. Second, the morphological diversity of the copulatory tubes is very high. For example, in some cases (e.g., some Dianous), it is composed of a very thick stick or rod; and thus, it looks as a different structure. In other cases, it is composed of a very long flexible, whiplike tube called “flagellum.” To make the matter more complex and inextricable, such a whiplike flagellum has evolved parallely many times in different species in Steninae, Euaesthetinae, Aleocharinae, etc. Consequently, the copulatory tubes in Staphylinidae are difficult to homologize. And, third, it is also difficult to understand the homologous relations of the parts of a copulatory tube among the various groups of Staphylinidae.

Here, I present a detailed study of the structures and functions of the endophallic copulatory tube in the family Staphylinidae. First, I discuss the general structures of the copulatory tube and the pseudocopulatory tube, together with the characterizations and terminology. Second, the distribution of the copulatory tubes in Staphylinidae is described and briefly discussed. Third, I describe the structures of the seven basic and some other unique forms of the copulatory tubes in detail in Staphylinidae, paying special attention to those of Steninae. Fourth, I describe or hypothesize the functions of the copulatory tubes in Staphylinidae, paying special attention to those of Steninae.

2 Some Morphological Terms

Some morphological terms are not consistently used; and then the term (e.g., “endophallus”) has several different meanings because each has been previously used differently by different authors. Thus, they are ambiguous and difficult to use in a precise way. Some other terms also are so because some morphological parts (that are designated by those terms) are complex in structure and position. In order for readers to understand the precise meanings of some important morphological terms used in this paper, I discuss here some important details.

Endophallus

Following Nichols (1989: 239), the “endophallus” is described as follows: “In the male insects, the internal sac or tube of the phallus invaginated at the end of the aedeagus,… .” (Note here that the terminal membranous part of the aedeagus in insects has been in general called “vesica,” but Sharp and Muir (1912: 585) called it “internal sac.”) It seems that this characterization is incomplete. This is because, according to this characterization, the endophallus comprises of “internal sac” or “internal tube,” but in many cases, it is composed of “the internal sac and tube.” Namely, there exist within an aedeagus, both the internal tube (which is connected anteriorly with the reversible internal sac) and the internal sac (which is connected anteriorly with the rim of the apical foramen). Thus, I consider here that the endophallus consists of the internal structures of aedeagus containing the copulatory tube, internal sac, and its related sclerites.

Basal chamber of copulatory tube

Klimaszewski (1984) and Naomi (2006a, b) called the basal swollen portion of the endophallic flagellum “sperm(a) sac,” while Gack and Peschke (1994) called the inner sac of the spermatophore “sperm sac.” Thus, presently, the term “sperm sac” is a homonym. The latter usage of the term “sperm sac” seems to be adequate, given that the sperm is deposited within the inner sac of a spermatophore. Thus, to avoid the terminological confusion caused by the term “sperm sac,” I call the basal swollen portion of the copulatory tube “basal chamber.”

Dorsal side of aedeagus

In general, the dorsal side of the aedeagus in Staphylinidae is the side opposite to the parameral attachment; and thus it is sometimes called “non-parameral” or “abparameral” (Gusarov 2003: 9) side. However, in Steninae the dorsal side of the aedeagus is the upper side of the aedeagus on which the parameres are attached, because the aedeagus of Steninae is positioned in situ within the abdomen, in such a way that the parameres face dorsally. This is the reverse of the situation in many other Staphylinidae.

3 Endophallic Copulatory Tube and Pseudocopulatory Tube

In the taxonomic studies of staphylinids, the intromittent sclerite of the aedeagal median lobe has been called as “basal tube” (Naomi 2006a, b; Naomi et al. 2017), “copulatory piece” (Maruyama and Klimaszewski 2004; Maruyama 2008), “dorsal copulatory piece” (Brunke et al. 2016), “endophallus” (Nomura 2001a; Irmler 2005), “flagellum” (Ashe 1984, 1992), “guide sclerite” (Löbl and Calame 1996), “internal structure of male genitalia” (Hammond 1973), etc. These endophallic structures are certainly highly diverse morphologically within Staphylinidae, but due to the existence of various intermediate conditions, we can trace the morphological transformation series from the original forms (simple, short setal structures) to one extremity (very thick sticks) or to the other extremity (whiplike, very long flagella). Furthermore, as mentioned later, we can also understand that these structures basically consist of same morphological components. Thus, they are considered homologous structures from the genuine morphological point of view. Note however that all endophallic sclerites that have been previously called variously by the aforementioned terms in descriptive studies are not necessarily copulatory tubes.

First, all these intromittent sclerites are used and function during a copulation; and second, although they are highly morphologically diverse in Staphylinidae, ranging from the thick and stiff rods to the whiplike, very long, flexible flagella, they all are tubular structures. Note here that the tube is open only at the dorsal side in Aleochara (Gack and Peschke 2005: 309, Fig. 1C) or only at the ventral side in Stenus (Naomi 2006a, b), so that it is a U-shaped tube in cross section. Given the function and structure of the endophallic intromittent sclerites, they are here collectively called “copulatory tubes.”

Precisely, what is the endophallic copulatory tube in Coleoptera? In order to identify the copulatory tube, we need to settle the landmark for its circumscription. The ejaculatory duct is formed by an ectodermal invagination; and the position of its opening is supposed to be settled in the early stage of genital morphogenesis (Hemming 2003). Thus, the position of the gonopore (i.e., the opening of the ejaculatory duct: Fig. 13.1a, b) can be considered an important landmark for identifying the endophallic copulatory tube in Coleoptera (see Matsumura and Yoshizawa 2012 in the case of the flagellum). By using the position of gonopore as its landmark, we will be able to characterize the coleopterous copulatory tubes. When characterizing it in an appropriate way, we must consider the following two points: first, the copulatory tube can be regarded as an endophallic tube that is distal from the opening of the ejaculatory duct; and second, the characterization must be applied to the morphologically diverse forms of the tube located at the apex of the endophallus. The copulatory tube of Coleoptera can be thus characterized as “a sclerotized, rod-, tube-, or whiplike terminal extension of the ejaculatory duct which is distally located from the opening of the ejaculatory duct (i.e., primary gonopore”; Fig. 13.1a, b) in such a way that we can regard a thick, stiff stick as well as a long, whiplike tube of the endophallus, as the copulatory tube.

Fig. 13.1
figure 1

Diagrams of the endophalli in Staphylinidae (lateral views). (a) Copulatory tube (= flagellum) in which the primary gonopore corresponds in place to the functional gonopore; (b) copulatory tube in which the primary gonopore does not correspond in place to the functional gonopore; (c) pseudocopulatory tube and copulatory tube

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A major problem here is that the endophallic copulatory tubes are highly diverse morphologically; and thus, coleopterists have used various descriptive terms that are suitable for the conditions of copulatory tubes that they studied. This is really the case for the “flagellum” (Fig. 13.1a), which is in some cases (e.g., Peschke 1978) called “virga” (Snodgrass 1935: 622). The endophallic flagellum of insect is characterized by Nichols (1989) as “sclerotized terminal prolongation of the ductus ejaculatorius, usually concealed within the internal sac when in repose, but sometimes very long and constantly protruding through the ostium of the penis.” The flagellum of Coleoptera is similarly described as “internal sac of a very long, slender eversible type” (Crowson 1981) or by other slightly different wordings in textbooks. I think that these traditional characterizations of the flagellum represent reasonable attempts made during the expansion of the knowledge of insect morphology; and in those senses, they can apply to the terminal, tubular, or whiplike modifications of the ejaculatory duct and thus are useful for descriptive purposes.

However, those characterizations are not precise, because we do not know, by using such wordings, how to homologize the flagellum or how to distinguish the flagellum from the ejaculatory duct. Note here that the so-called flagellum is a form of the copulatory tube; and consequently, it is the long or very long, whiplike form of the copulatory tube. The proper characterization of a flagellum may be obtained by slightly modifying the characterization of the copulatory tube as shown above. The endophallic flagellum is therefore characterized as “a whiplike, flexible terminal extension of the ejaculatory duct which is distally located from the opening of ejaculatory duct (i.e., the primary gonopore”; Fig. 13.1a). Note also that the term flagellum, by its characterization, designates the whiplike tube that includes, if present, the basal (swollen or ovoidal) chamber.

The above characterizations seem to be sufficient for identifying the endophallic copulatory tubes of Staphylinidae, but an important point is that, in the endophallus of a few Staphylinidae (e.g., some Stenaesthetus species), there exist another terminal sclerotizations of the ejaculatory ducts which are nonhomologous to the copulatory tubes. This endophallic sclerotization (which I call “pseudocopulatory tube” in this paper; Fig. 13.1c) is similar in structure to the true copulatory tube because it is tubular. In insects, the epithelial wall of the ejaculatory duct is surrounded by a strong muscular sheath (Snodgrass 1935: 572). Given that the position and structure of the pseudocopulatory tube is the same as in the muscular sheath, the pseudocopulatory tube may be identical with a special form of the muscular sheath, which may aid in ejaculating the sperm (enveloped in a spermatophore). It just forms the apical portion of the ejaculatory duct, but it is distinctly different from the true copulatory tube because of the following two reasons: first, the opening of the ejaculatory duct (i.e., primary gonopore) is located at its tip, but not at its base (Fig. 13.1c), and, second, it is located just proximal to the true copulatory tube (e.g., Stenaesthetus apterus Puthz 1988a, Fig. 6); in other words, the pseudocopulatory tube is directly connected at its most distal portion with the base of the true copulatory tube.

Since this paper strictly deals with the true copulatory tubes (but not with the various tubular structures of the endophallus including pseudocopulatory tubes), the pseudocopulatory tube should be clearly distinguished from the true copulatory tube, to correctly identify the latter in Staphylinidae.

4 Distribution of Endophallic Copulatory Tubes in Staphylinidae

Since, in Staphylinidae, the distribution of the species that have the aedeagus with an endophallic copulatory tube is very characteristic and interesting, I describe it here before describing and dealing with the matter of structures and functions of the copulatory tubes in Staphylinidae.

In Staphylinidae, the simple form of the endophallus is composed of a simple reversible internal sac (Fig. 13.3a); and it is often furnished (or covered) internally with spines and/or spinules (Fig. 13.3e). These primitive conditions are widely found in many groups of staphylinids including Cerapeplus (Löbl and Burckhardt 1988), Dasycerus (Löbl and Calame 1996), Glypholoma (Thayer and Newton 1978; Thayer 1997), Neophonus (Thayer 1987), Habrocerus and Nomimocerus (Assing and Wunderle 1995), Pseudopsis (Herman 1975), Trichophya (Ashe and Newton 1993), Xantholinus (Bordoni 2002, 2011), etc. The endophallic copulatory tube is not found in the aedeagus of these staphylinids.

However, the species that have the aedeagus with a copulatory tube are widely scattered in the various subfamilies of Staphylinidae (Table 13.1): omaliine group (Omaliinae, Proteininae, Dasycerinae, Pselaphinae); oxyteline group (Trigonurinae, Oxytelinae, Osoriinae, Scaphidiinae); tachyporine group (Tachyporinae, Phloeocharinae, Aleocharinae), and staphylinine group (Oxyporinae, Paederinae, Staphylininae, Scydmaeninae, Megalopsidiinae, Euaesthetinae, Steninae, Leptotyphlinae). Major characteristics regarding the distribution of the copulatory tubes in Staphylinidae are as follows:

  1. 1.

    The copulatory tubes are found in both, the basal subfamilies (e.g., Trigonurinae, Omaliinae, Tachyporinae, Oxyporinae) and more advanced ones (e.g., Pselaphinae, Aleocharinae, Euaesthetinae, Steninae, Leptotyphlinae). However, the subfamilies with many component species having the aedeagus with a copulatory tube belong to the evolutionally advanced subfamilies (i.e., Pselaphinae, Aleocharinae, Euaesthetinae, Steninae, Leptotyphlinae). As far as I know, there is no staphylinid subfamily, with all species having the aedeagus with a copulatory tube.

  2. 2.

    The distribution of the species with a copulatory tube in a given subfamily (in which species with a copulatory tube are found) is rather biased; that is, the copulatory tube is in general restrictedly found in the species of some particular groups of the subfamily. For example, in Paederinae, the copulatory tube is found in many species of Scopaeus (Frisch 1997, 1998, 1999, 2003). In Aleocharinae, the copulatory tube/flagellum is found in many species of Gyrophaenina (Ashe 1984), Bolitocharina (Ashe 1992), Aleocharini (Klimaszewski 1984; Yamamoto and Maruyama 2012), etc. In Staphylininae, the copulatory tube is found in many species of Othiini (Assing 1999) and Cyrtoquediina (Brunke et al. 2016).

  3. 3.

    In small-sized genera, only one or several species have the aedeagus with a copulatory tube. For example, out of 17 worldwide species of Dasycerus , only one species D. angulicollis has the copulatory tube (Löbl and Calame 1996) among the large genera. The genus Ocypus seems to be also an example of this. Out of 56 species of the genus Ocypus distributed in the west part of Palaearctic region, only one species O. similis possesses the aedeagus with a copulatory tube (Coiffait 1974).

  4. 4.

    When many species of a genus possess the copulatory tube, then various forms of the copulatory tube are found within the genus under consideration. This is, for example, true for Stenus and Dianous (Table 13.1).

Table 13.1 Representatives of the higher taxa of Staphylinidae, in which the species with an endophallic copulatory tube are included
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5 Structures of Endophallic Copulatory Tubes in Staphylinidae

5.1 General Structure of the Copulatory Tube

In Staphylinidae, the endophallic copulatory tube is in general composed of a basal chamber and a main tube (Fig. 13.2). The basal chamber is very small to moderate or large in size; and it is usually ovoidal or fusiform when it is swollen. It is, however, often missing (e.g., Kathetopodion, Leschen and Löbl 2005). If present, the basal chamber is usually demarcated by a basal constriction from the main tube (Fig. 13.2). The main tube is very often divided into two components: the basal tube and the apical tube (Fig. 13.2); in other words, the copulatory tube is tripartite (basal chamber, basal tube, and apical tube) in such cases. The copulatory tube is easily distinguishable from the ejaculatory duct in many cases, because the base of copulatory tube (i.e., the basal chamber) is more or less swollen (Fig. 13.2). Even in cases of the copulatory tube without a basal chamber (e.g., Pinophilus; Abarbanell and Ashe 1989), it may be easily distinguishable from the ejaculatory duct in most cases, because the proximal portion of the copulatory tube is more or less thick than the ejaculatory duct.

Fig. 13.2
figure 2

Diagram of the general structure of the copulatory tube in Staphylinidae

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In the copulatory tube of Staphylinidae (Fig. 13.1a, b), the opening of the ejaculatory duct (first gonopore) is not necessarily identical with the opening, through which the spermatophore (or sperm) is ejaculated during copulation (second gonopore). In some species of staphylinids, the first gonopore seems to substantially correspond in its position to the second (Fig. 13.1a). Namely, the first gonopore is located at the bottom of the basal chamber, and it is through this opening that the spermatophore (or sperm) is transferred to the vagina during copulation (e.g., Aleochara ; Gack and Pescke 1994). However, in some other species of Staphylinidae, the first gonopore does not correspond in position to the second (Fig. 13.1b). Namely, the first gonopore is located at the bottom of the basal chamber, whereas the second is at the middle of the copulatory tube or at or near its tip (e.g., some Stenus : Fig. 13.6g; Naomi 2006b). Thus, these two openings should be precisely distinguished in the morphological studies of Staphylinidae. In this paper, the first gonopore is termed “primary gonopore,” whereas the second one is termed “functional gonopore.”

5.2 Seven Basic and Some Other Unique Forms of Copulatory Tubes

The endophallic copulatory tubes are highly morphologically diverse in Staphylinidae, as mentioned above, but they seem to be basically classified into seven forms. In this subsection, the structures of these seven basic forms are first described in detail; and then some other unique forms of the copulatory tubes are described.

5.2.1 Form 1 (or Original Form) of Copulatory Tube

Concerning the original forms of the copulatory tube in Staphylinidae, empirical data suggest that there seems to be two candidates. First, a simple bulbous chamber (Fig. 13.3b) should be considered an original form of a copulatory tube, because it is an extension of the ejaculatory duct which is distal to the primary gonopore. It consists only of the basal chamber; and it is here regarded as “form 1r.” It is found in some Thinobius (Makranczy and Schülke 2001), etc. Second, a simple, seta-like sclerite (Fig. 13.3f, g) is also regarded as the other original form of a copulatory tube (“form 1s”). It consists of the simple main tube (i.e., a seta-like sclerite), with (Fig. 13.3f) or without (Fig. 13.3g) setulae around it. These two are here regarded as “form 1sa” and “form 1sb,” respectively. The form 1s is found in the endophallus which is covered densely with spines and spinules. It is rare in Staphylinidae; and it occurs in some Xantholinus (Coiffait 1972: 241) and some Scaphidiines (Leschen and Löbl 2005: 34).

Fig. 13.3
figure 3

Diagrams of the endophalli in Staphylinidae (lateral views). (a) Internal sac without modification; (b) internal sac with the copulatory tube (form 1r); (c) internal sac with the copulatory tube (form 2ra); (d) internal sac with the copulatory tube (form 2rb); (e) internal sac with setulae; (f) internal sac with the setulae and copulatory tube (form 1sa); (g) internal sac with the setae and copulatory tube (form 1sb); (h) internal sac with the setae and copulatory tube (form 2s)

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5.2.2 Form 2 of Copulatory Tube (“Copulatory Piece Auctorum”)

The form 2 of a copulatory tube is composed of the basal chamber and a simple main tube (Fig. 13.3c, d, h). The basal chamber is small (Fig. 13.3c), medium (Fig. 13.3d, h), or large in size (e.g., some Diaulota ; Ahn 1996). The basal constriction is indistinct (Fig. 13.3c, h) or distinct (Fig. 13.3d). The main tube is basically short and thin or moderately thick; it simply tapers toward the pointed apex (Fig. 13.3d, h) or is more or less curved (Fig. 13.3c). There often exist variations of form 2. They vary in structure, namely, they are thin to moderately thick, short to moderately long, and straight or curved, but they do not have a basal chamber (e.g., Dasycerus : Löbl and Calame 1996). They are classified into the “form 2v” in Fig. 13.3 and Table 13.1. The form 2 (and also 3) are rather different in structure from the typical whiplike flagellum and also from the thick, sticklike tube; and they have been called by various terms (e.g., “copulatory piece,” “guide sclerite,” and “endophallus”) in descriptive studies.

The form 2 of the copulatory tube is common in Staphylinidae (Table 13.1), but it is considered that the origins of these copulatory tubes must be different, because there are two different original forms described above. The “form 2ra” (Fig. 13.3c) and “form 2rb” (Fig. 13.3d) are here considered as derived from the form 1r, by extending posteriorly the distal portion of basal chamber. The “form 2s” (Fig. 13.3h) and its variations are here considered as derived from the form 1sa or 1sb, by enlarging its base and extending the main tube posteriorly (e.g., Xantholinus linearis ; Coiffait 1972: 236). The form 2s and its variations are found in the endophallus which is covered densely with spines and spinules (e.g., in Xantholinine genera; Bordoni 2002, 2011; Coiffait 1972). (Note here that once a copulatory tube has evolved into the form 3 or other advanced forms (i.e., whiplike form 4 and thick forms 5–7), we cannot see in general whether the copulatory tube is derived from a simple seta-like sclerite or from a simple basal chamber.)

5.2.3 Form 3 of Copulatory Tube (“Copulatory Piece Auctorum”)

The form 3 of the copulatory tube is in general tripartite; and it is composed of the basal chamber and the main tube (basal tube + apical tube) (Figs. 13.4 and 13.5c–e). It is basically long; and it is sometimes a little shorter than the whole length of the aedeagus. The proximal part of the copulatory tube is swollen to form a basal chamber; and when seen from the ventral side, it consists of “two thin rods” in some Stenus (Naomi 2006a, b). The main tube is U-shaped in cross section; and the basal tube is thin (Fig. 13.5c, d) or moderately thick (Fig. 13.5e), while the apical tube is whiplike (Fig. 13.5c) or moderately thick (Fig. 13.5e). The main tube is almost straight (Fig. 13.5c) or weakly (Fig. 13.5d) or strongly curved, but it basically tapers apically, and the demarcation between the basal tube and apical tube is indistinct.

Fig. 13.4
figure 4

Diagrams of the endophallic copulatory tubes (forms 3–7) in Staphylinidae. The lines are drawn, which show homologous parts of the different forms of the copulatory tubes

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Fig. 13.5
figure 5

(a, i) Aedeagi (ventral view); (b–g) copulatory tubes (ventral view); (h) spermatheca. (a) Stenus riukiuensis Puthz; (b) S. miroku Naomi; (c) S. unagi Hromádka; (d) S. gagyumontis Naomi; (e) S. ohtoensis Naomi; (f) S. olliformis Naomi; (g)–(i) S. ebisu Naomi (a, c: original illustration; b, d–i: Naomi 2006b) Scale 1: 0.1 mm for (a); scale 2: 0.1 mm for (b)–(h); scale 3: 0.2 mm for (i)

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There are variations of form 3. The “form 3v” is slightly different from the typical form 3 (Figs. 13.4 and 13.5e). For example, in some cases, the main tube is hardly divided into the basal and apical tubes (Fig. 13.5c, d); or it is entirely sticklike so that it has the same or similar thickness from the base to the apex. The basal chamber is missing in some other cases. In cases where a copulatory tube is thin, moderately long, and weakly tapers apically (e.g., Aphaenochara; Maruyama and Hlavác 2003), the copulatory tube may be still regarded as a variation of form 3, but due to the various intermediate conditions, it is sometimes difficult to separate the form 3 from the form 2 or 4. The form 3 and its variations are, as in the form 2, common in Staphylinidae (Table 13.1).

5.2.4 Form 4 of Copulatory Tube (“Flagellum Auctorum”)

The form 4 of the copulatory tube has been called “flagellum” in descriptive studies because it is whiplike. It consists of the basal chamber and the main tube (Fig. 13.4). The proximal part of the flagellum is more or less swollen (Klimaszewski 1984; Gack and Peschke 2005: 309, Fig. 1A: bf; Naomi 2006a, b) to form a basal chamber. The main tube is thin (Fig. 13.5b) to very thin (Fig. 13.5a), moderately long (Fig. 13.5b) to very long (Fig. 13.5a), straight (Fig. 13.5b), weakly or strongly curved, or irregularly, loosely coiled several times (Fig. 13.5a). In rare cases (e.g., Othius bhutanensis: Assing 1999; Aleochara tristis: Gack and Peschke 2005; Stenaesthetus afer: Puthz 2011a), it is very thin, extremely long, and regularly coiled many times. The tube is almost even in thickness from the base to the apex (Fig. 13.5a), or it weakly becomes thinner toward the apex (Fig. 13.5b). A unique flagellum is found in Stenus paludivagus Puthz 2000b. It is very long, wide, flat, and loosely coiled many times; and furthermore, it gradually widens toward the apex (Puthz 2000b). The form 4 is sporadically found in some species and genera of Aleocharinae, Euaesthetinae, Steninae, Othiini, etc. (Table 13.1).

5.2.5 Form 5 of Copulatory Tube

The form 5 of a copulatory tube is tripartite; and it is composed of the basal chamber and the main tube (basal tube + apical tube) (Fig. 13.4). Forms 3 and 5 are relatively similar in structure and thus sometimes difficult to distinguish from each other, but in the form 5 (Fig. 13.4), the basal tube is much thicker, and the basal and the apical tubes are in general easily distinguishable by a more or less distinct constriction between them. The basal chamber is small to large (Fig. 13.5f) or very large (Fig. 13.5g). The basal tube is thick (Fig. 13.5f) to very thick (Fig. 13.5g) and short to moderately long (Fig. 13.5f, g); and the apical tube is thin to feebly thick (Fig. 13.5f, g), feebly tapers toward its tip (Fig. 13.5f), or has almost the same thickness (Fig. 13.5g). The form 5 and its variations are most frequently found in Steninae, often in Pselaphinae, and in some genera of Euaesthetinae and Leptotyphlinae, etc. (Table 13.1).

5.2.6 Form 6 of Copulatory Tube

The form 6 of a copulatory tube consists of the basal chamber and the main tube (Fig. 13.4). Forms 5 and 6 are difficult to classify in some cases, but in the form 6, the apical tube is reduced into a small apicomedian protuberance. The basal chamber is medium to large (Fig. 13.6g) or very large sized (Fig. 13.6a, e). The basal tube is thick (Fig. 13.6a, 8) to very thick (Fig. 13.6d, e) and short to moderately long (Fig. 13.6f) or long (Fig. 13.6c). The apicomedian protuberance is small, thin, and simply pointed (Fig. 13.6a, e, g) or minutely bifurcate (Fig. 13.6d); and it is single (Fig. 13.6a, e, g) or rarely double (Fig. 13.6d). The form 6 is found in Pselaphinae, Steninae, Euaesthetinae, etc. (Table 13.1).

Fig. 13.6
figure 6

(a, d, e, g–i) Copulatory tubes; (b) basal portion of spermatheca; (c, f) aedeagi in ventral view. (a)–(c) Stenus ichihashii Naomi; (d, e) S. nakanei Hromádka; (f, g) S. gyrosus Naomi; (h) S. yasuhikoiellus Naomi; (i) Dianous coeruleovestitus Puthz (a–c, e–g: Naomi 2006b; d: Naomi 1997b; h: Naomi 2010; i: original illustration) Scale 1: 0.1 mm for (a, b, d, e, g, h); scale 2: 0.2 mm for (c, f); scale 3: 0.1 mm for (i)

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5.2.7 Form 7 of Copulatory Tube

The form 7 of a copulatory tube consists of the basal chamber and the main tube (Fig. 13.4). The basal chamber is in general stout and large (Fig. 13.6h) to very large (Fig. 13.6i). In some cases, it is modified with very thin, apicolateral projections (Fig. 13.6i), and/or others. The main tube is moderately thick (Fig. 13.6i) to thick or very thick (Fig. 13.6h) and large (Fig. 13.6h) or very large (some Stenus ; Naomi 2010); and it usually lacks the constriction between the basal and the apical tube. It is sometimes reduced into a short and small tube or rod (Fig. 13.6i). The main tube has accessory lobes, protuberances, etc. in some cases. For example, in Stenus yasuhikoiellus, the main tube has ventrally a deeply bifurcate lobe (Fig. 13.6h); and in Dianous coeruleovestitus, it has a pair of pointed lateral projections (Fig. 13.6i). The form 7 is, as far as I know, found only in Steninae.

5.2.8 Other Forms of Copulatory Tube

There certainly exist some other unique forms of copulatory tubes in Staphylinidae. Some representatives of the unique forms are as follows:

Corkscrew Form

The copulatory tube is strongly, tightly coiled in some Holotrochus (Irmler 1981, 2005); and in some Scaphobaeocera (Hoshina and Sugaya 2003; Löbl 2015), it is thin and very loosely coined like a corkscrew. In some Octavius (Puthz 1989c), the copulatory tube is long and moderately thick; and just like a corkscrew, the tube is strongly, tightly coiled at least in its basal half, although the entire tube is almost straight. In some Brachida (Ashe 1984), the basal part of the main tube is thick and strongly, tightly coiled.

Bifurcate Form

In some Octavius (Puthz 1977), the copulatory tube is long and moderately thick, with the apical half of main tube split longitudinally; the right lobe is thicker than the left one, and the mesial margin of the right lobe is furnished with an irregular line of 4, 5, 6, or 7 denticles. In Cyrtoquedius (Brunke et al. 2016) and Euplectus lapponicus (Löbl and Mattila 2010), the apical part of the main tube is split longitudinally.

Deformed Form

In Pselaphinae (e.g., Jeannel 1959; Nomura 1991), the copulatory tube is often morphologically highly deformed, for example, in Pselaphogenius (Nomura 2001b) and Octomicrus (Nomura 2010), the main tube branches with spine-like lobes of various forms; and it is spatulate, strongly curved, asymmetric and/or sinuous. However, even in these cases, the copulatory tube yet retains some basic characteristics. Namely, it is rodlike at least at its base; and the cross section of the tube is U-shaped at least at its base.

6 Functions of Endophallic Copulatory Tubes in Staphylinidae

The status quo is that the functions of the endophallic copulatory tube are not studied in most groups of Staphylinidae. However, the function of the copulatory tube in Aleochara (as “extension tube guiding rod”) was unraveled by Gack and Peschke (1994, 2005), whereas in some Stenus , the other function (as “spermatophore or sperm depositor”) was inferred from the information obtained during my morphological studies by discovering that the spermatophores were retained in the male copulatory tube (Figs. 13.5e and 13.6e, g) and also a spermatophore was found in the female genital chamber, which was probably pasted by a male during copulation (Fig. 13.7; Naomi 2006b). Therefore, based on the present observations and descriptions of the copulatory tubes, together with available information from the published papers, it is certainly possible to reasonably hypothesize the functions of the copulatory tubes in many cases of Staphylinidae. Thus, in this section, the functions of the seven forms of the copulatory tubes are separately described and discussed. I hope that the descriptions, observations, and possible hypotheses about them in this section are not only useful for descriptive studies of Staphylinidae but also form a discussion basis for further morphological studies on them.

Fig. 13.7
figure 7

Gonocoxites and the basal part of the spermatheca, with a spermatophore deposited in the vagina by a male (ventral view). Note that the spermatophore (which was once pasted to the basal pouch of the spermatheca by a male) comes out from the basal pouch. Stenus ichihashii Naomi (2006a). Scale: 0.1 mm

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In this section, the term “gonopore” means the “functional gonopore” (but not the “primary gonopore”’), because the position of the functional gonopore (Fig. 13.1a, b) is more useful for efficiently classifying the various forms of staphylinid copulatory tubes.

6.1 Functions of the Seven Forms of Copulatory Tubes

6.1.1 Form 1 (or Original Form) of Copulatory Tube

Regarding the form 1r (Fig. 13.3b), the gonopore is located at the base of the basal chamber, which functions as a chamber for storing a spermatophore (or sperms). Given its bulbous structure, the copulatory tube is supposed to play a role as a “spermatophore (or sperm) depositor” (see “form 6” of this subsection with respect to its characterization). Regarding the form 1sa and 1sb (Fig. 13.3f, g), the gonopore is supposed to be located at or near the base of the seta-like sclerite. Given its thin structure, it must function as an intromittent organ, which is inserted into the basal portion of the spermathecal duct during copulation.

6.1.2 Form 2 of Copulatory Tube (“Copulatory Piece Auctorum”)

In the form 2 (2ra, 2rb, 2s) (Fig. 13.3c, d, h), the gonopore is located at the base of the basal chamber. In cases where the copulatory tube has a thin, attenuate main tube (2rb, 2s), it is supposed to be inserted into the spermathecal duct during copulation. However, the function of the form 2ra with the relatively thick main tube is unclear.

6.1.3 Form 3 of Copulatory Tube (“Copulatory Piece Auctorum”)

The apical tube is whiplike so that it is too thin to transmit a large spermatophore to its tip (Fig. 13.5c, d); and thus, the gonopore is supposed to be located at or near the base of the copulatory tube. Since the whiplike apical tube is obviously thinner than the basal part of the spermathecal duct of female in some Stenus species (Naomi 2006a, b), the apical tube seems to have a role as an intromittent organ, which is inserted into the spermathecal duct during copulation.

6.1.4 Form 4 of Copulatory Tube (“Flagellum Auctorum”)

The function of the form 4 (i.e., the flagellum) of Aleochara was studied in detail by Gack and Peschke (1994, 2005). The gonopore is located at the base of the flagellum. The flagellum is an intromittent tube that is inserted into the spermathecal duct of the female during copulation. It plays the role as the guiding rod of an extension tube (or sperm). Note here that the extension tube is a very thin tube growing out from the spermatophore, to go through the spermathecal duct (Gack and Peschke 1994: Fig. 3).

In Phanerota (Ashe 1986), the copulatory tube is basically thin, and about as long as or distinctly longer than the whole length of the aedeagus, but the main tube seems to weakly differentiate into the stiff basal tube and the flexible, whiplike apical tube (e.g., Ashe 1986, Figs. 1, 8A and 9A). It belongs to form 3 or 4 (Fig. 13.4). At the dorsal side of the demarcation part of the copulatory tube between the basal and the apical tube, there exists a small pointed hook. On the other hand, the spermatheca of the female is moderately long and loosely coiled; and there exists a small triangular chamber at one side of the opening of the spermathecal duct (Ashe 1986, Fig. 7). Since the hook of the male copulatory tube nearly fits the triangular chamber of the spermatheca in size, the triangular chamber seems to function as the container for receiving the hook during copulation. It is thus hypothesized that the flexible, whiplike apical tube only is inserted into the spermathecal duct; and the pointed hook of the copulatory tube is pocketed into the triangular chamber of the spermatheca so that the copulatory tube tightly fixes in position during copulation.

6.1.5 Form 5 of Copulatory Tube

Given the thickness of the basal tube of the main tube in the form 5 (Fig. 13.4), the basal tube is considered to function as the chamber for storing a spermatophore (or sperm), as in the form 6 (e.g., Fig. 13.6g). It means that a spermatophore goes through the basal tube until near its tip before it is transmitted to the female genital chamber. Thus, the gonopore is supposed to be located at or near the tip of the thick basal tube in Stenus (Fig. 13.4). In Stenus ebisu (Fig. 13.5g, i) and its allied species (e.g., S. olliformis; Fig. 13.5f), the apical tube seems to play a role as an intromittent organ into the spermathecal duct; and it may also serve as a rod guiding an extension tube (growing out from a spermatophore), because of the following two reasons: first, the length and width of the apical tube (Fig. 13.5g) just fit the length and width of the female spermathecal duct from the opening to the base of the basal valve (Fig. 13.5h); and second, the cross section of the apical tube is U-shaped (Fig. 13.5g) as in the flagellum of Aleochara , which functions as the guiding rod of an extension tube.

6.1.6 Form 6 of Copulatory Tube

It was observed during my morphological studies that a spermatophore is deposited near the apex of the main tube in a male of Stenus gyrosus Naomi 2006b (Fig. 13.6f, g). This certainly implies that the gonopore of this Stenus species is located at or near the tip of the main tube (Fig. 13.4). The copulatory tube of S. gyrosus is so strongly sclerotized and rigid that a spermatophore may safely pass through the main tube to the gonopore, without its deformation.

Given the thickness of the main tube, it is apparent that the form 6 (Fig. 13.4) is not an intromittent organ into the spermathecal duct. In a female of S. ichihashii (whose male has a form 6 of a copulatory tube), a large spermatophore was observed that is probably pasted by a male at the opening of the spermathecal duct during copulation (Fig. 13.7). This observation suggests that the copulatory tube is supposed to function as the spermatophore (or sperm) depositor; here the spermatophore (or sperm) depositor means a rod or stick that directly deposits a spermatophore (or sperm) in the vagina or in the female basal pouch or infundibulum (i.e., a bowl-like pouch located at the opening of the spermathecal duct; Fig. 13.6b). Thus, in Stenus the sperm is transferred from the vagina to the spermatheca after the copulation, as in Aleochara (Gack and Peschke 1994). The apicomedian protuberance (Fig. 13.6a, d, e, g) probably has a function of fixing the main tube in position during copulation, by putting it into the opening of the spermathecal duct, because the size of the apicomedian protuberance just matches the size of the opening of the spermathecal duct. One might compare, for example, the size of the apicomedian protuberance of the copulatory tube in S. ichihashii, (Fig. 13.6a) with the size of the opening of spermathecal duct of the same species (Fig. 13.6b).

6.1.7 Form 7 of Copulatory Tube

In Dianous (Fig. 13.6i; Puthz 2000a) and Stenus (Fig. 13.6h; Naomi 2010), there exists a large opening at the apicomedian part of the copulatory tube (Fig. 13.4), which is considered the gonopore. A spermatophore (or sperm) is stored in the large basal chamber, whereas the main tube is supposed to play the role of fixing a copulatory tube in position during copulation, given the occurrence of various modifications (e.g., accessory lobes and projections) at the lateral and/or apical parts of the main tube. When considering the aforementioned function, together with the thickness of the main tube, the form 7 is considered to function as a spermatophore (or sperm) depositor.

In some Dianous species with an atrophied main tube (Fig. 13.6i), the basal chamber seems to become very large as if it were the main tube; and thus a possible interpretation of it is that the atrophy of the main tube is compensated by the enlargement of the basal chamber. If my interpretation is correct, then it seems in such cases that the basal chamber of form 7 functionally plays a similar role in the basal tube of form 6 (e.g., Fig. 13.6g), while the reduced main tube of form 7 plays a similar role in the apicomedian protuberance (as a reduced apical tube) of form 6 (e.g., Fig. 13.6g) in a sense that it has a role of fixing the copulatory tube in position during copulation.

6.2 Summary on the Functions of Endophallic Copulatory Tubes

What we understood and hypothesized about the functions of copulatory tubes in Staphylinidae are here summarized.

The functional gonopore is located at the base of the copulatory tube in the form 1, 2, and 4, at or near the base of the copulatory tube in the form 3, at or near the apex of the basal tube in the form 5, near the apex of the main tube in the form 6, and at the apicomedian part of the main tube in the form 7.

We can certainly demonstrate that the forms 1–7 of the copulatory tube all have the function of transferring a spermatophore (or sperm) to the vagina and then to the spermatheca of a female. However, strictly, the presented analyses suggest the following functions of copulatory tubes: (1) the form 1r (e.g., some Thinobius ) and 6 and 7 (e.g., some Stenus, Dianous ), which each comprises of a thick or very thick copulatory tube, are not intromittent organs into the spermathecal duct. Each serves simply as the spermatophore (or sperm) depositor, that is, a stick or a swell that directly deposits the spermatophore (or sperm) into the vagina or into the basal pouch located at the opening of the spermathecal duct; (2) the forms 1sa, 1sb, 2rb, 2s, and 3–5 each play a role as an intromittent organ into the spermathecal duct, because the apical portion or the apical tube of the copulatory tube is made up of a thin tube. The spermatophore (or sperm) is deposited in the vagina by the copulatory tubes of these forms. At least the forms 3–5 each are, given their structures (Fig. 13.4), supposed to serve also as an extension tube (or sperm) guiding rod, that is, a rod that guides the spermatophore extension tube (or sperm) into the spermathecal duct, as shown in Aleochara by Gack and Peschke (1994). Thus, the functions of the copulatory tubes are double in such cases (e.g., some Aleochara, Stenus ): a “spermatophore (or sperm) depositor” and an “extension tube (or sperm) guiding rod”; (3) the function of the form 2ra is unclear.