http://www.collembola.org/publicat/dimorphy.htm - Last updated on 2017.01.09 by Frans Janssens
Checklist of the Collembola: Note on the sexual dimorphy in Dicyrtomina (Dicyrtomidae)

Frans Janssens, Department of Biology, University of Antwerp, Antwerp, B-2020, Belgium
Paul A. Ardron, ...
Ashley Bradford, 6203 Arkendale Rd., Alexandria, VA 22307, USA

Introduction

Sexual dimorphy is quite rare in Collembola. In most species, sexuality is only revealed in reproductive adults by the shape of the genital opening, ventrally on the fifth abdominal segment. In females, the genital opening is a transvers slit, forming as such an anterior and posterior lip each accompanied by a transvers row of setae. In males, the genital opening is a longitudinal key-hole shaped slit surrounded by several whorls of setae.

Due to the improved capabilities of digital macrophotography, it became possible to sex adult reproductive specimens on habitus pictures of some surface active species of Collembola.

In this short preliminary note, we will explore the possibility of a case of sexual dimorphism with respect to the facial pigmentation in Dicyrtomina.

Material and methods

Fig.2. Dicyrtomina minuta forma ornata from the USA, Northern Virginia.
Male: distinctly swollen genital papilla present.
2010.01.20 © Bradford, A.
Fig.1. Dicyrtomina minuta forma ornata from the USA, Northern Virginia.
Female: genital papilla absent.
2010.01.20 © Bradford, A.
Alive specimens of Dicyrtomina were digitally photographed in situ in lateral aspect at a magnification of at most 4:1. A low lateral aspect is required to be able to observe the presence or absence of extern genitalia. In adult reproductive male specimens, the genitalia are distinctly present as a raised, swollen papilla (Fig.2). In adult reproductive female specimens, no such papilla is present (Fig.1).

Caveat: also in nonreproductive nonadult specimens the raised genital papilla is absent.

The lateral aspect pictures are taken with a digital camera Canon Powershot S5 IS with lens Raynox MSN-202 having a 4x magnification. In addition a Canon Speedlight 430EX flash on a cable is used to light the specimes from a different angle than the camera.

The pictures were digitally post-processed using Paint Shop Pro Version 4.10.

Preliminary observations

Fig.3. Dicyrtomina saundersi from the UK, England.
Dark face form.
2006 © Jones, D.
It was clear from the collection of pictures of Dicyrtomina at Bellinger et al. (1996-2010), among others, that there is a great infraspecific variation in pigmentation. Pale colour forms as well as dark forms are commonly recorded. More aberrant dark face forms draw our attention since 2006 (fig.3).

The dark colour form of Dicyrtomina saundersi is not necessarily uncommon. It was observed regularly at sites around Sheffield, UK, at least under rotting logs and other pieces of timber. Furthermore, around the end of 2009, the second author, Paul A. Ardron, transfered a bonzi plant from oudoors to an indoors window ledge and was surprised when up to about twenty specimens of Dicyrtomina saundersi emerged and the majority were of the dark form. Also colonies of the species with a high proporsion of the dark form have been seen under bark flakes of Corsican pine, on stabalised sand dunes in Norfolk, UK.

Fig.4. Dicyrtomina saundersi from the UK, England.
Male, dark form.
2009.02.05 © Ardron, P.A.
It was the second author, Paul A. Ardron, who pointed out first that the dark colour form in Dicyrtomina saundersi might be a case of sexual dimorphism (fig.4).
Given the dark colour forms are recorded from European Dicyrtomina ornata and Dicyrtomina saundersi, as well as from North American Dicyrtomina minuta (forma ornata), the presence of the dark face form was verified in the American specimens.

Results

The junior author, Ashley Bradford, took a series of lateral aspect 4:1 macrographic shots of individual specimens of the same population of Dicyrtomina minuta (forma ornata) from Northern Virginia, USA. In total, 16 specimens were photographed: 3 dark face forms and 13 pale face forms.

Fig.n3. Dicyrtomina minuta forma ornata from the USA, Northern Virginia.
Female.
2010.01.20 © Bradford, A.
Fig.n2. Dicyrtomina minuta forma ornata from the USA, Northern Virginia.
Male, pale form.
2010.01.20 © Bradford, A.
Fig.n1. Dicyrtomina minuta forma ornata from the USA, Northern Virginia.
Male, dark form.
2010.01.19 © Bradford, A.

The 3 dark face forms turned out to be male, consistently. There was some variation in the dark face mask itself. In one specimen the mask was less complete: the anterior part below the eyepatch, behind the antennal base, was pale but the posterior part was dark (see Discussion).

Among the pale face forms, 4 were males and 9 were females. To sex the pale face forms the following rule can be used:
the males have an unpigmented anterior lower part of the face, while the females have a pigmented anterior lower part of the face.

Discussion

Dark cheek form.
Fig.m. Dicyrtomina minuta forma ornata from the USA, Northern Virginia.
Male, dark cheek form, dorsal aspect.
2008.02.23 © Bradford, A.
Fig.m. Dicyrtomina minuta forma ornata from the USA, Northern Virginia.
Male, dark cheek form, lateral aspect.
2008.02.23 © Bradford, A.

Photographic records of dark Dicyrtomina specimens (in 2006, 2008) ocassionally revealed specimens with distinctly dark cheeks (fig.3, fig.m).

Fig.dcc. Dicyrtomina minuta forma ornata from the USA, Northern Virginia.
Male, dark cheek close-up
2010.01.19 © Bradford, A.
Fig.dcp. Dicyrtomina minuta forma ornata from the USA, Northern Virginia.
Male, partial dark cheek close-up
2010.01.20 © Bradford, A.
The dark cheek mask shows some variation. In many specimens the cheek is completely dark (fig.dcc). In others, the anterior part, the area just below the eyepatch and behind the antennal base, is pale (fig.dcp). The latter form can be considered as being inbetween the complete dark cheek form (fig.dcc, fig.n1) and the pale cheek form (fig.n2).

Fig.ds. Dicyrtomina saundersi from the UK, England.
Male, dark cheek close-up
2009.02.05 © Ardron, P.A.
Fig.ds2. Dicyrtomina saundersi from the UK, England.
Male, partial dark cheek close-up
2006 © Jones, D.
Also in Dicyrtomina saundersi this variation in dark cheek mask can be observed. In some specimens the area below the eyepatch is pale (fig.ds2), while in others the mask is more complete (fig.ds). Note the estethic 'tears' in the mask of this specimen from the UK. Such 'tears pattern' is typically observed in North American Ptenothrix species.

Fig.do. Dicyrtomina ornata from the UK, England.
Male, dark cheek close-up
2008.01.10 © Farrell, S.
The dark cheek mask also occurs in Dicyrtomina ornata (fig.do).

As shown in this study these dark cheeks are diagnostic to adult reproductive male specimens. Given the dark cheeks are observed in different species of Dicyrtomina, this is a generic diagnostic character.

Pale cheek forms

Male
Fig.pcm. Dicyrtomina ornata from France, Gironde.
Male, pale cheek form.
2012.11.25 © Sandra.
Fig.pcm1. Dicyrtomina ornata from Belgium, Limburg, Opitter.
Male; adult.
2015.12.01 © Huskens, M.L.
Fig.pcm2. Dicyrtomina ornata from Belgium, Limburg, Opitter.
Male (*);
pale upper + lower cheek.
2017.01.04 © Huskens, M.L.
Fig.pcm3. Dicyrtomina ornata from Belgium, Limburg, Opitter.
Male;
close-up of pale lower cheek.
2017.01.04 © Huskens, M.L.
Fig.pcm4. Dicyrtomina ornata from Belgium, Limburg, Opitter.
Male; subadult.
2017.01.08 © Huskens, M.L.

The cheek can be divided in 3 parts: upper, middle, and lower part. In the pale cheek males, at least the upper and lower part is pale. The middle part is optionally pale (in subadults?).

Female

To be completed.

More sexual dimorphism in Dicyrtomina saundersi.
1. colouration differences in the small abdomen, yellow in males.
2. shape differences in the abdomen

To be completed.

Conclusion

Sexing European Dicyrtomina ornata, Dicyrtomina saundersi and North American Dicyrtomina minuta (forma ornata) based on lateral aspect habitus pictures seems feasible. The algorithm to be used to sex the specimens:
if upper lateral face is (partly) dark then male
if upper lateral face is pale then
   if lower lateral anterior face is unpigmented then male
   if lower lateral anterior face is pigmented then female

Acknowledgements

The authors wish to thank David Jones, Shane Farrell, and Marie Louise Huskens for their contribution.

References