http://www.collembola.org/publicat/morpholo/head.htm
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Last updated on
2024.06.16
by Frans Janssens
Frans Janssens,
Department of Biology, University of Antwerp, Antwerp, B-2020, Belgium
Abstract.
To be completed.
Introduction
Fig.Sn. Sminthurinus niger from Holland
2021.03.14 © Kamsteeg, G.-J.
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The form of the head of Sminthurinus
is basically a frontocaudallly flattened sphere
with a ventral downwards directed conical projection.
Apically at the conical projection,
the mouthparts can be partially extended out off the mouth opening.
In frontal aspect, the observed shape of the head is variable.
It depends on the downward/upward tilt of the head.
At least 3 different tilt positions can be distinguised :
head not tilted with mouth opening directed downwards (fig.Sa),
head tilted down with mouth opening directed backwards (fig.Sn),
head tilted up with mouth opening directed forewards (fig.Sa2).
To be completed.
Head not tilted
Fig.Sa. Sminthurinus aureus from Belgium
2024.03.20 © Huskens, M.L.
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Fig.St. Sminthurinus trinotatus from Belgium
2024.03.21 © Huskens, M.L.
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In the not tilted head the eyebrows are in-line with the horizon of
the top of the head,
the labrum is partly visible
and the apical part of the mouthparts may be exposed
(fig.Sa, fig.St).
The head appears circular with a downwards projected mouth cone.
The ratio of the height versus the width of the head H:W = 1.11:1.
The height includes the visible part of the labrum.
Hence the mouthcone prolonges the head downwards with 11%.
To be completed.
Head tilted down with mouth opening directed backwards
Fig.Sa3. Sminthurinus aureus from Belgium
2024.03.20 © Huskens, M.L.
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Fig.Sl. Sminthurinus lawrencei from Belgium
2024.03.06 © Huskens, M.L.
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In the downward tilted head
a narrow rim is visible that runs tranversally behind the eyebrows.
The labrum and the mouthparts are not visible
(fig.Sa3, fig.Sl).
In the aureus-group, the head appears circular
with a short downwards projected mouth cone (fig.Sa3).
In the niger-group, the back of the head (the occiput)
is distinctly more visible as a raised rim behind the eyes
and makes the top half of the head appear more rectangular (fig.Sl).
To be completed.
Head tilted up with mouth opening directed forewards
Fig.Sa2. Sminthurinus aureus from Belgium
2024.03.20 © Huskens, M.L.
 |
Fig.Sa4. Sminthurinus aureus from Belgium
2024.03.20 © Huskens, M.L.
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In the upward tilted head the eyebrows appear bulged in respect to
the horizon of the top of the head,
the labrum is visible
and the apical part of the mouthparts may be exposed
(fig.Sa2, fig.Sa4).
The head appears circular with a short downwards projected mouth cone.
To be completed.
Sminthurinus domesticus
Fig.Sd. Sminthurinus domesticus from Belgium
Head not tilted
2024.03.21 © Huskens, M.L.
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Fig.Sd2. Sminthurinus domesticus from Belgium
Head tilted down
2024.03.21 © Huskens, M.L.
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In Sminthurinus domesticus of the niger-group,
the transversal dorsal post-ocular rim
is already visible as a very narrow rim
when the head is not tilted down (fig.Sd).
When the head is tilted down,
the rim is distinctly visible.
In addition, the head appears distinctly wide (fig.Sd2).
To be completed.
Sminthurinus niger sensu Gisin 1963
To be completed.
Discusion
To be completed.
Conclusion
To be completed.
Acknowledgements
We would like to thank, in alphabetical order,
Marie Louise Huskens,
and
Gert-Jan Kamsteeg
for their kind permission for using their respective photographs
as illustrations.
References
