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Shamshev & Grootaert: A review of the genus Stilpon from the Oriental region
Thorax dark brown. Scutum entirely tomentose. Postpronotal
bristle long, inclinate. Dorsocentrals in multiple rows,
complete posteriorly. Acrostichals 2-serial, complete
posteriorly.
Legs with colour pattern: hind femur brownish in apical 1/
2; fore tibia, fore and mid tarsi, mid femur at apex, mid and
hind coxae brownish yellow. Mid coxa with 2 brown bristles
on outer side. Hind trochanter lacking spinules. Mid femur
(Fig. 76) slender, with 3 short black antero- and 2 longer
posteroventral spines in basal 1/2, bearing 2 long yellow
bristles in extreme base. Hind femur (viewed laterally) more
or less evenly thickened, with row of anterodorsal bristles
becoming longer toward apex and row of prominent dorsal
bristles. Fore tibia lacking prominent ventral bristles. Mid
tibia with ordinary setation, lacking ventral spinules. Hind
tibia unmodified.
Wing normally developed, covered with uniform
microtrichia; more or less uniformly, rather deep infuscate.
Costal vein with long setulae along anterior margin. Vein
R2+3 about 2.5 times longer than Rs. Distance between apices
of R2+3 and R4+5 1.5 times longer than distance between
apices of R1 and R2+3. R4+5 and M parallel and straight in
apical part. Halter with elongate, contrast black knob and pale
stem.
Abdomen largely dirty yellow, bearing mostly scattered dark
setulae which are longer on pregenital segments, with all
tergites (except segment 8) of subequal in length, tergites 1-
2 unmodified. Hardly prominent gland-like structures present
between tergites 6-5, 5-4, 4-3 and 3-2. Segment 8 with
moderately long bristles.
Hypopygium (Fig. 77) brown, large. Hypandrium with 2 short
bristles in apical part. Epandrium completely divided. Left
epandrial lamella small, fused to hypandrium, with 2 short
bristles in apical part. Left surstylus with upper lobe (Fig.
78) largely divided; lower part with greatly developed
surstylar comb, upper part digitiform, fused with lower part
basally. Right surstylus (Fig. 79) moderately large, sublinear,
more or less rounded at apex, bearing numerous bristles of
different length, lacking spines. Left cercus unbranched, short,
fairly broad, rounded at apex, lacking spines and ventral
bristle, with 2 very long left marginal bristles in basal part.
Right cercus unbranched, short, rectangular, lacking spines.
Phallus short.
Female. Unknown.
Measurements. – Body length 2.5 mm, wing length 2.2 mm.
Etymology. – In reference to the largest size of this species
Figs. 80-84. Details of male abdomen, dorsal view. 80, Stilpon lek, new species, 81, Stilpon lekkwar, new species, 82, Stilpon trilobatus,
new species, 83, Stilpon malayensis, new species, 84, Stilpon paradoxus, new species; T – tergite. Scale bar: 0.1 mm.
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THE RAFFLES BULLETIN OF ZOOLOGY 2004
among all others found until now from the Oriental region,
“yai” means big in Thai.
Phylogenetic relationships. – The relationships of S. yai are
unresolved. The main problem originates because of unknown
female of this species. The presence of 2 long marginal bristles
in basal part of cercus may indicate S. yai is closer related
to some species of the S. divergens species group. However,
it has no a long ventral bristle on the left cercus, whereas the
presence of this bristle appears to support a monophyly of
this group. The structure of the phallus resembles that in the
S. graminum and
S. seeluang species groups. However, this
character is insufficiently studied in many other Stilpon
species. S. yai has setose left cercus and 2 distinct bristles in
apical part of the left epandrial lamella. So, it cannot be
included in the S. graminum or S. seeluang species groups,
respectively. S. yai shares the same condition of the left
surstylus as in S. paradoxus (upper part largely separated).
Additionally, in S. yai the right surstylus appears to be
enlarged and cerci might exhibit some tendency to be
completely fused (presumably, the conditions toward those
found in S. paradoxus). However, the last two arguments are
hypotheses only. Whereas the condition of the left surstylus
noted is a too weak argumentation to discuss some
relationships between these two species at present. Like some
North American species, S. yai has the elongate costal setulae.
But, again, this is probably a quite homoplastic character
because the lack of the scutal tomentum supports well the
monophyly of these Nearctic species.
Distribution and seasonal occurrence. – Thailand. Known
from one locality. The only record is from the beginning of
May. Collected from riverbed.
DISCUSSION
The phylogenetic relationships within Stilpon
The phylogenetic relationships within Stilpon are difficult to
resolve with the data currently available. The main problem
deals with the unclear sister relationships of the genus.
Nevertheless, Cumming & Cooper (1992) discussed the most
evident phylogenetic patterns within Stilpon. We tested the
implication of the newly described species on their
preliminary conclusions. The hypothesised phylogenetic
relationships of the species of Stilpon are presented in Fig.
85. The cladogram is a result of the heuristic search performed
using the programs NONA (Goloboff, 1999) and, to submit
the data matrix, WINCLADA (Nixon, 1999). The data matrix
(Table 1) consisted of 23 morphological characters. We
should specially note here that the cladogram presented does
not pretend on a rigorous cladistic analysis. Such analysis is
impossible at present because the sister relationships of
Stilpon are unresolved. That was the reason why we did not
indicate the precise outgroup. To resolve this problem a
special study is needed that, however, is beyond of the scope
of this paper. Nevertheless, we considered that it would be
helpful for the future studies to visualise the possible
relationships within Stilpon. It is clear that this preliminary
analysis is mainly based on the characters which phylogenetic
value is more or less evident. In some cases they were checked
in other closely related taxa of the Drapetini.
The monophyly of Stilpon has been accepted here, following
Cumming & Cooper (1992), to be substantiated by two
apomorphies: linear to sublinear frons (character 1) and male
terminalia with a single ejaculatory apodeme (character 20).
Although, the first condition appears to present in some
closely related genera (Grootaert, 1994). Three informal
species groups of Stilpon have been previously recognised:
S. varipes group,
S. graminum group, and
S. divergens group
(Cumming & Cooper, 1992).
The lack of the scutal tomentum (character 4) and acrostichal
setulae (at least partial, character 6) was considered to support
the monophyly of the S. varipes species group. Our data
confirm this conclusion. All species treated in the present
paper share an entirely tomentose scutum and complete rows
of acrostichal setulae. The absence of the gland-like structures
on the male abdomen (character 9) could also indicate close
relationships of the species of this group. However, this
condition is present in some other species of Stilpon.
The monophyly of the S. graminum species group was
substantiated by the apomorphic development of apical spines
on the left cercus of the male terminalia. Within this clade
two monophyletic lineages have been recognised: S.
graminum, s. str., (upper lobe of the left surstylus completely
divided, female terminalia shortened, with the apex of sternite
8 hinged and at least partially separated from the base) and
S. nubilus (upper lobe of the left surstylus lacking comb,
sternite 10 of the female terminalia fused with anteroventral
margin of tergite 8).
The implication in the analysis of the newly described species
indicates that the S. graminum clade sensu Cumming &
Cooper (1992) is likely to belong to a broader branch of
Stilpon. This clade could be substantiated by the apomorphic
shortening of the female terminalia (character 21) and the
partial or complete division of the upper lobe of the left
surstylus of the male terminalia (character 12), assuming
subsequent reversal in S. nubilus Collin and its allies. The
clade appears to include two main lineages.
The first lineage agrees more or less with the concept of the
S. graminum group sensu Cumming & Cooper (1992)
(character 17). However, the relationships within the group
are not quite evident. S. spinicercus, the S. graminum and S.
nubilus complexes form one clade based on two characters
of the male terminalia, namely the presence of 2-3 apical
spines on the left cercus (character 17.2) and the prolonged
apically right surstylus (character 15). The S. graminum and
S. nubilus appear to be the sister complexes of the species.
This conclusion may be based on the shape of the female
cercus (character 22), which is broad oval in these species.
The absence of the abdominal gland-like structures could also
support this clade. However, this is a homoplastic condition
within Stilpon. The monophyly of the S. nubilus complex is
supported by the apomorphous presence of the apical spines