|
Sesamia Guenee
Type
species: nonagrioides Lefebvre.
Synonyms: Several genera are listed below that could be included within a broad
concept of Sesamia.
This genus belongs to a major group of amphipyrines where the larvae bore and
feed in the stems of grasses, reeds and rushes. They are found in approximately
equal diversity in both temperate and tropical regions of the Old World, though
generally restricted to semiarid habitats in the tropics such as savannah where
their host-plants are abundant.
Several features characterise the whole group, which appear to be unique within
the noctuids at least; the female genitalia features are not seen in other
monocotyledonous stem-boring groups such as those in the Pyralidae (Crambinae,
Schoenobiinae and Galleriinae). The forewings have the longitudinal, rather
striate, pale fawn, dull orange or grey patterning seen in other grass or
palm-feeding groups (e.g. the pyralids mentioned, Mythimna the Ambadra
Moore/Norraca Moore group of the Notodontidae and the genus Psalis Hubner
in the Lymantriidae).
In the male genitalia the tegumen is broad, the peniculi prominent. The valves
of temperate taxa at least have the harpe absent or small if present (in some
European taxa), its presence usurped by a distal process arising from a broad
sclerotisation of the costa that is directed towards the apex or, more
frequently, towards or across the ventral margin; there may also be an angled
process, sometimes developed into an acute spine, on the dorsal margin of the
costa. This is probably a development of the higher trifine costal process. The
development of these two processes is highly variable within the complex, and
one or other may be absent. The ventral process in tropical taxa, when present,
is possibly of alternative derivation as discussed below. The aedeagus apex is
often spined or scobinate, sometimes this being on distinct processes; the
vesica is usually globular with one or more sclerotised structures on it rather
than general scobination; these structures are usually localised, clearly
defined, sometimes massive.
The female genitalia have the ovipositor lobes much more darkly sclerotised than
in other trifines, conical-acute or more rounded-acute, usually with setae very
much shorter and more densely distributed.
These characteristics may reflect the oviposition behaviour of the taxa; eggs
are inserted under the leaf sheath, often in a mass (see also Mythimna above).
The temperate region taxa (e.g. Coenobia Stephens, Nonagria Ochsenheimer,
Arenostola Hampson, Oria Hubner) tend to have a corona retained on
the rather elongate male valve, with the ventral process of the costal
sclerotisation tending to lie along the length of the valve or only weakly
extending to the ventral margins. The ovipositor lobes tend to be rather swollen
below the acute apex (roundedly acute) with setae directed rather irregularly,
sometimes very short. A similar ovipositor is seen in the grass-feeding genus Apamea
(above), which may therefore be sister-genus to the temperate stem
borers at least.
Tropical taxa have the valve apex more or less rounded, lacking a corona; the
costa is angled or with an acute process dorsally (these features are also seen
in the Palaearctic genus Coenobia). A ventral process occurs in many taxa
but it is more strongly associated with the sacculus than in temperate taxa,
though also often linked to the costal process by a band of sclerotisation.
There is thus the strong possibility that it is the harpe and therefore not
homologous with the ventral structure of the costa mentioned for
temperate taxa. The features bringing together the tropical taxa as a possible
natural group are in the ovipositor lobes: these are arranged together in an
acute, conical structure, perhaps more densely setose than in temperate taxa,
and with the setae on the dorsal side of each, (often slightly larger than the
ventral ones) often directed away from the apex of the ovipositor.
The
female features just mentioned are shown by the type species of the
following genera: Sesamia Guenee, Busseola Thurau (= Calamistis
Hampson), Conicofrontia Hampson, Poeonoma Tams & Bowden,
Tridentifrons Warren, Poecopa Bowden, Carelis Bowden, Manga
Bowden, Sciomesa Tams & Bowden. The features are not seen in Speia
Tams & Bowden (type species vuteria Stoll, S. Africa).
The
indications are that these genera form a natural group distinct from Palaearctic
stem-borer genera, defined on the female features and on differences in form of
the costal process of the male valve and the position and strength of the harpe
more distally on the sacculus.
The
whole complex needs further review and might, as with Mythimna, best be
treated as Sesamia sensu lato until the characters within it can be more
completely assessed. Some sections of it might be considered plesiomorphic and
therefore possibly paraphyletic. They exhibit characters such as: more typical
trifine patterning (cf. grass-like); a weak costal flange to the valve (cf. a
spine-like process); trifine band-like signa in the female bursa; presence of a
saccular harpe (both loss and greater development might be considered apomorphic).
Consideration of these characters arranges the genera (as represented by their
type species) as follows:
(A) Costal
process of valve acute; harpe strong, spinose; forewing pattern grass-like;
signum absent. All these characters are considered apomorphic.
Sesamia Guenee.
(B)
Costal process of valve acute; harpe lost; forewing pattern grass-like; signum
weak (Acrapex) or lost (Poecopa).
Acrapex Hampson,
type species prisca Walker (Sri Lanka).
Poecopa Bowden,
type species mediopuncta Bowden (N. Africa).
(C)
Costal process of valve a transverse flange (plesiomorphic); harpe lost (apomorphic);
forewing pattern plesiomorphic; signum lost (except in Sciomesa).
Carelis Bowden,
type species albula Bowden (W. Africa).
Poeonoma Tams
& Bowden, type species serrata Hampson (E. Africa).
Tridentifrons
Warren, type
species insularis Warren (Java).
Sciomesa Tams
& Bowden, type species mesioscia Hampson (S. Africa).
(D)
Costal process of valve a flange; harpe present on valve; forewing pattern
plesiomorphic though tending to grass-like in Conicofrontia; signum
absent
(Conicofrontia) or present (other genera).
Conicofrontia Hampson,
type species sesamioides Hampson (S. Africa).
Busseola Thurau,
type species sorghicida Thurau= fusca Fuller.
Manga Bowden,
type species basilinea Bowden (W. Africa).
The male antennae may be ciliate or bipectinate, occurrence
of the latter
being more frequent in Sesamia sensu stricto
Features of the aedeagus and anellus require investigation, as does the presence
or not of apodemes on the male eighth tergite. Apodemes are seen in a group of
Oriental taxa in group (C) with plesiomorphic patterning but male genitalia
tending towards those of Acrapex with the flange extended into an acute
costal process, and no harpe: grisescens Hampson (N.E. Himalaya); insularis
Warren (= microsema Hampson syn. n.) (Java, Bali); nigropunctata
Wileman (Taiwan); leucaneura Hampson (Burma). The last two have a
slight corona to the valve, as does Xylostola robusta Hampson (N.E.
Himalaya; below), a species more typical of group (C) and lacking strong
apodemes.
Another group of Oriental species resembles the African Busseola phaia Bowden
in valve characters and hence could be placed in group (B): punctivena Wileman
(Taiwan); praepallens Hampson (India); fumea Hampson (N.E.
Himalaya). The Bornean Busseola (see below) has a stronger, spinose harpe
on the valve.
All of the complex are probably stem-borers of Gramineae, the eggs being laid,
often in groups, between the leaf sheath and the stem. The young larvae may bore
immediately into the stem or wander up the plant to enter through the funnel of
young leaves. The larvae have crochets in a mesoseries as in other Noctuidae,
and usually have the prothoracic shield prominently sclerotised. Pupation is
usually in the stem. There appears to be great variation in the pupal cremaster
within the group, and this may also provide a key to its classification.
<<Back
>>Forward <<Return
to Contents page
|
