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Thosea Walker

Type species: unifascia Walker.
Synonym: Autocopa Meyrick (type species, monoloncha Meyrick).
This genus is at present under review (Cock, Godfray & Holloway, in press) as it contains numerous important coconut pest species. As set out by Hering (1931) it contained a large proportion of species, perhaps 50%, that were misplaced. African and Neotropical species placed in Thosea are also unlikely to remain there. Many of the misplaced Oriental species are found in Borneo and will be assigned to more realistic genera in this work. Others will be dealt with in a separate publication of a broader review of Thosea. A large proportion of the species falls into the genus Aphendala Walker Gen. rev, and this name could be used as a temporary repository for taxa evicted from Thosea as strictly defined here.

The genus has the general characters of the ‘crescent signum' group. All Bornean species have the white patch present on the apex of the fore tibia, though it is only small in T. lutea Heylaerts. Other Thosea species, including all those from Sulawesi and the Papuan Subregion, have lost it. The bipectinations of the male antennae taper away distally relatively gently.

The species generally have grey forewings with a black discal spot and an oblique, dark, straight postmedial distal to which the ground colour is darker. There are in some species darker shades: a bar medially on the dorsum and a blotch in the centre of the marginal zone.

In the male genitalia the uncus tapers to a point rather than having a ventral spur just subapically. The valves bear each a long whiplash-like arm or furca from the base of the sacculus, often longer than the valves themselves. This is often associated with a membraneous flap on the valve, central or subcostal, that probably acts as a guide for the furca in articulation. The furca is basally flexed sharply, the curvature extending forward into the abdomen beyond the vinculum.

The larva is also characteristic, being an ovoid ‘lozenge' shape with complete rows of small, even-sized spined tubercles, the laterals longer than the dorsolaterals. The colour is usually green with a narrow, complete dorsal band of another colour, mostly white or yellow, but sometimes blue.

Thosea vetusta on oil palm in Peninsular Malaysia (M.J.W. Cock)

'Thosea' obliquistriga in Hong Kong, showing adult resting posture (M.J. Bascombe)

There are nine species in Borneo, six of which belong to a Sundanian subgroup in the genus defined, in the male genitalia, on the absence of a membraneous flap on the valve, a bulging somewhat more heavily sclerotised sacculus, a rather spine-like, slender, straight distal portion to the gnathus (except the trio mentioned below) and an uncus that is sharply divided into a basal, somewhat shouldered setose portion, and a distal, slender, tapering acute one. Three of the six comprise an endemic Bornean trio with a bifid or trifid gnathus.

Of the remaining species one, lutea, is Sundanian with a sister species or group of species in the Lesser Sunda Is. It has strong sinuous furca arms, a distinctive valve shape and central flap. The others are T. vetusta Walker with a very oblique postmedial, sexual dimorphism, a distinctively angled sacculus to the valve and an elongate subcostal flap, and a new species possibly allied to vetusta.

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