View Image Gallery of Tribe Orthostixini

This family-group concept is based on the western Palaearctic genus Orthostixis Hübner, a smaller, more delicate taxon than Naxa, but where the wings are white with a similar distribution of dark grey spots. The male antennae in Orthostixis are weakly serrate rather than bipectinate in both sexes, as in Naxa, but the forewing radial system includes an areole between R1 and Rs based well short of the angle at the apex of the cell whence M1 branches off. In Naxa the areole is relatively larger, with R4 and R5 connate at its apex (Bornean species; Fig 4) rather than arising from it on a long common stalk. The hind tibia in Orthostixis has one pair of short spurs: these are lacking in Naxa (Inoue, 1961).

The male genitalia of Orthostixis have simple valves somewhat as in some Boarmiini. The juxta is elongate, flask shaped. The uncus is expanded gently towards the apex, finely setose. The aedeagus is a simple tube with no cornuti in the vesica. There are no setae on the third sternite. The genitalia in Naxa are similar, but the valves lack robust setae along the costal margin, the uncus is slender, acute, and the gnathus, strongly present in Orthostixis is absent.

The signum in female Orthostixis is ennomine-like, elliptical, with a serrate margin. In Naxa it is weaker, more irregular.

The larva in Orthostixis has the setae prominent as in Naxa. A full description of larva and pupa was published by Rebel (1903: 260). The larva is
stout, with strong bulges and folds, the setae set on tubercles. The host-plant is Scutellaria (Labiaceae). The pupa is bone white with a black pattern and yellow spots as in Naxa.

There are thus a number of features to suggest a relationship between Naxa and Orthostixis, such as the facies, areole, general structure of male and female genitalia, strong larval setation and striking pupal markings, but no really unambiguous synapomorphy. Neither genus should be placed in the Oenochrominae sensu stricto.

Biology: Orthostixinae
The mature larva, pupa and host-plant of Naxa guttulata Warren were observed by the author and colleagues at Poring Hot Springs during March, 1997 (See Systematic Account). Three pupae retained emerged as adults and the specimens are in the collection of Universiti Malaysia Sabah. Whilst congeners have been reared from Oleaceae (Holloway 1996a), guttulata was found feeding on the fronds of large ferns growing on the forest floor. Only one species appeared to be utilised, identified (Chew Ping Ting and J.M. Camus, pers. comm.) as Diplazium ?megistophyllum (Copel.) Tagawa (Athyriaceae). The appearance of larva and pupa and their occurrence in a small colony suspended in a web under the arches of the fronds, is exactly as described for congeners, the pupae suspended head upwards in the web. Both are illustrated below. Only one web was occupied but a few others were observed on the same fern species in the vicinity. An adult disturbed from the undergrowth came to rest on the underside of one of the fronds of the tenanted fern. Fern-feeding is an unusual and specialised habit in the Lepidoptera, only known otherwise in the Geometridae in the ennomine tribe Lithinini.

Larva (left) and pupa (right) of Naxa guttulata Warren in webbing under the arch of a frond of a Diplazium fern at Poring Hot Springs (photos by Dr D.J.L. Agassiz). 

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