The Syntominae and Euchromiinae were placed as subfamilies of the family Ctenuchidae by Watson, Fletcher & Nye, (1980) following the arrangement of Forbes (1960), who used Euchromiidae as the family group name. This family was also recognised by Inoue et al. (1982) with no subfamilial divisions. The group as a whole, with the possible exception of Ctenucha Kirby itself, consists of taxa with elongate forewings, reduced hindwings, and often banded abdomens, probably mimicking day-flying Hymenoptera. Ctenucha, Euchromia Hübner and all Bornean syntomines have veins Sc and Rs completely fused in the hindwing.

Minet (1986) accepted that presence of a metepisternal tymbal organ was the strongest definitive apomorphy for the Arctiinae and, on the strength of it, included the Euchromia group and the Neotropical Ctenuchinae in the Arctiidae. The syntomids have reduced or atrophied tympanal organs and lack a tymbal organ so he excluded them as the family Syntomidae. He also noted that larvae of the first two groups had heteroideous crochets, a derived feature in the Arctiidae, whereas syntomid larvae had homoideous crochets, the plesiomorphic condition. He regarded this larval condition as suggestive that tymbal organs have never been present in the syntomid lineage rather than secondarily lost.

However, syntomid females possess paired, often dichotomous dorsal glands associated with the ovipositor lobes of the female genitalia as do Euchromia species and other arctiids. Thus, in view of the uncertainty over whether the syntomids have lost or never had tymbal organs, this character of the female genitalia is used to place them at subfamily level, Syntominae, within the Arctiidae. The characters discussed by Minet point strongly to paraphyly within the Ctenuchidae as recognised by Watson, Fletcher & Nye (1980), hence Euchromia is perhaps best placed in its own subfamily, Euchromiinae, until its relationship to Ctenucha can be ascertained. The fusion of Sc and Rs in the hindwings is, however, a potential synapomorphy for the broader concept of the Ctenuchinae including all the Ctenuchidae of Watson, Fletcher & Nye (rather than the genus Ctenucha and allies, with most Neotropical taxa possibly assigned to the Euchromiinae on grounds of other shared hindwing venation features), and also the Thyretinae (see below).

Support for this fusion of veins as a synapomorphy is provided by consideration of the hindwing venation of other wasp-mimetic groups such as the Sesiidae and Zygaenidae where the area of the hindwing has also been reduced. In the Sesiidae veins Sc, R1 and Rs on the costal margin of the hindwing are greatly reduced, closely parallel, and concealed within the upfolded costal edge, not fused or missing (Duckworth & Eichlin, 1974). In Zygaenidae with narrowed hindwings and syntomine build (e.g. Tarmann, 1984) Sc and Rs are usually separate distally though they may be fused for some distance within the cell. Therefore neither of these groups has developed a parallel fusion of veins to that seen in the arctiid groups.

Ctenucha has all median and anterior cubital vein branches present in the hindwing; two anal veins are present. In Euchromia two anal veins are also present but M2 and one of the anterior cubital branches (possibly CuA2) have been lost, or CuA1 and CuA2 are fused (Fig. 1). In the Syntominae M1 is lost or fused with Sc + Rs, and there is only one anal vein; M2 is lost, and one of the anterior cubital branches may also be lost (Figs. 1-6). Thus venation reduction in the hindwing consequent on its overall reduction in size in these wasp-mimetic groups has followed a somewhat different course in the Syntominae and Euchromia.

The African Thyretinae, placed as a subfamily of the Notodontidae by Minet (1986), are undoubtedly arctiids, having a tymbal organ (A. Watson, pers. comm.) and a typically arctiid larva (J . E. Rawlins in litt. to A. Watson). The female genitalia (Thyretes Boisduval examined) have a pair of short, broad, slightly branching dorsal glands to the ovipositor lobes, another arctiid feature. In the hindwing venation Sc, Rs and M1 are usually fused as in Syntominae, with M2, M3, CuA1 and CuA2 arising independently from around the posterior angle of the hindwing cell. There is only one anal vein. Hence the Thyretinae may be sister-group to the Syntominae. However, in some thyretine genera, such as Pseudapiconoma Aurivillius, M1 is present connate with Sc + Rs as in Euchromia (though with only one anal vein). The venation of several genera was illustrated by Hampson (1898), who included the group within his concept of Syntominae; this also included the Ctenuchinae and Euchromiinae.

Only Euchromia of the Euchromiinae is present in the Bornean fauna; the full composition of the group remains to be investigated.

There are, however, numerous taxa of the Syntominae. The work of Obraztsov has done much to establish firm definitions of the genera using genitalic characters, though his work did not cover all Oriental groups in the subfamily.

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