Much of the taxonomy of the Drepanidae, has in the past, been inaccurate and very incomplete. This study has attempted to remedy the deficiency in the genus Ditrigona Moore.

The genus Ditrigona has been entirely reconstructed such that the number of species and subspecies has been increased from two to 52 by the addition of 18 new species and five subspecies, and the amalgamation of three other genera–Peridrepana, Leucodrepana and Leucodrepanilla which are considered to be junior synonyms of Ditrigona. A number of species have been transferred from the family Geometridae where they were incorrectly placed.

A complete bibliography is given for each species, together with information regarding the value and accuracy of references. The difficulties in nomenclature have been rectified.

All existing type material has been examined. In the case of D. quinaria, where the original type no longer exists, a neotype has been designated. Where syntypes occur lecto-types and paralectotypes have been designated. The necessity for authors to nominate holotypes and lectotypes has been demonstrated in this work. In a number of instances a series of syntypes have been shown to comprise more than one species.

There is much external resemblance between species in the genus and diagnosis can often only be made by reference to the genitalia–usually the male genitalia provide the most valuable characters. The genitalia of each species have been examined for the first time. Microscope slides of the male and female genitalia have been prepared and figured.

Descriptions of new species have hitherto been inadequate. A full description is given for each species, species-group and the genus. A diagnosis follows each species description in order to separate the species from those with which it is likely to be confused. Keys are provided to facilitate identification, and 97 characters for each species are tabulated– these were used in the numerical taxonomy.

The distribution of Ditrigona is centred in the Indo-Chinese subregion of the Oriental region where the Drepanidae are considered to be most prolific. No species from this genus are known from South India, Ceylon and the Malay peninsula where the genus Teldenia Moore occurs. Distribution maps are given.

Ditrigona is a large genus and as such has a diversity of species. They are divided into four species-groups based on adult morphology. Whilst diverse the species comply with the conditions of the generic description and are all seen to belong to Ditrigona.

The Ditrigona species-group derocina comprises three very closely allied species, but the taxometric results show them to have a comparatively low phenon level with the other groups.

Until now there has been much confusion over the identity of D. pruinosa, sericea, quinaria and obliquilinea. These species are here clearly defined.

The hind wing ‘tail’ modification of D. triangularia and regularis is thought to be a convergent trend which does not necessarily indicate a close relationship between them, and is most unlikely to be a generic character as has once been suggested. The numerical taxonomy shows the two species to be distantly related.

The mytylata species-group, being so large, can be regarded as consisting of two subgroups typified by mytylata and virgo with berres as the link between them. Species in the mytylata subgroup fall into two categories–those with buff fasciae on the one hand, and those with greyish brown fasciae on the other.

Four species–D. qidnaria, obliquilinea, lineata and conflexaria–are divisible into subspecies. The subspecies are diagnosed mainly on the basis of small but consistent differences in the male genitalia, which are also characteristic of different regions.

Variation has been shown to occur in Ditrigona. Geographical variation occurs in D. quinaria which has a wide distribution–its movement probably occurs along mountain ranges. The valleys may form a barrier preventing the intermingling of populations thus bringing about the development of the five subspecies.

Seasonal variation is exhibited by D. typhodes and D. quinquelineata which both have two distinct forms–one in spring and early summer, the other in late summer and autumn.

Sexual dimorphism occurs in the antennae and the genitalia. In D. typhodes the males are much more lightly and finely speckled with grey than are the females.

Considerable individual variation occurs in D. sericea which although being a widespread species its variation does not appear to be related to geographical distribution. D. virgo is similarly variable.

The findings of numerical taxonomy have, in the main, supported the classification made by using orthodox taxonomic methods, with a small number of interesting exceptions.

There has been no weighting of characters and this decision does not seem to have affected the majority of species. However, a case could be made in favour of weighting, particularly when consideration needs to be given to aspects of phylogeny and evolution.

Numerical taxonomy only correlates known measurable characters whilst orthodox taxonomy also takes into consideration evolution and phylogeny, which are mainly speculative. Thus the results produced by the two methods may be different, but used together the study of classification will have greater stability.