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Volume 79, 1951
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[Read before the Wellington Branch, May 30, 1951; received by the Editor, May 30, 1951]

All the known species of the endemic earthworm fauna of New Zealand belong to the family Megascolecidae, and within that family to the three sub-families Acanthodrilinae, Megascolecinae and Octochaetinae. It is with the last of the three sub-families, the Octochaetinae, that this paper is concerned.

The present definition of the sub-family was set down by Stephenson (1915). As set out by this author in his monograph (1930, p. 841) it runs:

“Arrangement of chaetae from pure lumbricine to pure perichaetine. One oesophageal gizzard in one simple segment, or two in two simple segments, or one in a space which represents two or more fused segments; in the last two cases calciferous glands in the region of x–xiii. Excretory system of meganephridia along with micronephridia, or micronephridia alone, the latter never having the form of sacs. Sexual apparatus from pure acanthodriline to pure microscolescine.

“Distribution: Throughout India, but more sparsely in the north; New Zealand; South Madagascar.”

The distribution of the sub-family, particularly the genus Octochaetus, which is found in India and New Zealand, has been widely discussed by biologists. Michaelsen (1921) considered that there must have been a continuous land bridge between India and New Zealand, not passing through Australia, at some time in the past, allowing Octochaetus to achieve its distribution. The postulation of such a land bridge, however, raises obvious difficulties, since if it had existed one would expect to find closer affinities between other elements of the present faunae of India and New Zealand. Michaelsen (1921) split the genus, mainly on geographical grounds, into two sub-genera, O. Octochaetus for the New Zealand species and O. Octochaetoides for the Indian species. Stephenson (1923) suggested that the two sub-genera may be quite distinct and may have evolved separately in the two regions, but in 1930 (p. 844) he allowed the two sub-genera to stand. It seems to the present author that Stephenson's opinion of 1923 gives the most reasonable explanation of the problem. The only essential difference between Octochaetus and a typical “acanthodriline” genus of the sub-family Acanthodrilinae is that the former has a micronephridial excretory system while the latter has a meganephridial excretory system. There is a common tendency throughout the family Megascolecidae for a change from the meganephridial to the micronephridial condition (e.g. in Megascolides and Spenceriella meganephridia and micronephridia frequently are found in the same species) and there is no reason why such a change should not have

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taken place in some New Zealand Acanthodriline genus, giving rise to a genus having the same characters as the Indian genus Octochaetoides. Evidence that such a change can occur in an Acanthodriline genus is found in Rhododrilus disparatus Lee, a species having all the characters of the genus Rhododrilus except that in ii–xx micronephridia are present together with meganephridia.

Five genera belonging to the sub-family Octochaetinae have been recorded from New Zealand. They are:

  • Octochaetus Beddard (1892)

  • Dinodrilus Beddard (1890)

  • Hoplochaetina Michaelsen

  • Conicodrilus Benham (1945)

  • Cryptochaeta Benham (1950)

To these five genera must be added Leucodrilus n.g., described in this paper.

The three genera Octochaetus, Dinodrilus and Hoplochaetina may mark three stages in an evolutionary trend, since they each have the acanthodriline arrangement of the reproductive organs and have respectively eight, twelve, and more than twelve chaetae on each segment. Leucodrilus is similar to Octochaetus except that it has the microscolecine arrangement of the reproductive organs. It might be said that Leucodrilus is then synonymous with the Indian genus Eudichogaster, but since it is apparent that the New Zealand Octochaetinae are distinct from the Indian group, Leucodrilus must be regarded as a genus distinct from Eudichogaster. Here must again be mentioned Rhododrilus disparatus, which by its possession of micronephridia in association with meganephridia in the anterior segments could be included in Leucodrilus. However, as its relationships lie more closely with Rhododrilus, it is better that it should be left in that genus. It does show how Leucodrilus could have been derived independently from the Acanthodrilinae and not necessarily from Octochaetus by the supervention of the microscolecine condition of the reproductive organs.

The genus Conicodrilus was established by Benham in 1945 for species which resemble Dinodrilus in all respects except that the prostatic papillae of Conicodrilus take the form of prominent, relatively long, pointed cones as opposed to the more usual low, rounded prostatic papillae generally found in Dinodrilus. Benham took as the type of his genus Dinodrilus gracilis Ude (1905), which he placed in the new genus together with Conicodrilus kanieriensis Benham (1945). The form of the prostatic papillae varies widely in many genera of earthworms, e.g. in Rhododrilus, one species (R. aduncocystis) has no prostatic papillae, each pore merely opening in the centre of a small brownish area on the ventral aspect of xvii, while in other species of Rhododrilus (e.g. R. macroseptus) the prostatic papillae are prominent dome-like structures. If Benham's differentiation of Conicodrilus and Dinodrilus on the form of the prostatic papillae is accepted, then R. aduncocystis must be differentiated as a new genus although it has all the characters of Rhododrilus. In many other cases new genera would have to be established. Such

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a situation would present many difficulties, and to preserve some degree of uniformity Conicodrilus should go back into Dinodrilus.

Cryptochaeta is a genus established by Benham (1950) for a species, C. microchaeta, which resembles Octochaetus in all respects except that the chaetae are arranged in four very closely spaced couples on each segment, instead of the generally widely spaced chaetae of Octochaetus. Now, according to Stephenson's definition of Octochaetus (1930) the chaetae are lumbricine in arrangement. The same author (1930, p. 13) defines the lumbricine arrangement of the chaetae as follows: “lumbricine—eight chaetae per segment, usually arranged in pairs as in Lumbricus; sometimes, however, they are not paired, the interval between the two chaetae of the dorsal couple, or of both dorsal and ventral couples, being as great as that between the ventral and the dorsal couples.” In other words, provided there are eight chaetae on each segment, the arrangement is lumbricine, regardless of the spacing of the individual chaetae. If spacing of the chaetae is taken as a character of generic importance, most, if not all, genera of earthworms would have to be split and once again a confusing situation would arise. So Cryptochaeta should be included in Octochaetus and C. microchaeta should become O. microchaetus.

There are, then, four genera of the sub-family known from New Zealand. The relationships of these genera are probably best expressed in the following diagram, taking as a root genus the “Notodrilus” ancestor of the whole family Megascolecidae:

Rhododrilus does not belong to the sub-family Octochaetinae, but is included in the diagram as a possible root genus for Leucodrilus.

The New Zealand genera of the sub-family Octochaetinae may be identified from the following key:

a1 One pair of prostatic pores on xvii Leucodrilus
a2 Two pairs of prostatic pores, on xvii and xix
b1 Eight chaetae on each segment Octochaetus
b2 Twelve chaetae on each segment Dinodrilus
b3 More than twelve chaetae on each segment Hoplochaetina
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