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Volume 42, 1909
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Art. XVI.—On an Isopod inhabiting Ants' Nests in New Zealand.

[Read before the Philosophical Institute of Canterbury, 3rd November, 1909.]

It has long been known that an isopod, Platyarthrus hoffmannseggii, Brandt, is found associated with several species of ants in England and Europe. It has been described by Bate and Westwood,* Schoebl,† Webb and Sillem,‡ and others. Lord Avebury§ has also published the result of his observations and experiments upon it, and has suggested that it acts as a scavenger in the ants' nest.

When I was preparing my paper on the terrestrial Isopoda of New Zealand in 1900, I heard from Mr. W. W. Smith that he had found two specimens of an isopod supposed to belong to the genus Platyarthrus in ants' nests in New Zealand, and that these had been forwarded along with the ants to specialists in Europe. My efforts to trace these specimens were

[Footnote] * “British Sessile-eyed Crustacea,” 11, p. 464.

[Footnote] † Sitzungsb. d. k. Akad. d. W. math. naturw. Cl. xl Bd., No. 9, 1860, p. 279.

[Footnote] ‡ “The British Woodlice,” 1906, p. 30.

[Footnote] § “Ants, Bees, and Wasps,” 16th edition, 1902, pp. 75, 90, and 407.

[Footnote] ∥ Trans. Linn. Soc., 2nd ser., Zool., vol. viii, p. 100.

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unsuccessful, and at that time I could merely note their occurrence in ants' nests. In 1902 Mr. J. MacMohan sent me numerous specimens of an isopod found in association with the nests of ants in the Rai Valley, in Marlborough, and later on Mr. Smith sent me similar specimens from New Plymouth. Although these specimens were undoubtedly associated with the ants, it was not quite certain that they were habitual guests in the nests, and I therefore waited for further evidence. Recently, however, Mr. Smith has found other specimens undoubtedly living in the ants' nests,* and associated with two species of ants—viz., Amblyopone cephalotes, Smith, and Huberia striata, Smith; and I am therefore now able to describe the isopod. It proves to belong not to Platyarthrus, but to Trichoniscus; it thus belongs to a different family of terrestrial isopods, and it is interesting to note that we have here the same habit of life arising independently in two quite different isopods. It is rather larger than Platyarthrus hoffmannseggii, being about 5 mm. in length, and, though very much lighter in colour than the ordinary terrestrial Isopoda, it is not quite white, but is generally marked with bands or patches of pale brown on a white ground. Moreover, it is not blind, but possesses fairly perfect eyes, provided with, apparently, the normal amount of pigment. The description of the species is given below.

Trichoniscus commensalis, sp. nov.

Body rather broadly oval, fairly convex, pleon not abruptly narrower than the peræon; whole dorsal surface thickly covered with spiny tubercles, which are arranged more or less regularly in transverse rows—viz., one row along the posterior margin of each segment, and two or three much more irregular rows on the anterior part of each segment. In the pleon there is a fairly well-marked row along the posterior margin of the third segment, the other portions of the dorsal surface of the pleon usually bearing only minute spines.

First segment of peræon the longest, and produced into two rounded lobes reaching as far as the eyes; the last segment of the peræon with the postero-lateral angles produced so as to include the first three segments of the pleon, and to reach almost to the end of the epimera of the 3rd segment; in the pleon the 3rd, 4th, and 5th segments bear fairly well-developed epimeral portions. Terminal segment with the sides concave, the angles rounded, posterior margin straight or very slightly convex and bearing four small spines.

Eyes fairly well pigmented, formed of three ocelli, fairly close together.

Antenæ stout, especially the penultimate segment of the peduncle, which is half as broad as long, the last segment longer than the preceding, and bearing fine setæ on the outer side and roughened tubercles on the inner; the flagellum as long as the last segment of the peduncle, composed of five joints, the last bearing a pencil of fine hairs. The uropoda fairly stout, outer branch stouter but not much longer than the inner, both covered with fine setæ and bearing a tuft of longer setæ at the extremity. In the last pair of legs the ischium is produced on the outer side into a prominent triangular process or plate bearing one or two stout setæ.

Colour white with pale-brown markings, usually more or less arranged in longitudinal lines; the colour, however, varies very considerably, some

[Footnote] * Mr. Smith says that on on occasion “the ants were in the act of carrying off the specimens when I took them from them.”

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specimens having the dorsal surface almost wholly covered with brown markings.

Length, 5 mm.; breadth, 2 mm.

Hab.—New Plymouth and Mount Egmont, in nests of Amblyopone cephalotes and Huberia striata (W. W. Smith); Rai Valley, in nests of ants (J. MacMahon). Probably widely distributed in the North Island and in the north-western portion of the South Island.

The specimens from Greymouth collected by Mr. R. Helms, which I had previously referred with hesitation to T. otakensis,* belong to the préent species. I have specimens also from Swanson, Auckland (H. Suter), and one from Kapiti Island (E. A. Newson). The last specimen is much browner than those actually taken in ants' nests by Mr. Smith and Mr. MacMahon, and it is possible that it was not living in association with ants. There is the same doubt with regard to the Greymouth and Auckland specimens.

[Footnote] * Trans. Linn. Soc., 2nd ser., Zool., vol. viii, p. 117.