Oniscus asellus Linné (The "Common Slater.") [Plate IX.]
1761 Oniscus asellus Linné (41), p. 500, No. 2058.
1792 Oniscus murarius Cuvier (9), p. 22, pl. XXVI.
1838 Oniscus fossor Koch (34), part 22, pl. XXII.
1868 Oniscus asellus Bate and Westwood (1), p. 468.
1868 Oniscus fossor Bate and Westwood (1), pp. 471-2.
1898 Oniscus asellus Sars (59), p. 171, pl. LXXV.
Oniscus asellus is one of the largest of our woodlice and it is also probably the commonest, though Porcellio scaber is in many places quite as abundant. The body of Oniscus is broad and expanded and the colour is usually a slate grey with yellowish markings more or less regularly arranged.
From the genus Porcellio the species with which we are concerned is at once distinguished by the three jointed flagellum. Porcellio has but two joints and has, besides, a prominent lobe projecting from the middle of the head, which is not seen in Oniscus. Philoscia, although it has three joints to the flagellum, has a narrow abdomen and lacks entirely the lateral lobes which are a feature of the other genera of Oniscidæ.
Oniscus fossor of Koch (34) was recognized by Kinahan and by Bate and Westwood as a species. Dr. Scharff submitted specimens to Professor Budde-Lund who found no differences between them and Oniscus asellus. The former (63) mentions, however, that the characteristics of the supposed species are those of young examples of Oniscus asellus, and Professor Sars (59, p. 173) seems to be of the same opinion. Many young examples of Oniscus asellus that we have examined have a curious whitish transverse band owing to the light colour of the dorsal plates of the first abdominal segments. The flagellum also does not seem to shew in young animals a distinct division into three joints.
BRITISH LOCALITIES:—
England: High Beach, Epping, including an albino; Maldon; Brightlingsea; Iver; Hanwell; Eton; Kew; Pamber Forest; Kingston-on-Soar; Bluebell Hill, Maidstone; (W.M.W.): Lynmouth; (W.M.W. from J.T.C.).
Scotland: (Scharff, 63). Dinnet, Aberdeenshire; (W.M.W. from Madame Christen).