1833 _Platyarthrus hoffmannseggii_ Brandt (3), p. 174, pl. IV., fig. 10. 1844 _Itea crassicornis_ Koch (34), part 36, pl. V. 1860 _Typhloniscus steinii_ Schöbl (66), p. 282. 1868 _Platyarthrus hoffmannseggii_ Bate and Westwood (1), p. 464. 1898 _Platyarthrus hoffmannseggii_ Sars (59), p. 175, pl. LXXVI., fig. 2. Up to the present this is the only woodlouse which has been found in the nests of British ants. It is small and oval, its colour is white, and its body is covered with tubercles. The edges of its side plates are toothed, its flagellum has but a single joint and it has no eyes. Miss Kate Hall tells us that, if very hungry, ants in captivity will kill and eat _Platyarthrus_. With regard to its own food, Lord Avebury has favoured us with the opinion that it lives on the spores of the lower plants, such as would be found in the ants' nest. [Illustration: FIG. 46.--FLAGELLUM AND LAST PEDUNCULAR JOINT OF THE ANTENNA OF _Platyarthrus hoffmannseggii_.] _BRITISH LOCALITIES:--_ _England_: Warley; Hanwell; West Drayton; Langley; Kingston-on-Soar; Bluebell Hill, Maidstone; (W.M.W.): Berkhamsted; Salcombe; Devon; Cheddar Cliffs, Somerset; (Norman, 49): Ide, near Exeter; (Parfitt, 53): Torquay; (Stebbing in 49); Lulworth Cove; (Rev. A. R. Hogan _teste_ Bate and Westwood, 1): Hammersmith; Oxford; Berry Head, Torquay; Plymouth; (Bate and Westwood, 1): In the nest of _Myrmica rubra_, Newton Ferrers (E. E. Lowe). _Scotland_: Banff; (Thomas Edward in 49). _Ireland_: Leixlip, Co. Dublin; Lismore, Co. Waterford; Glengariff, Co. Cork; (Scharff, 63): Bagenalstown, Co. Carlow; (64). _FOREIGN DISTRIBUTION:--_ _Europe_: France; (28): Spain; (12): Denmark; Germany; Holland; Bohemia; Austria; Tyrol; Helvetia; (59). * * * * * NOTE.--In the genera which follow, air-tubes or air-cavities (tracheæ) are present in the outer plates of the abdominal appendages, 1 and 2, or 1 to 5. The appendages in question have in consequence a milk-white appearance in the living animal owing to the fact that the enclosed air reflects white light. Considerable interest attaches to the study of these tracheæ, which have the same function as those of insects, but which have been independently developed. To emphasise the latter fact the structures are often termed "pseudotracheæ." _Genus_--=PORCELLIO= Latreille, 1804 (37), p. 45. _Flagellum, with two joints; abdomen, broad; frontal lobe projecting._