206 ON
ON PLATYMISCHUS DILATATUS,
PLATYMISCHUS THE
PARASITE
OF
A
WESTW.
DILATATUS, SEAWEED-FEEDING
B Y THE H O N .
WESTW., FLY.
SECRETARY.
IT is perfectly true that the old notion of every Moth having its own particular parasite is an exploded theory, (as I stated at Trans. Entom. Soc. 1933, p. 133); yet there certainly are a good many exceptions to such a rule, and the present Hymenopteron is among them. It is parasitic upon one species of Fly only and, because that especial Fly does not (we can not teil why) live in Suffolk, its parasite is equally unknown here. At least, neither has turned Up on our coast; but the main object of this paper is to draw Members' attention to the fact in order that they may make a closer search for both. The parasite is like a small Ant and inconspicuous, as one might expect from its utter lack of wings and small size which never reaches more than one-twelfth of an inch ! But, under a lens, it shows a highly polished and shining black body, girt around the narrow waist by a cosy-looking belt of für, with more or less of the legs and antennal base blood-red. This last feature is especially noticeable in the males, because in only that sex the first antennal joint, next the head, is broadly splayed out almost triangularly, rather like a tea-plate. Platymischus is known from all other genera of the Proctotrypid subfamily Belytinas, to which it belongs, by having :—Head nearly Square and distinctly flat, with neither discal ocelli nor elongate mandibles; eyes circular. Antennae not thirteen-jointed, in male with the first joint flat and very broad. Mesonotal notauli absent; scutellum basally foveate. Legs short and stout; hind tibise slender, and abruptly clavate at the apex. Wings always absent. The only small Hymenopteron that is likely to be confused with Platymischus, because occurring in similar situations, is the Cynipid Clidotoma subaptera, Walk. (Entom. 1932, p. 133), which isinstantly known by its short wings. P. dilatatus was originally described by Prof. Westwood in the Philos. Mag. i, 1832, p. 128 ; and six times since then. It is found nowhere but upon the sea-shore, as Haliday says, " commonly on Algce thrown up by the sea " in Ireland. " It swarms on the rocks and among the sea-weed near Plymouth; I found it also near Torquay, but there it was much less common. It moves slowly, like the Psili [Monelata parvula, Nees], and varies very much in size " from I i to two millimetres (Walker, Entom. Mag. ii, p. 117); Charmouth and Lulworth in Dorset (Dale). Three males, but no females, at Ventnor in I. Wight on 5 October 1920 (Dr. Haines); both sexes are abundant among sea-weed on the Milford Häven coast, but not equally so at the same time : in the early spring the female appears in great numbers, while the male