On Platymischus dilatatus, Westw., parasite of a Seaweed Fly

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

ON PLATYMISCHUS DILATATUS, WESTW.

207

is to be found only sparingly ; towards the end of October the case is exactly reversed (Marshall, E M M . iv, p. 166). " M r . Marshall believes the genus parasitic on Dipterous larvae inhabiting low marshes" (Ashmead, Proc. U S . N a t . M u s . xlv, p. 411 : which sounds like a libel!).

Its life history was for verv long a moot point, not set at rest tili 1920. D r . Kieffer t h o u g h t ( S p p . H y m . E u r o p . x, 894) it probably a parasite of the larvas of Diptera living in decomposed sea-wrack, such as Homalomyia fucivorax, Kieff. and Orygma luctuosa, Mg., the latter of which Flies is British. At T r a n s .

Entom. Soc. 1907, p. 11, I have mentioned that M r . N e w b e r y presumed it to attack the rove beetle Cafius xantholoma, Gr., at Plymouth during M a y 1895 ; and at Berlin Ent. Zeit, iii, p. 98, we find that M r . George Wailes undoubtedly observed a t N e w castle that Platymischus was parasitic on Micralymma brevipennis, Gyl., which is another rove beetle, living partly below sea-level.

Scarborough (Walsh). M y own collection contains specimens captured u p o n Lancing beach in Sussex about 1890 (A. Piffard) ; six taken among C. xantholoma at high-water mark on the beach at Freshwater Bay in I. W i g h t on 31 August 1906 (Frank M o r e y ) ; under a stone on the river-beach at T e i g n m o u t h in Devon during June 1909 (P. de la Garde) ; several f r o m Gerrans Bay on 22 August 1900 and a male f r o m Cremyl in August 1905, in Cornwall (E. A. Newbery). T h e male, in m y experience, is m u c h the commoner sex.

Thus we see that, with the exception of isolated Newcastle and Scarborough, the species' distribution extends f r o m Sussex along the south coast westward t h r o u g h H a n t s , Dorset, Devon and Cornwall to P e m b r o k e on the west Welsh coast. In Ireland, Haibert took several a m o n g the sea-weed Pelvetia canaliculata, Dec. (which occurs also in Suffolk), between limestone flakes in rrioist situations, on t h e rocky shore at Malahide near D u b l i n during September 1916 : indeed, a rocky coast seems necessary to its environment, for I have f o u n d it u p o n neither the Suffolk sands nor the shingle of L y m i n g t o n salterns.

Our somewhat sparse knowledge of the Belytids' economy leads one to expect a D i p t e r o u s h o s t ; and we have seen above that the choice lies between two Staphylinid beetles and Flies of two different families. M r . N e w b e r y strongly favoured the f o r m e r and wrote that " M r . Keys has sent m e bags of tidal refuse f r o m time to time, containing chiefly C. xantholoma b u t never any IJiptera, f r o m Batten near P l y m o u t h ; these bags always contained • dilatatus in n u m b e r s . I have myself collected at Batten and Cremyl, and in both I have always f o u n d the two insects associated in sea-weed. T h u s , t h o u g h I have not actually b r e d them, I feel no doubt as to their parasitic connection. It should not be f f i c u l t t 0 isolate a certain n u m b e r of C. xantholoma and actually

208

O N PLATYMISCHUS DILATATUS, WESTW.

find if the Platymischus emerge from that beetle " (in lit. 17 Dec. 1917). On the other hand, Mr. Lyle later wrote (in lit. 14 Feb. 1918) that he had " bred it in numbers during mid-December 1912 from the puparia of a Dipteren that had been found the preceding September under decayed seawced on the beach at Osmington Mills in Dorset," which Fly appears to have been erroneously named Orgyma luctuosa, Mg. (cf. EMM. 1920, p. 153, bracket). Hugh Scott's further breeding (loc. cit.) conclusively proves that this parasite emerged from one or other of two allied Flies, which were collected in September 1919 at Durlston Bay near Swanage in Dorset, Coelopa pilipes, Hai. or Fucomyia gravis, Hai. : the former I have taken in numbers at Ryde in I. Wight without seeing our Platymischus there, the latter has been sent me commonly from Penzance where no Hymenoptera were observed ; but, be it noted, neither is known in Suffolk. Since these Flies of the family Phycodromidas swarm in myriads on all rocky coasts, and none of that family's species occur throughout rockless E. Anglia, we need have no hesitation in accepting this Dipteron as the host of P. dilatatus, whose absence along with that of its single host goes to prove parasitism restricted to that host.

W I L D - F L O W E R S ' L O C A L I T I E S . — T h e following 1 9 3 3 additional habitats to Hind's Flora were sent in too late for insertion in our last Transactions, and have been selected from an interesting and much longer list:—Ajuga reptans, L., flore albo, Fakenham on 28 May ; Butomus umbellatus, L., Knettishall by the Ouse on 26 June ; Verbascum Blattaria, L., Elvedon to Culford road on 28 June ; the fine Linum perenne, L., in NC. Suffolk ( l o c a l i t y not to be bruited abroad !). Solidago virganrea, L., denied habitats

by Hind, is surely uncommon elsewhere than in merely the SW .: e.g. Palgrave and also it gilds a rod of ry-embankment uncomfortably near one of our towns.—E. H. K I R K B Y .

Mr. Powell reports o n 1 August the sudden o c c u r r e n c e of Datura stramonium, L., over three feet high and in good fruit, growing in the keeper's garden at Staverton Thicks in ButleyThis plant seems often to spontaneously generate ; it a p p e a r e a thus in a piece of meadow taken into her Semer garden by Mrs. Donkin ; in Mr. Bevan's garden at Tye House in H a d l e i g h ; a similar spot in Blakenham Magna ; and in 1 9 3 3 Miss R a w l i n s had the same experience in Layham.—R.B.