Pseudacteon. 427 



short, costal cilia moderately sliort; third vein somewhat strong 

 with the apical part thickened; fourth vein evenly curved. Halteres 

 blackish or brownish. 



f 



Fig. 126. Wing of Ps. formicarum q . 



Female. Quile similar; the ovipositor pointed, black and shining; 

 the hairs at the hind margin of sixth segment conspicuous. 



Length 0,7 to fiilly 1 mm. 



Ps. formicarum does not seem to be common in Denmark, but 

 this is probably only because it has not been saught for in the right 

 way; I have only taken it on Bogø south of Sealand on ^7? — Vs 

 1917 ; I took it here with the net in grass in a garden, and it was present 

 somewhat numerously; I have also seen a specimen in Lyngby Mose 

 on ^Ve 1918. I have, however, never searched for it at the nests of 

 Lasiiis tiiger; it is known to be parasitic on this ant; Wasmann men- 

 tions (Biol. Zentralbl. 38, 1918, 317) that when in summer material 

 from the nest of the said ant is spread out, the females of the Phorid 

 occur numerously, hovering over the worker-ants, rushing down on 

 them to sting them between the abdominal segments and here laying 

 the egg. Donisthorpe has mentioned that the species also chases 

 Lasiiis flaviis^ umbratiis., fuliginosus, Formica sanguinea, Tapinoma 

 erraticum and Myrmica lohicornis, but Wasmann gives good reasons 

 for the opinion that its normal host is Lasius niger, and this author 

 gives 1. c. upon the whole an interesting account of its biology to whicli 

 I refer, My specimens seem to be darker than those originally described, 

 tliey have the legs yellowish to nearly brownish yellow, and the 

 halteres blackish or brownish, while Verrall described both legs and 

 halteres as pale yellow, and so also Strobl; a Dutch specimen from 

 Pater Schmitz is, however, similar in colour to my specimens. 



Geographical distribution : — Resides from Denmark known from 

 Britain, Holland and down into Styria, and it is probably widely 

 distributed. 



