220 The Museum Gazette 



whitish substance, should be examined under a microscope, 

 when the fructifying branches of the fungus will be easily 

 seen. It is supposed that the spores are carried in the air 

 to other flies, but it is possible, that, as has been lately shown 

 in the case of the potato disease, the mycelium may be 

 latent in its host until favourable climatic conditions prevail. 

 It is noteworthy that diseased flies apparently congregate 

 before death. In August, two years ago, we found large 

 numbers of a species of Horse-fly 1 on the flower-heads of 

 the common Plantain (P. lanceolata) , on each flower were 

 resting ten to thirty corpses. 



The Horse-flies are carrion feeders : it may be that flies 

 feeding habitually on carrion are more prone to the disease. 



The fungus of the house-fly is known as Empusa (Sporodo- 

 nema) muscce. Other fungi of the same genus attack wasps 

 and aphides. 



Spangle Galls. 



The spangle galls on the underside of oak leaves are very 

 beautiful objects, and should be carefully observed with a 

 pocket lens, or better still a microscope fitted with a low- 

 power objective. There are only three or four British species. 

 These galls are the alternate generation of the currant and 

 pea galls of June and July ; and from them only female flies 

 will emerge (in March and May). 



The commonest is Neuvotems lenticularis . It is button-like, 

 almost flat, but very slightly raised in the centre, and is 

 ornamented with reddish star-shaped hairs. In some seasons it 

 is extraordinarily abundant ; in August, 1904, we counted 286, 

 379 and 326 galls on three leaves from the same oak. The 

 alternate generation is the currant gall (Spathegastev baccavum) 

 appearing on the male inflorescence of the oak in May. 



The smooth spangle (N. Icevinsculus) has a knob in its 

 centre and the edges are slightly turned up. It is either 

 quite smooth or with hairs around the umbo. The spring 



1 Metanomostia mellinum. 



