S a 1 m o n , On Specialization of Parasitism in the Erysiphaceae. 265 



and it was then found that in the majority of instances the conidio- 

 phores bore only a Single spore, which frequently could be seen, 

 under the microscope, apparently in the process of becoming deta- 

 ched. Occasionally, however, a conidiophore occurred which bore 

 two conida at its apex. The conidiophores of the Oidium produced 

 by the sowing of these conidia on seedling plants of T. pratense 

 (see Table 1 3) bore, however, as a rule, chains of 4 - 8 spores. 

 Occasionally only a Single spore was found, but in all probability 

 such represented the last remaining spore of a chain, or the first 

 of a chain beginning to be formed. The ripe conidia feil off, on 

 the leaf being shaken, in chams of 4—8, as well as singly. Spores 

 separated from the conidiophores but still held in chains were readily 

 obtained by shaking a leaf with powdery masses of spores over a 

 dry glass slide; the chain of spores separated at once on water 

 touching it. Similarly, the Oidium obtained on Pisum arvense by 

 the sowing of the conidia of the Oidium on P. sativum (see Table 14) 

 bore on the eleventh day after inoculation conidiophores producing 

 spores either singly or in chains of 2—4. It may be that the 

 number of spores, i. e. whether the spores are borne in chains or 

 singly, is determined by favourable or unfavourable external conditions. 

 It is, however, possible that the Oidium of E. Polygoni, like that of E. 

 Cichoracearum ^ always produces, when fully grown, a chain of 

 spores, and that my previous drawings of the former, in which the 

 conidiophores are shown as bearing only a Single spore at their 

 apex (see 2, p. 187, PL 9, ff. 1, 2, 8) represent the immature 

 stage only. 



A biological fact of some interest may be mentioned here. This 

 is the apparently frequent association of the larva of some Dipterous 

 insect with the Oidium of several species of the Erysipheae. 

 I first observed these larvae in the Spring of 1902 feediag on the 

 Oidium of Sphaerotheca mors-uvae on some gooseberries sent from 

 Ireland. A larva was carefully watched, at intervals, for a day, 

 during which time it was observed to be feeding exclusively on the 

 conidia of the Oidium. Its method was either to feed upon the 

 little powdery masses of loose spores or to break off the conidio- 

 phores near their base and then draw the whole chain of spores 

 into its mouth. Identical larvae were afterwards observed, dming 

 August and September, to be very commonly associated with the 

 Oidium of E. Graminis on grasses growing in the Botanic Gardens 

 at Cambridge. Theii^ habits were watched, and it was again found 

 that they were feeding exclusively on the conidia of the Oidium. 

 The larvae were seen with the Oidium on several species of Bromus 

 (B. interruptus, B. hordeaceus etc.), on Festuca elatior var. pratensis, 

 on several species of Avejia, and on one or two other grasses. 

 Later, the same larvae were sent to me fi'om Albourne, Sussex, 

 feeding on the Oidium of E. Polygoni on Pisum sativum. At the 

 end of September I observed the larvae feeding on the conidia of 

 Sphaerotheca pannosa on Roses at Reigate, Surrey» It certainly 

 appears as though the Erysiphaceae are commonly attacked on the 

 one side by an animal, as they are on the other side by the vege- 

 table parasite Ampelomyces quisqualis Ces. Some larvae were 

 reared, and the chrysalis State obtained, but the imago has not yet 



