A. E. Cameron 71 



family, but in the present case of the two families Chenopodiaceae and 

 Solanaceae, it lays itself rather open to attack, unless one extends the 

 content of " specific chemical stimulus " to mean not merely that 

 exerted by any one organic substance, but rather by certain of them 

 wliich have similar groupings of thcii- component atoms {i.e. specific 

 kmd of chemical stimulus). Such for instance are the nitrogen bases 

 of which betaine, guanine, liyoxanthine in the beet, atropine in the 

 belladonna, and hyoscinc and hyoscyamine in henbane, all possessing 

 marked pliysiologicsil or toxic properties, are well-known examples. 

 But it must not be inferred that these substances aie attractive to 

 P. hyoscyanii, foi tlieic is no direct proof on this point, but, hypotheti- 

 cally, their jiiesence in these different plant species would satisfactorily 

 explain the varied food habits of the larva. 



Since completing this paper, Dr Imms suggests to me the possibility 

 of hetae being one species confined to the Chenopodiaceae and Ityoscyami 

 a second species confined to the Solanaceae. The possibility of their 

 being physiological species, which have undergone more or less morpho- 

 logical separation, appears to him as well as to myself, on the sum total 

 of evidence, to be quite a feasible and justifiable proposition. 



Summary. 



The species P. hyoscyami has been recorded at various times by 

 various authors, and it has often been described under different names, 

 partly because of its having been reared from a fairly wide range of 

 food plants. The belladonna leaf-miner is the larva of this species, 

 found during the summer throughout Europe, United States of America 

 and Canada. 



The injury to the plant consists in the destruction of the paren- 

 chyma which the maggot greedily devours, the leaves assuming a 

 blistered appearance in consequence. The leaves thus attacked quickly 

 flag and wither during dry weather. In this way excessive damage to 

 the various food plants often results in their total loss, heavily affecting 

 the agriculturist pecuniarily where they are grown as cultivated crops. 



Other food plants besides belladonna are mangolds, beet and henbane. 



The number of the larvae in one leaf varies wath the size of the latter 

 and, roughly speaking, directly as the size. 



The ravages are periodic and often quite localised, resulting in 

 diminished yields of the products of the different crops attacked. The 



