70 Belladonna Leaf-Miner 



feeding maggots and liberated in cages containing potted plants of 

 belladonna. Again the results were quite negative. From the slight 

 evidence thus obtained one would not be prepared to jump at a general 

 conclusion, but it may just be possible that within the limits of a single 

 polyphagous species, certain well-defined " biologic " species may be 

 established, each of which shows a marked tendency towards one of 

 their food plants. Consequent on this preferential adoption of a host, 

 slight variations may arise such as in the colour, a fact which at one 

 time led to the establishment of the variety hetae as a species distinct 

 from hyoscymni. As has been already noticed the imagos of hyoscyami, 

 the larvae of which have had henbane and belladonna for food plants, 

 are distinctly lighter in colour when compared with those the larvae of 

 which have fed on the leaves of beet and mangold. 



At Dartford in Kent, where henbane and belladonna are grown 

 on a large scale for the sake of their alkaloid bases, it has been found 

 that, whereas in some years as much as 80 per cent, of damage is done 

 to the former crop by the maggot, the latter remains unaffected, although 

 in close proximity. It would appear that the fly has become thoroughly 

 established on the henbane to the exclusion of the allied belladonna. 

 Of course, it may be that some definite organic substance specific to 

 heiibane and not present in belladonna is chemotropic to the fly, 

 which would account for its ovipositing on the one host rather than on 

 the other. When henbane is absent belladonna proves quite attractive. 



Much interest, again, is derived from the question why P. hyoscyami 

 should select members of the widely different families of Chenopodiaceae 

 (beet, mangold) and Solanaceae (henbane, belladonna) as its host 

 plants. Is it possible that they all exert the same sort of chemical 

 stimulus inducing the one species to oviposit on any of them ? Dr 

 Trilgardh (p. 116) says : " If the food of the larva consists of several 

 species of one and the same genus, or of different genera within one 

 or several families, then it is an organic union, or group of such, common 

 to all these, to which the species reacts positively." Again he says 

 (ibid.) : " The odour of organic matters [to which the flies orient them- 

 selves and are attracted^] is due to the occurrence of certain specific 

 chemical combinations, e.g. organic acids, amines, terebines, phenols, 

 glycosides, etc. which are characterised by a certain structure and strati- 

 fication of the atoms." One can easily perceive the validity of the 

 argument where one is confined to a consideration of a single vegetable 



^ The italics aie mine (A. E. C). 



