290 THE PRACTICAL APPLICATION OF ECOLOGY 



The discovery by Lai (1934), in Edinburgh, of two biological 

 races of the common hemipterous insect Psylla mali, is of con- 

 siderable interest. The forms P. peregrina of the hawthorn and 

 P. mali of the apple have hitherto been regarded as two separate 

 species. Lai has shown that the imagines of both forms are 

 identical and the only differences are those shown by the nymphs. 

 In peregrina the last instar nymph has a brown streak on the 

 developing wings which is absent in mali. Also, in peregrina 

 the seta? surrounding the abdominal margin are all of one kind — 

 long and spatulate : in mali they are of two types, long, pointed, 

 variously curved setae and small setae of spear-like shape. In 

 peregrina the dorsal surface bears round tubercles which are 

 replaced in mali by very minute setae, which cover the body of the 

 latter. The two forms are closely confined to their individual 

 food-plants : they do not interbreed and neither will oviposit on 

 the host of the other. The nymph of either, when transferred to 

 the food-plants of the other, does not survive long. In view of the 

 foregoing evidence, it is pointed out that P. mali Schmidt has a 

 race peregrina Forst which has previously been regarded as 

 another species. This example, wherein similar adults betray 

 nymphal differences, affords an instance of what Giard termed 

 poecilogony, which is a rare phenomenon among insects. 



So far the subject of biological races has only been considered 

 with reference to their expression in differences of food-preference. 

 Evidence of such races with very different characteristics requires 

 mention. Thus, the moth Ephestia kuhniella appears to occur as 

 two strains or races with marked different rates of development. 

 Payne (1933) found that the slow strain was able to complete its 

 developmental cycle between 8° and 34° C. and the fast strain 

 between 10° and 32° C. A number of observers had previously 

 noted marked differences in the rates of development of larvae 

 hatched upon the same day. The most recent worker, Ahmad 

 (1936), has confirmed Payne's observations and points out that 

 this insect is only susceptible to parasitisation by Nemeritis 

 during the last larval and pupal stages, but the mixture of strains, 

 he states, results in the continuous presence of the host in the right 

 condition for parasitisation. 



