154 THE SCOTTISH NATURALIST 



grown spun up in corners of the cages and remained there 

 during the winter, as caterpillars under a silk covering. It is 

 hoped to obtain from these adult saw-flies during the coming 

 spring. 



Parasitism. — This formed the most interesting result of 

 the observations on this insect. It was found that several 

 larvae towards the end of August, while having the character- 

 istic orange colour of the full-fed caterpillar, were very 

 much undersized and smaller than those previously obtained. 

 These smaller larvae wandered aimlessly around the cages 

 and eventually spun up in corners. This went on regularly ; 

 the young forms fed and reached a certain stage and then 

 finally spun up long before they had attained the size of a 

 normal full-fed caterpillar. Such a larva was isolated in 

 a box, and after a few days the contents of the box were 

 found to be a shrivelled larval skin and a dipterous cocoon, 

 which in due course opened and a Tachinid fly issued. 

 Tachinids issued from saw-fly cocoons in the cages at 

 intervals of two to three days, from 19th August till ist 

 September. This was clear evidence that the undersized 

 and sickly appearance of the larvae was due to parasitism. 

 They had evidently been parasitised before being brought 

 into the laboratory. The parasite was identified by Major 

 Austen of the British Museum as Ptychoinyia selecta, Meigen. 



This parasite probably also accounts for the great 

 difficulty that was experienced in keeping alive the youngest 

 forms in the laboratory. These larvae gradually died off. 

 A wilt disease was also suspected. 



In conclusion, it may be stated that there is evidently 

 little risk of this insect appearing in large numbers and 

 damaging young poplars. Parasitism effectively prevents 

 this. The great majority of the caterpillars, reared in the 

 laboratory, were parasitised. It was apparent that the 

 observations on this insect had been made on the larvae 

 of the second brood of summer 192 1, and it is hoped to 

 ascertain during the coming spring the time of emergence 

 of adults of the first brood and their subsequent life-history. 



