HABITS OF TWO PARASITES OF BLOW-FLIES. 



229 



never obtain any. The females have a better chance with theii 

 short jerky flight. 



Ejfective Parasitism. — Under this term Girault and Saundei-& 

 refer to an interesting phenomenon, and one which was also 

 observed by the w^riter. It is, that the attack of lYasonia can be 

 effective upon the host pupa within 15 to 24 hours prior to the 

 moment when the adult fly would have emei-ged, the length of 

 tlie period being ap2)roximately such time as is required for the 

 depositing, development, and hatching of the egg, and which in 

 the instances observed by tlie writer would be from 30 to 74 hours. 

 In a specific instance, and one which must be about the extreme 

 limit of effectiveness, a-puparium was noticed to be cracked 

 along the crease at the anterior end, but not sufficiently to have 

 allowed a fly to emerge ; it vv^as opened, and found to contain 

 seven lai'vse feeding upon a perfect pupal fly which had cracked 

 its enveloping skin anteriorly and the puparium, but which had 

 failed to get any further, its organ of locomotion — the ptilinum — 

 apparently having lost its efficiency gradually as the larv?e fed. 



Percentage of Parasitization. — The writer has no figures rela- 

 tive to the percentage of parasitization under natural conditions, 

 and the data relative to laboratory conditions cannot be taken as 

 a true index of the ca,})acity of Nasonia, as the number of host 

 puparia presented for attack were woi-ked out on the basis of 

 20 per female, and the numbers of the latter in the breeding-jars 

 or main cage were only approxim.ately ascertained. 



Upon reference to Girault and Saunders the following is 

 found : — " Further, the local abundance of this parasite is indi- 

 cated by the fact that in at least a portion of the experiment just 

 mentioned, a portion selected at random, the percentage of 

 parasitism was as high as 90 per cent. We have evidence to 

 show, on the other hand, that this percentage of mortality of the 

 host was by no means general, but was considerably lower on 

 the average for this season of the year.'" (6.) 



Hibernation. — This parasite hibernates as full-grown larvje 

 within the puparia of its various hosts. It emerges in the spring, 

 the earliest I'ecorded emergence, in England, being the end of 

 April (4). 



Attraction to Light. — The females are freely attracted to artificial 

 light, the males very slightly. 



Z/osifs.— The difterent species of puparia given to Xasonia 

 hrevicornis to parasitize, and from which it was successfully bred 

 are as follows : — 



Musca clomestica Linn. 

 Calliphora erythrocephala Meig. 



vomitoria Linn. 

 Phorm ia grcenlandica Ztt. 

 Lucilia ccesar Linn. 

 ,, sericata Meig. 



IG*^ 



