HABITS OF TWO PARASITES OF BLOW-FLIES. 229 



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

 short jerky flight. 



Effective Farasitisiii. — Under this term Girault and Sannderi 

 x-efer to an interesting plienomenon, and one which was also 

 observed by the writer. It is, that the attack of JVasonia can be 

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

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

 the period being approximately such time as is required for the 

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

 the instances observed by the wi'iter would be from 30 to 74 hours. 

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

 limit of eflfectiveness, 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 v/as opened, and found to contain 

 seven larvse feeding upon a perfect pupal fly which had cracked 

 its enveloping skin anteriorly and the puparium, but Avhich had 

 failed to get any further, its organ of locomotion — the j)tilinum — 

 apparently having lost its efficiency gradually as the larvae fed. 



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

 tive to the percentage of parasitiza,tion under natuial conditions, 

 and the data i-elative to laboratory conditions cannot be taken as 

 a true index of the capacity of Nasonia, as the number of host 

 puparia presented for attack vvere worked out on the basis of 

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

 or main cage were only approximately 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 liave 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 

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



Hibernation. — This parasite hibernates as full-grown larvae 

 within the puparia of its vario\is hosts. It emerges in the spring, 

 the earliest recorded emergence, in England, being the end of 

 April (4). 



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

 light, the males very slightly. 



Hosts. — The diflferent species of puparia given to Ahisonia 

 hrevlcornis to pai-asitize, and from which it was successfully bred 

 are as follows : — 



Musca domestica Linn. 

 Galliphora erytlirocephala Meig. 



,, vomitoria Linn. 



Phormia grcenlandica Ztt. 

 Lucilia ctesar Linn. 

 ,, sericata Meig. 



16** 



