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able to reproduce their kind parthenogenetically, and probably 

 frequently do so in a state of nature. In one case that was 

 noticed, that of a species of Psendogonatopns, of the offspring 

 thus produced only one in forty was of the male sex. 



THE ECONOMIC VALUE OF DRYINIDAE. 



The parasites of the group now under consideration have un- 

 til lately been considered as more or less rare insects, and there- 

 fore it is of interest to consider the extent of their economic 

 value. A fairly true estimate of this value can be made by 

 carefully watching some common species of leaf-hopper, which 

 is subject to their attacks, over a considerable period of time. It 

 must be understood that the following remarks apply to species, 

 which are found in countries where the winters are not cold 

 enough to put a cessation to insect activity. If we keep watch 

 on a species of hopper from the time when its numbers are at a 

 minimum, it will usually be observed that the individuals become 

 more and more numerous with each generation. At first the 

 parasite is also very scarce, or for a time may not be observed 

 at all, but it likewise increases in number with the increase of the 

 hopper, so that both frequently attain their greatest abundance 

 at the same time. In the case of a common Australian species of 

 grass eating Libiiriiia and the Psendogonatopns parasitic on it, 

 which were observed for about three consecutive months, the 

 visibly parasitized individuals, when the hoppers were most 

 abundant, amounted to as high as. 50 per cent of the whole. 

 Many individual hoppers carried two to four, some even six 

 parasites. Adults and all the earlier instars were attacked alike. 

 This statement however would give a very inaccurate idea of the 

 true value of the parasite. Firstly, many of the apparently 

 sound hoppers were afifected by the parasite, which had not yet 

 shown externally ; secondly, an enormous proportion of them 

 were in the young stages and incapable of breeding, and, for 

 days or weeks to come, were Uable to be parasitized before they 

 could reach maturity. It will not therefore be surprising to 

 learn that a few weeks after the parasites were at their maximum, 

 the hoppers from their numerical maximum fell suddenly away 

 to a minimum, being represented, in places where they had 

 swarmed previously, by only scattered specimens. 



It might have been expected that now the parasite would 

 have been swarming in the locality, where so many dryinized 

 hoppers had lately existed, the more so as, on account of their 



