224 GNATS OR MOSQUITOES — CHAPTER VIII 



exception of paraffin, and an aniline product called " larvi- 

 cide," prepared by Weiler-ter-Meer, of Uerdingen, none are 

 sufficiently powerful, in proportion to their cost, to be of 

 any use on a practical scale. To these may be added tar, 

 but the reports I have received as to the efficacy of this 

 latter are so contradictory that there can be no doubt that 

 it is very uncertain in its action, and it is hardly cheaper 

 than parathn, as to the efficiency of which, when properly 

 applied there can be no doubt. Paraffin, it must be remem- 

 bered, kills the larvae, not by acting^ as a poison, but by 

 choking them, the thin film which it forms on the surface 

 preventing their getting access to the air ; and therefore to 

 use it with success, care must be taken that it is made to 

 spread over the entire surface. No very large quantity is 

 required, but it is better to err on the side of liberality ; 

 and according to my experience, the best and easiest way is 

 to sprinkle it over the surface b}' one or two rapid sweeps 

 from a fine-rosed watering pot. So emploj'ed, I have never 

 known it fail, the larva3 being all found dead in the course 

 of a few hours. 



"Larvicide" on the other hand acts as a poison, killing 

 all larvae within twenty-four hours, in a strength of about 

 one in 7,000. The consignment I sent for had not come to 

 hand when I left India, so that I know nothing of its powers 

 from personal experiment ; but Celli appears to regard it as 

 the only agent of the poisonous class that is cheap enough 

 for use out of the laboratory. He appears to consider it as 

 the most promising agent we have as yet at our disposal, 

 and states that the cost of disinfection per cubic metre of 

 water varies from lire 0*0056 to 0'0012, which is equivalent 

 to saying that a shillingswoxth is sufficient for from 7,350 

 to 34,300 cubic feet of water. It is said to be not in the 

 least injurious to plants, and that the water containing it 

 may be drunk with impunity by men and cattle, but never- 

 theless to be lethal to most insects which are injurious to 

 crops, so that it is particularly suitable for the treatment of 

 such portions of rice swamps as are dangerously near 

 habitations. 



It has the further advantage over paraffin in the fact 



