38 GNATS OR MOSQUITOES — CHAPTER II 



by infected insects are desirable, there is no need of such 

 an experiment to estabhsh the biological relation between 

 Mosquito and parasite, for if it is found that the parasites 

 ingested by the Mosquito with the blood of the infected 

 vertebrate undergo developmental changes, it may be taken 

 as proved that the species used is capable of acting as 

 intermediate host, and is therefore dangerous to the verte- 

 brate from which the infected blood was derived. It is well 

 to make use of a fair number of insects in each experiment 

 as all may not bite, and in any case some ten or a dozen will 

 be required for dissection day by day in order to follow out 

 the changes undergone by the parasites within the ^losquito. 

 The method of dissecting out the stomach of the Mosquito 

 is described elsewhere, and it only remains to add that 

 the best medium in which to examine is ordinary "normal 

 saline" solution. If the stomach be very full of blood it is 

 well to puncture it, and shake it to and fro in a watch-glass 

 of saline solution, with the point of a needle, or which is 

 safer, to irrigate it with drops of solution as it lies on the 

 slides. 



For permanent preparations Major Koss found nothing 

 so suitable as formol solution, as balsam or glycerin render 

 everything so transparent that all details are lost. Such 

 experiments are, when negative, conclusive only when con- 

 ducted at the season of the year at which the parasitic 

 disease under investigation is rife, as at any other season 

 it may possibly be merely owing only to unsuitable climatic 

 conditions that the parasites fail to continue their cycle 

 within the Mosquito. 



