17 



Libellula larvae, Corysca, Nepa, tadpoles, and other 

 water animals, respectively, to a series of bottles con- 

 taining equal numbers of larvae. Note the rate at 

 which they are devoured, if at all. The carnivorous 

 forms Nepa, Corysca, Libellula rapidly devour larvae. 

 Hydrophilidae beetles, tadpoles, etc., do not destroy 

 larvae. Observe that some species of fish are much 

 more active devourers of larvae than others. 



N.B. Wherever possible introduce small fish into tanks, 

 ponds, etc. 



Note that weeds often protect larvae from being 

 consumed by small fish. 



8. Make experiments with different chemical 

 and other bodies, and note the absence or presence of 

 culicidal power. 



(a) Note that chemical bodies in solution kill only 

 with difficulty, as a rule ; e.g., corrosive sublimate. 

 Ammonia, however (i in 4,000 of water), will kill 

 mature larvae according to Waddell. 



() Note that oils rapidly kill larvae by blocking 

 the air tubes. Treat larvae by pouring a little olive 

 oil upon the water. Stain with osmic acid and note 

 globules of oil within the air tubes. 



9. Add some paraffin oil to a small Anopheline 

 pool, observe the presence next morning of dead female 

 mosquitoes that have come to lay their eggs. Observe 

 the effect of paraffin on different kinds of natural water, 

 and the great efficacy in some cases and futility in 

 others. 



10. Observe that pools covered with Lemna 

 (duckweed) are very frequently, if not always, free from 

 larvae. The action of the Lemna is said to be mecha- 

 nical. 



N.B. Wherever possible grow duckweed in ponds, tanks, etc. 



