77 
Libellula larvae, Gorysca , 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 , Cory sea, 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. 
(b) 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. 
