REPORT ON MOSQUITOES. 85 



The Enemies of Larva. 



While the adults are subject to numerous enemies and en- 

 counter dangers that kill off a large percentage of specimens 

 before they have a chance to reproduce, the larva or wriggler 

 faces yet greater dangers, and its chances to reach the adult 

 stage are, on the whole, less than even. Numerous entire broods 

 mature, of course, but on the other hand, numerous others are 

 completely wiped out. The effect of weather conditions, which 

 dry up or otherwise eliminate breeding places while swarming 

 with larvje, is elsewhere referred to, and this, after all, is the 

 most effective general check. 



Next to the weather comes disease; but of these diseases we 

 know little. We know that sometimes almost every larva in a 

 pool will die before it is mature, and I have had breeding jars in 

 which it seemed impossible to bring any to the pupal condition. 

 I place diseases as second in effect because they reach those cases 

 of temporary pools where few if any other enemies can reach 

 them. 



Fish, which come in here, deserve separate consideration. 



Pollywogs do not eat wrigglers except by accident, and this 

 applies in general to those of both toads and frogs. It is pos- 

 sible that some of the other batrachians or reptilians may take 

 them in one stage or another, but there is no evidence to that 

 effect. Mr. Seal tested the pollywogs thoroughly and the con- 

 clusion that they are of no practical value is based upon his work. 



His report of the experiments on the bull-frog is as follows : 

 "A number of tadpoles of the bull-frog {Rana pipiens Latr.) 

 were placed in a perfectly clean aquarium tank with clear water, 

 in which were a large number of mosquito larvae; but the tad- 

 poles were not detected in eating any of them. The experiment 

 was repeated and carried to the extent of depriving the tadpoles 

 of all food for several weeks ; but they were not even then seen 

 to eat anv of the larvae. They do eat small crustaceans like 

 Daphnia. They do not seek for them, but suck them in if they 

 happen to come within range of their suction. The mouth of the 

 tadpole is very small and adapted to sucking, and it is possible 

 that the spines in mosquito larvae are offensive to them. As the 

 tadpole of Rana pipiens is the largest and most voracious species, 

 it is probable that there is no tadpole that is destructive tO' mos- 

 quito larvae." 



Some of the shore birds eat wrigglers in considerable quanti- 

 ties. Mr. Viereck sent me the stomachs of a ring-necked plover 

 {^gialitis semipaliiiata), of a least sand piper (Fringa mimi- 



