THE REACTIONS OF MOSQUITOES TO LIGHT IN DIF- 

 FERENT PERIODS OF THEIR LIFE HISTORY 



S. J. HOLMES 

 From the Zoological Laboratory, University of Wisconsin 



Everyone is familiar with the quick wriggling downward of 

 mosquito larvae upon one's approach. Pass your hand over a 

 jar in which the larvae are hanging from the surface film and 

 they will quickly dart towards the bottom. The movement is 

 not an effort to get away from the source of the shadow. It is 

 not simply a tropism. It is a specific reaction to shadows, or a 

 sudden diminution of light intensity, by swimming downward. 

 Nevertheless there seems to be an element of negative photo- 

 taxis in the behavior of the larvae when stimulated by shadows, 

 although otherwise they may show a positive reaction to light. 

 These conclusions are borne out, I think, by the following 

 experiments : 



A glass jar containing numerous larvae of Culex territans 

 and C. pipiens was placed before a window through which the 

 light fell upon the jar obliquely from above. A dark object was 

 passed over the jar; the larvae quickly swam downward and 

 toward the side of the jar away from the light. Numerous 

 repetitions of the experiment gave the same result. When the 

 larvae were brought into a dark room and illuminated from one 

 side a shadow thrown upon them would cause them to go down- 

 ward and away from the light as before. The direction of the 

 light rays is therefore a factor in causing the direction of the 

 movements of the larvae. I have often noticed that specimens 

 in a jar before a window swam dow^nward and away from the 

 light upon my approach, regardless of the direction in which I 

 came. This led me to ascertain whether the larvae would swim 

 away from an approaching object or would simply swim down- 

 ward and away from the light when stimulated by a shadow 

 or large object in their visual field. The latter was found to 

 be true. In jars before a window the wrigglers would repeatedly 

 swim directly towards my hand if it was moved toward them on 

 the side away from the light. Specimens taken into the dark 

 room and illuminated from one side by an incandescent electric 

 lamp would react in the same way. A jar of larvae was then 



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