248 Annals Entomological Society of America [Vol. VIII, 
over a body of larve, in this case the movement is downward 
and not transverse. They remain quietly on the bottom 
for several minutes. Pup are the more restless, and cannot 
remain below the surface as long as do the larvee. When the 
pipe line dredge poured silt into the marsh, larve were not 
found in the silt-laden waters, but always just in front of it. 
When silt became generally distributed, the reduction .of 
larvee was very pronounced. 
At the laboratory several tests were made to learn the 
reactions to such stimuli as gases passed over the surface of 
water containing larve. <A large basin was used; it contained 
pup and larve and water from the breeding place. Three 
glass jets of about 1 mm. diameter and 2 cm. apart, were placed 
2.5 em. above the surface of the water, at the middle of the basin, 
and in such a way that the streams of gases passing through 
these jets shall pass parallel to the surface of the water. The 
gases used were oxygen, carbon-dioxide, hydrogen sulphide 
and chlorine. Each test consisted of five minutes of constant 
stream of gas. Two hours elapsed between each test. There 
was no marked reaction toward oxygen. When CQ: was 
used, no reaction was noted during the first two minutes, 
but after that all larve slowly separated toward the two ends. 
This test was repeated several times, varying each time the 
light at both ends, but the reactions were alike. Similar 
separation occurred with H.S and Cl, being most marked 
and almost instantaneous with the latter. 
Similar experiments were made, excepting that the gases 
were allowed to diffuse in the water. The modifications in 
apparatus consisted in the addition of a small fan-shaped 
jet near each end through which fresh water flowed into the 
basin; opposite these two jets were small outlets for overflows. 
At the middle was a small fan-shaped jet through which the gasses 
passed. All gases were introduced for periods of five minutes, 
and a new lot of larve and water were used for each test. The 
flow of gas was regulated so as not to agitate the water. The 
number of larvee were never less than a hundred. 
When oxygen was introduced, the larve nearest the stream 
of gas became more active, but except for this, no other reaction 
was observed. With carbon-dioxide the larve separated for 
both ends, and at the end of 4.5 minutes they wriggled to the 
