74 F. ALEX. McDERMOTT 



of only a few millimeters between them, though more usually 

 they placed themselves so as to have the head in the direct 

 air current. Those in other portions of the dish assumed posi- 

 tions with the axis parallel to the stream lines, with the head to 

 windward, even though they happened to be in a slight eddy 

 current. A thermometer was placed in the dish, inclined toward 

 the fan; a few insects climbed up this toward the fan, but the 

 current appeared to be too strong for them. Increasing the tem- 

 perature of the air current to 40° C. caused scattering and finally 

 flight, though the flies seemed reluctant to go, rather attempting 

 to back away slowly, before taking to flight. Sudden stopping 

 of the current .of air caused immediate dispersal. While most 

 of the insects took to flight at once on being disturbed mechani- 

 cally, as with the bulb of the thermometer, a few of them would 

 allow themselves to be pushed about and even pressed down 

 tightly on the bottom of the dish, with the thermometer bulb, 

 without taking flight. The writer believes that the observation 

 has been made that flies lighting on moving vehicles usually turn 

 with the axis parallel to the direction of motion, and with the head 

 forward, but he is not aware of any observation of the kind here 

 recorded. Unfortunately, means were not at hand to try the 

 effect of wider and lower variations of the temperature of the air. 



