384 F. H. KRECKER. 



and it consequently did not so easily cause strain or seriously 

 disturb the attachment and there was therefore no turning 

 reaction. 



It would appear from the foregoing experiments that the 

 Ephemeridae do not change position under the stimulus of a 

 breeze until a strain is exerted on the organs of attachment. 

 That this does not merely mean that the response was delayed, 

 until a breeze of a given intensity developed is shown by the fact 

 that a comparatively weak breeze directed against the wings 

 alone had the same effect as was caused by a somewhat stronger 

 breeze against the thorax. There is, therefore, evidence, I 

 believe, for concluding that Ephemeridae do not orient positively 

 to a breeze because of sensations derived from the breeze per se 

 but that they react positively to tension exerted on the muscles 



of attachment. 



REACTIONS TO GRAVITY. 



The position of Ephemeridae when resting upon a perpendicular 

 surface is negative with regard to the earth's surface and usually 

 approximately vertical to it, although variations as great as 

 45 degrees occur. On comparatively smooth surfaces the orien- 

 tation is more generally an approximation to the vertical, 

 whereas on surfaces such as a wire screen, which affords a good 

 foothold at any angle, variations from the vertical may occur 

 in 50 per cent, of the individuals concerned. Individuals picked 

 up by the wings and replaced head downward, if they are not so 

 disturbed as to fly away, will struggle to gain a foothold. The 

 position of the claws, which are adapted to a vertical position, 

 make attachment rather difficult. This difficulty is increased 

 by the fact that the long abdomen is thrown forward and down- 

 ward and thus tends to destroy equilibrium. On comparatively 

 smooth surfaces such as a planed board the insects rarely suc- 

 ceeded in maintaining their equilibrium long enough to gain a 

 footing. On a wire screen they were more often successful and 

 once they gained a footing and their equilibrium they retained 

 the new position. The picking up process caused so many of the 

 insects to fly away that other methods were tried. Several in- 

 dividuals were placed in a vertical position on a straw hat held 

 perpendicularly and then the hat was slowly revolved until the 



