306 



ENTOMOLOGY 



Geotropism. Gravity frequently determines the orientation and 

 direction of locomotion of an animal. A freshly emerged moth hangs 

 with the abdomen downward and remains in this position until the 

 wings have expanded. Certain dolichopodid flies found on the bark 

 of trees "rest or walk with the long axis of the body perpendicular to 

 the earth and parallel with the long axis of the trunk of the tree and the 

 head pointing upwards. When disturbed they fly off, but very sooi 

 alight nearer the earth and again walk upward." (Wheeler.) Cocci- 

 nellidae (lady beetles) and cockroaches are also negatively geotropic. 



The latter insects, as Loeb 

 has observed, tend to leave 

 a horizontal surface but 

 come to rest on a surface 

 that is vertical or as nearly 

 so as possible. 



Wheeler says, " Geotro- 

 pic as well as anemotropic 

 orientation is not altered 

 for the sake of response to 

 light. Even if the insect 

 be strongly helio tropic, as 

 is the case inmost Diptera, 

 it orients itself to the wind 

 or to gravity no matter 

 whence the light may fall." 

 Experiments by W. H. 



FIG. 296. A, tracks made on paper by a larva of Cole show that the pomace 



Lucilia ccesar moving out of a spot of ink under the n 7-. . 7 / . T 



influence of light; A and B show respectively the first "7* UrOSOphlld am pel 0- 



and second directions of the light. B, tracks made in 'hJiHd when Creeping 

 the dark. After POUCHET. 



reacts negatively to grav- 

 ity, to a centrifugal force which is equal to or slightly greater than 

 gravity, and to air currents without regard to other stimuli. Gravity 

 is, then, a kinetic as well as a directive stimulus. The stimuli caus- 

 ing these reactions are probably received by the sensory nerves of the 

 leg muscles. (Cole.) 



Phototropism. It is a matter of common observation that house 

 flies, butterflies, bees and many other diurnal insects fly toward the 

 light; arid that cockroaches and bedbugs avoid the light. These are 

 familiar examples of phototropism (heliotropism) or the "control of the 

 direction of locomotion by light. J) The photo tropic response is 



