286 



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 soon alight 

 nearer the earth and again walk upward." (Wheeler.) Coccinellidae 

 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 sur- 

 face 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 heliotropic, as 

 is the case in most Diptera, 

 it orients itself to the wind 

 or to gravity no matter 

 whence the light may fall." 

 Phototropism. It is a 

 matter of common observa- 



FIG. 293. A, tracks made on paper by a larva of 

 Lucilia ccesar moving out of a spot of ink under the in- 

 fluence of light; A and B show respectively the first and 

 second directions of the light. B, tracks made in the 

 dark. After POUCHET. 



tion that house flies, butter- 

 flies, bees and many other 

 diurnal insects fly toward 

 the light; and that' cock- 

 roaches and bedbugs avoid 



the light. These are familiar examples of phototropism, or the "control 

 of the direction of locomotion by light." The phototropic response is 

 either positive or negative according as the organism moves, respectively, 

 toward or away from the source of light. Maggots of Lucilia ccesar and 

 of many other flies are negatively phototropic as a rule (Fig. 293, A), 

 but in the absence of light (other directive stimuli being excluded, of 

 course) wander about indifferently (Fig. 293, B}. 



Do the different rays of the spectrum differ in phototropic power? 

 This question has occurred to many investigators, who have found that, 



