NO. 10 TROPISMS OF LEPIDOPTERA — McINDOO 5 



response, comparable to the turning of a plant toward light. In reply 

 to the question as to why moths fly toward a candle at night and not 

 toward the moon, Mast (p. 227) replies that in moonlight there are 

 large illuminated areas all about, whereas in candle light the objects 

 are so faintly lighted that moths do not react to light reflected from 

 them. In reply as to why mourning-cloak butterflies fly toward a large 

 illuminated patch of flowers rather than toward the sun which is much 

 brighter, Parker (69) says it is because the patch of flowers makes a 

 larger " spot on the retina," All of these responses Mast considers 

 adaptive regardless of the explanations given. 



(c) Is orientation accomplished by selection of trial movements? — 

 Loeb (42, p. 57) exposed blowfly larvae in front of a window. He 

 found them to be photonegative and to crawl with mathematical 

 precision. Other investigators have repeated these tests, but they 

 failed to find that blowfly larvae, or in fact any other insects, respond 

 to light with mathematical precision. Mast (56, p. 196) says that 

 blowfly larvae are excellent examples of animals which are guided 

 fairly directly on their courses by successive trial movements. 



Loeb (42, p. 24) tested caterpillars of Euproctis (Porthesia) 

 chrysorrhoea and he found them to be strongly photopositive, creeping 

 in a straight line toward the light. Lammert (40) in 1925 tested three 

 other species of caterpillars which were also photopositive, but crawled 

 in wavy lines. The present writer's results (p. 13) with codling-moth 

 larvae agree with those of Lammert. 



(d) Hoiv do light rays bring about orientation? — Loeb in 1888, 

 according to Mast (56, pp. 54, 57, 228-235), claimed that orientation 

 in animals is controlled by the direction in which the rays pass through 

 the tissue. In 1889 he further said that symmetrically located points 

 on the photosensitive surface must be struck by light at the same angle. 

 Later he abandoned the idea of the importance of the angle between 

 the sensitive surface and the light rays and substituted the view that 

 orientation is brought about by absolute difference of intensity of the 

 light on symmetrically located points on the sensitive surface. 

 Jennings, Mast, and others claim that orientation is accomplished by 

 changes of intensity on the sensitive surface. Loeb believed that light 

 acted constantly as a directive stimulation similar to the action of a 

 constant current of electricity, while Jennings and Mast believe that 

 it acts only when the animal turns out of its course so as to produce 

 changes of intensity. 



(e) Do circus movements support Loeb's theory? — Mast (56, 

 pp. 215-218) states that several workers have found that if one of 



