ANATOMY AND PHYSIOLOGY I I 5 



bees frequently select blue flowers; white butterflies (Pieris) 

 prefer white flowers, and yellow butterflies (Colias) appear 

 to alight on yellow flowers in preference to white ones (Pack- 

 ard). In fact, the color sense is largely relied upon by insects 

 to find particular flowers and by butterflies to a large extent to 

 find their mates. To be sure, insects will visit flowers after 



FIG. 144. 



Alimentary tract of a collembolan, Orchesella. F, fore gut; H, hind gut; M, mid 

 gut; c, cardiac valve; cm, circular muscle; Im, longitudinal muscle; p, pharynx; py, 

 pyloric valve. 



the brightly colored petals have been removed or concealed, 

 as Plateau found, but this does not prove that the colors are 

 of no assistance to the insect, though it does show that they 

 are not the sole attraction the odor also 'being an important 

 guide. 



Problematical Sense Organs. As all our ideas in regard 

 to the sensations of insects are necessarily inferences from our 

 own sensory experiences, they are inevitably inadequate. 

 While it is certain that insects have at least the senses of touch, 

 taste, smell, hearing and sight, it is also certain that these 

 senses of theirs differ remarkably in range from our own, as 

 we have shown. We can form no accurate conception of these 

 ordinary senses in insects, to say nothing of others that insects 

 have, some of which are probably peculiar to insects. Thus 

 they have many curious integumentary organs which from 

 their structure and nerve connections are probably sensory 

 end-organs, though their functions are either doubtful or un- 

 known. Such an organ is the sensillum placodeum (p. 95), 

 the use of which is very doubtful, though the organ is pos- 

 sibly affected by air pressure. Insects are extremely sensitive 



