ANATOMY AND PHYSIOLOGY I15 
bees frequently select blue flowers; white butterflies (Pveris) 
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 
Fic. 144. 
Alimentary tract of a collembolan, Orchesella. F, fore gut; H, hind gut; M, mid 
gut; c, cardiac valve; cm, circular muscle; /m, 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 
