1919] Mclndoo: Olfactory Sense 67 



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RESPONSE TO CHEMICAL STIMULI. 



While feeding silkworms it is easy to determine that they 

 can smell; this is done by placing fresh mulberry-tree leaves 

 near them so that they can neither see nor touch the leaves; 

 immediately the silkworms move their heads, work their 

 mouth parts and begin crawling toward the leaves, and some- 

 times a hungry silkworm for several minutes will follow a leaf 

 dragged slowly just in front of its head and the writer believes 

 that the larva is guided solely by means of the odors emitted 

 from the leaf rather than by seeing the leaf. Silkworms may also 

 be induced to eat other leaves which they will not ordinarily 

 even "taste." This was proved by the following test: Leaves 

 from peach trees, cherry trees, plum trees, and apple trees were 

 dipped into the juice from mulberry-tree leaves and were then 

 fed wet to silkworms; all of these leaves were eaten, although 

 some of them apparently were not really relished. When other 

 leaves of the same trees were dipped into water, the silkworms 

 did not even "taste" them. 



To test the responses to other chemical stimuli the various 

 larvae used were confined in small observation cases and the 

 sources of the odors were usually kept in small vials which were 

 held directly beneath the larva in the case. The following larvae 

 were used in the experiments: Tent caterpillars, fall web- 

 worms, tussock-moth larv£e, army worms and larvae of a. 

 butterfly {Papilio polyxenes). The following sources of odors 

 were employed and the average reaction time of the above 

 larvae to them are: Oil of peppermint, 14.6 seconds; oil of 

 thyme, 9.5 seconds; oil of wintergreen, 17.6 seconds; dried 

 leaves of pennyroyal, 20.1 seconds; dried leaves of spearmint, 

 22.9 seconds; wild cherry-tree leaves, 42.5 seconds; fresh grass, 

 19.1 seconds, old honey and comb, 51.5 seconds; protruded 

 thoracic glands of above Papilio larvae, 22.8 seconds; and as a 

 control — a clean and empty vial, 60 seconds (totally negative). 

 The details pertaining to these experiments are as follows: 



In each set of experiments 10 larvae were used, one larva 

 .being confined in a case. As a rule, the more the larvae were 

 handled the more satisfactorily they responded to odors, but 

 generally speaking these larvae were the most unfavorable 

 insects that the writer has ever used for testing the responses to 

 •odors. When placed in the experimental cases, all of them, 



