ATTRACTION BY ODORS TO F00D-PLANT6. 73 



necessity of eyes is not apparent, for the sexes never can see one an- 

 other. Not only is tiie greater part of tlic caterpillar life of the female 

 passed within its bag or basket, but inside of its walls it attains its 

 growth, it undergoes its pupation, it changes to the mature insect, the 

 eggs are fertilized, and the mother dies. It never emerges from its 

 cocoon. Sight alone could not have brought about the fertilization of 

 the eggs, nor could other sense than the one with which we believe in- 

 sects to be, for necessary uses, specially endowed. 



When we recall larval growth, and the source from which the larva 

 derives the material which forms its entire structure, it Avill seem but 

 natural, that Avhen, through transformation, it has attained its perfect 

 state, it should be peculiarly sensitive to the presence of its food-plant 

 and be led to select it for ovipositiou, through the exercise of its most 

 acute sense. Take, for illustration, the familiar caterpillar of the to- 

 mato plant, known as Sphinx quinquemaculata. In its larval state it 

 lives but to eat and grow. Its skeleton — its outer integument — can 

 not grow fast enough to meet its insatiate demand for food, and there- 

 fore, at intervals — four several times — it is cast off and a new and 

 more capacious one is supplied. Observe the play of its powerful jaws ; 

 unless frightened into temporary quiet, they almost give us a ])erpetual 

 motion ; they rest not day or night ; their sweep is like a mower's 

 scythe, and a tomato leaf disappears before them as a meadow falls be- 

 fore a mowing machine. The creature is scarcely more than an ani- 

 mated eating apparatus. Its life is passed on a single plant, if that 

 suffices for its appetite. It never tastes other food. And Avhen it has 

 finally attained its beautiful winged form, every particle entering into 

 its composition, of fluid, nerve, muscle, and chitin, is buttransmutated 

 tomato leaf. Is it strange, then, that the moth should be able to 

 know tomato when it smells it ? Would it not be more strange if its 

 organs of smell should respond to any other odor ? This alone would 

 be familiar to it — others, perhaps, might be acquired. 



Having now offered plausible reasons, why, from physiological causes, 

 insects may be supposed to be extremely sensitive to the odor of their 

 natural food-plant, — having shown the important pur])oses that the 

 possession of this sense in an acute form may serve them in the per- 

 petuation of the species — and having presented some marvelous mani- 

 festations of insect attraction, sexual and otherwise, which are only 

 explicable upon the theory that they are directed by the sense of smell, 

 -^need we hesitate to accept the opinion advanced, that many of our 

 more injurious insects are guided in the deposit of their eggs by the 

 sense of smell alone. 



There are classes of insects in which ovipositiou would not be con- 

 10 



