12 BULLETIN 1324, iU>. S. DEPARTMENT OF AGRICULTURE 



it as the stimulus responsible for the oviposition of insects. From 

 present knowledge, however, it seems doubtful whether a free- 

 living insect can ever be induced to oviposit by means of an odor 

 stimulus alone. 



The reaction of Drosophila to odor concentration is interesting. 

 It has been shown by Adolph (1, p. 334, 335) that odor concen- 

 trations are never so low that they fail to call forth positive re- 

 sponses, and even very faint odors have full stimulating value. 

 If this proves true of many insects it will perhaps explain how 

 the faint odors emanating from the green portions of some plants 

 may possess great stimulating value, particularly when the insect 

 is near by. 



In captivity, some species will oviposit on almost any convenient 

 surface, but others hold strictly to specific substances and refuse 

 to oviposit in their absence. Among Lepidoptera, for example, 

 there are species (Satyrus dry as, Carpocapsa pomeneUa) which lay 

 their eggs at random on the walls or floor of the cage, and others 

 which refuse any but a particular food plant for this purpose 

 (Papilio machaon L.. Pieris hrassicae., Arginnis selene Schiff., and 

 others, 24, 43, 69). These results show the difference in oviposition 

 behavior that may occur in the same family of insects. 



It has been observed, however, that there are occasional errors 

 of judgment on the part of female insects which have specialized 

 food plants; that eggs are, in fact, sometimes placed upon plants 

 which can not nourish the larva?. Knoll (46) observed the habits 

 of Macroglossum stellatarum, in captivity ,, the larva of which is 

 closely restricted to plants of the genius Galium. After retaining 

 the eggs a long time, the female will deposit them on any avail- 

 able green portion of a plant, regardless of its botanical relation- 

 ships. And more recently Schwarz (70) concludes from observa- 

 tions on Catocala extending over a number of years that such 

 mistakes in oviposition are a phenomenon of old age and a sign 

 of physical exhaustion. 



The question now arises, how has the female insect obtained the 

 ability to respond to these stimuli which lead it almost unerringly 

 to the specific larval food? Is it impelled by a series of tropisms, 

 or by an instinct which is the result of natural selection, or by an 

 acquired instinct now hereditarily fixed? The tropistic view has 

 been advanced by Loeb (48, 49), Tragardh (79), Howlett (37, 38), 

 and others. Brues (11) and Loeb (49, p. 160) have mentioned the 

 possible relation qi natural selection to food selection by the female 

 insect. Bachmetjew (2) believes that the female insect must have 

 an acquaintance with the taste of the larval food plant which it has 

 inherited from the larva. To use his own words (p. 713). 



Allein der Geruehsempfindung bei der Wahl der betreffenden PflanzegiDg die 

 Geschmacksempfindung gescliichtlich voran, denn urn zu wissen, wo er seine 

 Eier ablegen soil, musste der Falter zuerst mit dem Geschrnack der betreffen- 

 den Pflanze bekannt gewesen sein. resp. dieg von der Raupe geerbt haben. 



Wheeler (82, p. 71-72) states that oviposition and feeding upon 

 the host blood in the parasitic Hymenoptera are congenitally or 

 hereditarily conditioned reflexes. Little of an exact nature seems 

 to have been done to elucidate this important question. However, 

 the very suggestive experimental investigation of Craighead (14, 15) 



