i BULLETIN 13824, U. 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 (7, 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 dryas, Carpocapsa pomenella) which lay 
their eggs at random on the walls or fioor of the cage, and others 
which refuse any but a particular food plant for this purpose 
(Papilio machaon L., Pieris brassicae., 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. 
Tt 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 larve. Knoll (46) observed the habits 
cf 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 (71) and Loeb (49, p. 160) have mentioned the 
possible relation of 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. 7/3). 
Allein der Geruchsempfindung bei der Wahi der betreffenden Pflanzeging die 
Geschmacksempfindung geschichtlich vyoran, denn um zu wissen, Wo er seine 
Bier ablegen soll, musste der Falter zuerst mit dem Geschmack der betreffen- 
den Pfianze bekannt gewesen sein, resp. dies 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) 
