THE OVIPOSITION RESPONSE OF INSECTS 11 
sity and wave length of light, temperature, and humidity, rate of 
movement of the medium in which she lives, odor, and the physical 
and chemical character of surfaces aid in bringing the gravid female 
insect into contact with the specific larval food and induce her to re- 
lease the eggs. A. given set of stimuli is not effective for all species. 
Thus, for Drosophila melanogaster the stimuli may be roughly clas- 
sified in the following ascending order of effectiveness: Odor, mois- 
ture, taste, odor and taste, texture, texture and odor, and a combi- 
nation of texture, taste, and odor. In comparison with Drosophila, 
the house fly is more dependent on the odor of the medium; most 
substances which do not liberate ammonia probably seldom, in na- 
ture, evoke egg deposition. The response of Macroglossum to green 
and yellow lhght presents a reaction at present apparently unique 
among insects, but which further study may show to be widespread 
in those species which lay their eggs on green plants. 
The experimental evidence at hand suggests, then, that a chain 
of stimuli is, in many species, necessary to induce egg deposition. 
‘Adolph (1, p. 38) sets forth this view in the following words: 
HKgg laying in itS nature is a complete response (‘all or none”); that is, 
partial stimulation can not be measured. A single potent factor in the chain 
may never lead to the extrusion of eggs. 
A similar view is gained by Knoll (46, p. 349) from his study of 
‘Macroglossum, by Picard (60) from observations on Pimpla insti- 
gator ¥., by Hase from studies on Habrobracon juglandis, and the 
results of the writer’s experiments on the house fly are concordant 
with it. 
Loeb (49) seems to favor the idea than an odor stimulus is suffi- 
cient to produce oviposition in certain free-living insects. He says 
ip tGO) : 
The fact that eggs are laid by many insects on material which serves 
‘as a nutritive medium for the offspring is a typical instinct. An experi- 
‘mental analysis shows again that the underlying mechanism of the instinct 
is a positive chemotropism of the mother insect for the type of substance 
serving her as food; and when the intensity of these volatile substances 
is very high, that is, when the insect is on the material, the egg-laying 
mechanism of the fly is automatically set in motion. Thus the common house 
fly [see footnote, p. 8] will deposit its eggs on decaying meat, but not on 
fat; but it will also deposit it [them] on objects smeared over with asafetida 
on, whieh the larve can not live. * * * It seems that the female insect 
“lays her eggs on material for which she is positively chemotropic, and this 
-is generally material which she also eats. 
Fabre’s observations on the blow-fly Calliphora vomitoria empha- 
‘size the predominance of odor in this response, and Howlett’s re- 
‘sults with Sarcophaga would appear to leave little doubt that odor 
~alone can induce insects to oviposit. It must be said, however, that 
Howlett’s experiments are given in little detail and might be acci- 
“dental or unusual rather than the customary response of the fly 
“in question. And the observations of Loeb and Fabre do not exclude 
“effects due to the surface with which the flies came in contact. At all 
events, it is desirable that thoroughgoing evidence be obtained before 
“accepting as fact the proposition that free-living Diptera can be 
induced to lay eggs solely by means of an odor stimulus. It seems 
‘necessary to stress the dependence of chemotropism upon other fac- 
tors at this point because certain entomologists have rather accepted 
