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 light 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. 338) sets forth this view in the following words: 



Egg 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 F.. by Hase from studies on HabrobroGon 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 

 (p. 160) : 



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 which the larvae 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 C alliphora 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- 

 tor- ill this point because certain entomologists have rather accepted 



