8 BULLETIN 1324', IT. S. DEPARTMENT OF AGRICULTURE 



ing" insects is not the morphological character of the host insect but 

 of the gall which it inhabits. In some other parasites mentioned 

 by this author, the stimulus seems to be furnished by the silken 

 cocoons or webs of the host insects. 



ODOROUS SUBSTANCES 



A number of observations are on record which stress to a greater 

 or less degree the importance of odor as a factor in oviposition. 

 Scudcler (71), in discussing the so-called botanical instinct of butter- 

 flies, excludes taste and sight but believes the oviposition behavior is 

 in keeping with the idea that the larval food plant is detected by 

 means of the olfactory sense. Tragardh (79) places great emphasis 

 on chemotropism, and Picard (59) also emphasizes its importance 

 but recognizes that light, temperature, humidity, and other physical 

 factors play a part. Brues (11) states that there is much in the 

 behavior of certain species to suggest that food plants are selected by 

 the female insect on the basis of odor. In addition, Brues recog- 

 nizes "some attribute of the plant, perhaps an odor, but far less pro- 

 nounced to our senses than odor or taste " as a factor in the attraction 

 of insects to plants. Grevillius (30) thought it probable that the 

 choice of a food plant on the part of the brown-tail moth ( Euproctis 

 chrysorrhoea) was determined by the olfactory sense. The cotton 

 worm moth (Alabama argillacea Hbn.), which lays its eggs on the 

 leaves of the cotton plant (Gossypium sp.), may be attracted by the 

 nectar glands on the leaves (12). In fact, moths were seen alter- 

 nately feeding from these glands and ovipositing. It was found, 

 however, that no preference was shown for the portion near 

 the glands on the involucre. This fact induced Comstock to 

 question whether oviposition was here determined by the pres- 

 ence of the nectar glands. Studies by McColloch (50) on Heli- 

 othis obsoleta show that it deposits 60 per cent of its eggs on the 

 silks when the corn plant is in silk. Artificial silks made of cotton 

 twine soaked in the fresh juice pressed from corn silk received 79 

 per cent of the eggs laid, while the controls (untreated cotton twine) 

 received 21 per cent. Thus odor appears to be important in this 

 case, but surfaces, according to McColloch, must also be considered. 

 Knoll (Jf.6) emphasizes the effect of odor upon Macroglossum stella- 

 tanim.when the moth is close to the plant upon which the eggs are to 

 be laid. But green or yellow light is necessary to attract the moth 

 to the plant from a distance. The potato tuber moth is attracted by 

 the odor of certain plants (61), but, as previously mentioned, the 

 character of the surface is also highly important. Pewits (20) 

 thought the vine moth (Cochylis ambiguella) might be attracted and 

 induced to lay its eggs upon or near the buds of the grapevine by the 

 odor poured from the nectaries. But, in addition, he recognized the 

 possible effect of contact stimulation. Loeb (49, p. 100) states that 

 the blowfly e is attracted to and will oviposit on decaying meat but 

 not on fat. It will also deposit eggs on objects smeared over with 

 asafetida. A positive chemotropism is responsible, according to this 

 author, for oviposition. Fabre's observations on the blow-fly, Calli- 



e It is here called " the common house fly," but the reference is undoubtedly to one 

 of the Calliphoras (cf. Loeb, 48). 



