491 



food or uot. In some species tlic factors of the sensory stiiii- 

 uhis are so iiiniierous or so ])iirticiilar lliat the species will 

 react only under verv narrow Jiniits while in others the range 

 of reaction is much broader and in snch species we tind these 

 "lapses of instinct" occurring. These are the species in which 

 we find considerable adaptability of habits and the wide range 

 of reaction is of value to th(> species since the eggs laid almost 

 at random serve to find for the species additional sources of 

 food which would be missed by a species reacting within nar- 

 rower limits. 



The adaptability so secured may, as has been sliown by 

 C'ushman, serve to carry over a species on unusual food when 

 its preferred food-plant for any reason fails to seed. This 

 adaptability may also serve to permit wider dispersal of the 

 species. Thus the adaptability of the BrucJiiis iJruminus per- 

 mitted it to shift to Leucaena gJaiica from Acacia after having 

 shifted from Olneya tesota to the Acacia, and it has been able 

 to establish itself in the Islands while the more narrowdy re- 

 acting Bruclius pisoriun appears as yet to have failed. 



One of the curious things in experimental work with B)u- 

 cliidae is that very often, as shown by breedings made under 

 natural conditions, a species will not naturally breed in certain 

 host plants but Avhen confined with the seeds will oviposit and 

 develop in them. From this it seems to me we must distin- 

 guish between the sensory stimuli which cause the Bruclius 

 to approach and alight upon the larval food and the ovipositicm 

 stimuli proper. The visual faculties of the insects appear to be 

 most prominent in the former, at any rate I have seen what 

 were apparently attempts of a Brachus to settle on seeds with- 

 in a glass tube and resting on the glass over them \vhen actual 

 contact was impossible. In the oviposition reflexes tactile 

 stimuli through the antennae and perhaps the tarsi must play 

 an important part. 



In some cases it is possible to analyse the complex reflex 

 of oviposition with interesting results. Mr. Timberlake has 

 been able to show that there is an olfactory element in the 



