i6o ANIMAL ECOLOGY 



14. On the other hand, some marine fish react to other 

 factors than salinity. Thus, the mackerel in the Black Sea 

 are said to migrate in response to a change in temperature. 

 Whether this is the actual factor atwork or not, it appears certain 

 that they do not react to salinity. ^^* With birds, the precise 

 way in which they find their right habitat is not known, but 

 in many cases they appear to know simply by a rather elaborate 

 process of experience and memory, in others by some more 

 mysterious sense of direction. Many insects have definite 

 chemotropic reactions which lead them to choose the right 

 habitat either for feeding or for egg-laying. For instance, 

 Howlett ^^^ made Sarcophaga oviposit in a bottle with scatol in 

 it, this being a decomposition product of albuminous sub- 

 stances ; while Richardson ^^^ made house-flies oviposit in re- 

 sponse to ammonia together with butyric and valerianic acids. 

 Barrows ^^'^ found that the positive reaction of Drosophila to 

 fermenting fruit was due largely to amyl, and especially ethyl, 

 alcohols, acetic and lactic acids, and acetic ether. 



But although a considerable amount of work has been done 

 in this direction, and many more examples could be quoted, 

 the fact remains that we are hardly able in any case to say how 

 a particular insect does manage to find the right habitat to 

 live in. That is to say, we can see that it inhabits certain 

 vegetation and physical conditions, and that these are best 

 suited to its physiological endowments, but it is hard to find 

 out, and usually unknown, by what indication it is enabled 

 to find these optimum conditions. This point has already 

 been touched upon in the chapter on environmental factors 

 (p. 40). 



The third method of finding habitats when dispersal is 

 in progress is by general broadcasting combined with local 

 directive movement. This appears to be practised by some 

 birds (homing pigeons amongst others) and by a number of 

 insects. Thus, a butterfly may undertake a huge migration 

 whose direction is only determined by the particular winds 

 blowing at the time ; but if it arrives at any place at all like its 

 original home it will then be able to find the right food-plant 

 for its larvae, by chemotropic or other means. It is possible 



