THE MFORATION AND mSPBRSAL OF INSECTS. 211 



tho winter in their now homo, under now conditions of onviron- 

 mont, in the same manner as their ancestors have dono for lonp: 

 ages in the homo from which they came. Restriction of area 

 covered by the food-plant and climatic conditions are two of the 

 greatest barriers to the more general dispersal of insects. 



It has been stated that many species of insects can subsist in tho 

 larval state on only one species of plant, and hence, whatever dispersal 

 powers such insects may have, arc rendered more or loss nugatory by 

 the fact that they have no chance of colonising permanently any 

 district where the food-plant does not exist, and the range of the 

 species is strictly limited by the geographical range of the food-plant. 

 When any such species migrates, its permanent settlement depends 

 initially upon the presence of this particular (or a very closely allied) 

 food-plant, on which the progeny may feed. But even given the neces- 

 sary food-plant, climate may be an efficient barrier. In cases where the 

 insect is more or less polyphagous, this want of a special food-plant 

 offers no barrier to the spread of the species ; yet wo often find 

 instances in which insects with more or less polyphagous larvae are 

 restricted in their distribution. Climate usually proves in such cases as 

 those, the predominating barrier determining the range of each species. 



The influence of climate as a barrier is rarely applied to the 

 migrating individuals directly, i.e., heat and cold do not kill the 

 insects that actually migrate, it is the progeny that suffers. In sub- 

 tropical regions, hybernation is not known ; insects in these districts 

 are often multiple-brooded, and the dominant and migrating species 

 are almost always so. They take their habits to temperate regions, 

 and attempt to do there, what it has been the habit of the species to 

 do for innumerable generations under far ditterent conditions. They 

 are consequently exterminated almost to an insect every time they 

 attempt to spread. This phase of the subject will be more fully dis- 

 cussed hereafter. 



Another factor is the dependence of certain insects on others on 

 which they prey, or on which, in some other Avay, they are directly 

 dependent. Thus the range of the parasitical Hymenoptera and 

 Diptera is directly dependent upon the range of their hosts. The 

 " wax " moths, such as (idllrrla ccrcuna, Achroea (/n'sfl/a, and 

 Aphomia sociella, are entirely dependent on the bees and wasps, in 

 whose nests they lay their eggs, and on the stores of wax and honey 

 on which the larvfc of these moths exist. All these various causes 

 tend to show the reason why even predominant species often fail to 

 establish themselves outside more or less local areas. 



The peculiarities which are everywhere evident in the distribution 

 of insects depend very much on their habits and mode of life, and 

 these factors are pre-eminent in limiting the extension of their present 

 boundaries by migration. We have already shown that physical 

 barriers which would prove insuperable in the case of many animals 

 are practically obliterated in the case of insects. They cross wide 

 arms of the sea, vast tracts of ocean, and even high ranges of 

 mountains, and such barriers as these usually form, thus, oftentimes, 

 prove ineftectual. Their oiTective migration is, however, frequently 

 limited by organic and climatic conditions to an extent unknown 

 among other groups of animals, and their permanent settlement in a 

 now country entails an adaptation to such a complex series of 



