BITING AND NOXIOUS INSECTS OTHER THAN MOSQUITOES 43 



falls, or when the supply of food is less in quantity, or after a period of deliberate abstention 

 from food, sexual individuals are produced and fertilized eggs are laid which hatch in the 

 spring, and the phenomena are repeated. In other cases these phenomena are added to or 

 rendered more complicated by the intercalated partheuogenetic generations exhibiting well- 

 marked metamorphosis, of kinds such as occur in apterous or in winged insects ; while again 

 the habits of successive generations may differ greatly, the individuals of some generations 

 dwelling in galls, while those of other generations live underground on roots. 



As regards the physiology of production of winged and wingless individuals there has 

 been but little exact inquiry. Vast numbers of individuals may be produced without any 

 winged forms occurring, while on the other hand these latter are occasionally so abundant as 

 to float about in swarms that darken the air ; the two forms are probably, however, 

 determined by the supply of food. The winged forms are less prolific than the apterous 

 forms; and Forbes has noticed in Apliix iiiuiiH-niiHria, where the generations consist partly of 

 apterous, and partly of winged individuals, that when the corn begins to flag in consequence 

 of the attacks of the Aphis, then the proportion of winged individuals becomes large. The 

 appearance of winged individuals is frequently accompanied by a peculiar change of habit ; the 

 winged indiviiluals migrating to another plant, wliicli in many cases is of a totuUv different 

 botanical nature from that on which the apterous broods were reared ; for instance, . l/*/a'.s imdl, 

 after producing several apterous generations on apple, gives rise to winged individuals that 

 migrate to the stems of corn or grass, and feeding thereon commence another cycle of 

 generations. On the whole, it would ajipear tliat the appearance of winged forms is a 

 concomitant of decreasing nutrition. It is a very remarkable fact that the sexually perfect 

 females are invariably apterous, and this is frequently also the case with the males. It is 

 also highly remarkable that the sexually perfect individuals are of comparatively small size. 

 There are at least three kinds of males in Aphidae : — 1, winged males ; 2, wingless males 

 with mouth well developed ; 3, wingless small males with mouth absent. 



We have already alluded to the fact that the mode of reproduction of Aphids leads to 

 an unrivalled increase. This, however, is not due to the prolificness of the individual, which, 

 in point of fact, appears to be considerably below the average in insects, but ratlier to the 

 rapidity with which the young begin to reproduce. This has been discussed by Huxley, 

 Buckton and others. The first-named naturalist calculated that the produce of a single 

 Aphis would, in the course of ten generations, supposing all the individuals to survive, 

 " contain more ponderable substance than five hundred millions of stout men ; that is, more 

 than the whole population of China." It has since been contended that Professor Huxley's 

 calculation was much below the mark. Although it is somewhat difficult to make a 

 calculation dealing adequately with the actual facts, yet it is clear that the increase 

 of Aphids is such tliiit, drawing as they do their nutriment directly from the plant 

 in its growing state, in the course of two or three years there would be no nutriment 

 available for other animals, except such as might be derived from plants not attacked 

 by Aphids. The numbers of Aphidaj would be so great that they could not be expressed by 

 ordinary numerical methods, and their increase would be actually limited onlv by the 

 relations existing between different kinds of plants and between plants and Aphids. This 

 result is avoided by the fact that Aphids are themselves the victims of a whole army of 

 insect enemies. They have the numerous members of a special group [Braconidrc, 

 AphiiUides) of minute Ili/ini-iinpteni to live inside their bodies, and manv Aculfntc 

 IlymcHupfi'm depend entirely on the Aphidte as the source of food for their own progeny, '^phis 



Enemies of the 



