INSECTA. 



235 



after birth. The only change which these fertile female larv» after- 

 wards undergo is increase of size and development of the repro- 

 ductive organs. In the last generation, which is the seventh, the 

 ninth, or the eleventh, according to the species of Aphis, the fer- 

 tilising influence would seem to have expired, and developmental 

 force exhausts itself in more frequent and numerous moultings, in 

 the formation of wings, and in the modification of the female organs 

 already described. Many males, which, like the females, acquire 

 wings, form part of the produce of the last brood, which takes place 

 in Autumn. They rise in the air, frequently migrate in incalculable 

 numbers, unite, and the females then produce eggs, which are glued 

 to twigs and leaf-stalks, retain their vitality throughout the winter, 

 are hatched in the spring, and give birth to the apterous and larvi- 

 parous females, which continue to produce successive generations of 

 similar females until the close of summer. 



But why, it may be asked, should there be this strange combination 

 of viviparous generation at one season and of oviparous generation 

 at another in the same insect? The viviparous or larvi parous gen- 

 eration effects a multiplication of the plant-lice adequate to keep 

 pace with the rapid growth and increase of the vegetable kingdom 

 in the spring and summer. No sooner is the weather mild enough 

 to effect the hatching of the ovum which may have retained its vi- 

 tality through the winter, than the larva, without having to wait for 

 the acquisition of its mature and winged form, as in other insects, 

 forthwith begins to produce a brood, as hungry and insatiable, and as 

 fertile as itself. The rate of increase may be conceived by the fol- 

 lowing calculation : — 



The Aphis lanigera produces each year ten viviparous broods, and 

 one which is oviparous, and each generation averages 100 indi- 

 viduals. 



1st generation 1 aphis produces 



2d 100 



3d 10,000 



4th 1,000,000 



5th 100,000,000 



6th 10,000,000,000 



7th 1,000,000,000,000 



8th 100,000,000,000,000 



9th 10,000,000,000,000,000 



1 0th 1 ,000,000,000,000,000,000 



If the oviparous generation be added to this you will have a thirty 

 times greater result. 



hundred, 

 ten thousand, 

 one million, 

 hundred millions, 

 ten billions, 

 one trillion, 

 hundred trillions, 

 ten quatrillions. 

 one quintillion. 



