16 THE BOOK OF NATURE STUDY 



the peculiarity mentioned. They owe their extraordinary abund- 

 ance to the special powers of reproduction during favourable 

 weather. The eggs are laid in the autumn in crevices of the 

 bark or other sheltered positions ; they hatch in the following 

 spring, producing wingless females only. These females give 

 birth to others like themselves, and that without any intercourse 

 with males, for at this season there are no males, and without 

 depositing eggs, but produce their daughters alive viviparously. 

 This viviparous, parthenogenetic (virgin-birth) method of repro- 

 duction continues for several generations ; the daughter aphis 

 herself becoming a mother in about ten or fifteen days. 

 Hence the offspring of a single individual become in a few weeks 

 extremely numerous. In the course of these generations there 

 may occur some individuals with two pairs of well-developed 

 wings, but these are usually females, and perhaps make their 

 appearance the better to secure the dispersal of the species. As 

 the autumn sets in and supplies of food become less abundant 

 males and females, both possessing wings, are born. These 

 pair, and fertilised eggs are laid which hatch out in the spring, 

 and so the cycle is repeated year after year. 



Ants are very frequently found in attendance upon clusters 

 of plant-lice, and are seen to caress the abdomen of the aphis 

 with their antennae. This they do for the sake of a sugary liquid 

 which is secreted from a pair of tubular processes projecting 

 from the dorsal side of the fifth abdominal segment. The sticky 

 deposit (" honey-dew ") often found on the leaves of trees, or 

 even dripping on to the ground, is of the same origin. 



There are many insects which feed themselves and their 

 young upon plant-lice, and which are therefore the friends of 

 the gardener ; chief among these are wasps, lady-bird beetles of 

 all sorts, and both as larvae and imagines, and the larvae of the 

 lace-wing fly, and of syrphid dipterous flies (hover-flies), not to 

 mention minute hymenopterous parasites that live within the 

 body of the aphis itself. 



Some of the plant-lice, including Phylloxera, are responsible 

 for the formation of galls by plants, the insects passing part 

 or the whole of their lives in these curious excrescences. One 

 of the commonest of such galls in this country is the " false 



