1 64 THE BIOLOGY OF INSECTS 



and devour leaves as their parents do. A very familiar 

 example of this likeness of the young to the adult in habit 

 and in form is afforded by the Aphids or " greenfly " (Fig. 

 42). In the spring and summer virgin female broods of these 

 abundant insects, the mother may be seen on a leaf of her 

 food-plant surrounded by her large family of newly or 

 lately born young. They have the same general aspect as 

 their parent, the same tapering abdomen with its prominent, 

 paired cornicles, and they feed in just the same way by 

 piercing the plant-tissues and sucking thence a continual 

 supply of sap. Newly born aphids are, of course, all 

 wingless, and it is interesting to find that in a large pro- 

 portion of the spring and summer females (Fig. 42, D) — 

 produced in a series of virgin generations — ^wings are never 

 developed, so that the adults, never wandering far from 

 their birthplace over their native plant, remain wingless 

 like the new-born young. In many of the aphid summer 

 females, however, wings are developed (Fig. 42, B, C) 

 from outward rudiments that increase in size after each 

 moult, and these winged individuals can fly away to other 

 plants so as to extend their feeding-ground. Among the 

 aphids the absence of wings accompanies passivity of habit, 

 and the same connection is still more strikingly shown by 

 whole groups of insects that pass their lives, from egg to 

 adult, on the bodies of animals, deriving thence their food- 

 supply, such as the Anoplura or blood-sucking lice, and the 

 Mallophaga or biting lice which nibble at the hairs or 

 feathers or bite the skin of their mammalian or bird hosts. 

 These groups are entirely wingless throughout life and afford 

 interesting examples of the association of winglessness with 

 the parasitic habit among insects. Here, as might be 

 expected, the adult differs from the newly hatched louse in 

 little except size and the development of the organs of 

 reproduction. As no wings appear, the series of moults 

 through which the insects pass is marked by the smallest 

 possible change of form. 



Where, however, the insect in its earlier stages lives 

 among surroundings or under conditions differing from 



