286 



INTRODUCTION TO ZOOLOGY 



CHAP. 



food supply. When this is running short, owing to the large 

 number of wingless individuals produced, then winged forms 



appear, which 

 migrate to a fresh 

 plant and there 

 found a new 

 colony. 



Young are pro- 

 duced by these 

 winged forms also 



FIG. 212. Aphis off Sycamore. (Winged form. ) 



parthenogeneti- 



cally, and they 

 closely resemble their parent. The wings of any winged 

 individuals grow gradually with the successive moults of 

 the skin. 



In some eases these winged individuals go though their 

 gradual development protected by a delicate web of silk 

 threads stretched across part of the leaf. In the autumn, 

 however, there always appear normally sexual individuals. 

 The females are always wingless, and usually smaller than 

 the members of the previous summer broods. The males are 

 usually winged, though 

 wingless males do occur. 



The fertilised eggs are 

 laid amongst the scales of 

 the buds of the plant on 

 which the Aphis feeds, and 

 these eggs are able to en- 

 dure the cold of winter 

 which kills the adult insects. 

 They hatch in the spring, 

 and so start the race afresh. 



Sometimes in FIG. 213 - 



ApnlS the autumn > A dead Aphis that has been attacked by the 

 Parasite. 



under syca- 

 more, lime, or fern leaves, 

 the dead outer skin of a 

 winged Aphis may be found 

 fixed on to a light-brown 

 disc formed of interwoven silken threads (see Fig. 213, b). 

 Within this disc may be seen moving a little legless grub 



hymenopterous parasite Praon, one of the 

 false Ichneumon-flies ; ?>, disc woven of 

 silk threads concealing the larva of Praon 

 which has now left the body of the Aphis. 

 Lower on the leaf a disc is shown in sur- 

 face view. 



