390 THE BIOLOGY OF INSECTS 



Later, aphids are born which develop into winged autumn 

 migrants ; these fly back to the apple-trees, where the 

 sexual forms are born and, when mature, pair in preparation 

 for the winter eggs. 



The bark-lice (Chermesidae), a family of Hemiptera 

 closely allied to the Aphidae, afford interesting examples of 

 insects with a complex Hfe-cycle adapted to two distinct 

 types of coniferous trees, with the formation in certain 

 generations of remarkable galls. Chermes differ from 

 Aphids in its mode of reproduction, as egg-laying is 

 practised by all the virgin generations as well as by the mated 

 females. In most species of Chermes whose life-cycles have 

 been investigated, by A. Cholodkowsky (1907), C. Borner 

 (1908), H. M. Steven (1917), and others, the sexual forms 

 are developed on the Spruce, which is consequently known 

 as the '' principal host " plant of the insects. Both sexes 

 are wingless, and after pairing each female lays a single egg 

 which hatches in late summer ; the first-stage young winter 

 on the buds of the tree, their long piercing stylets inserted 

 into the tissues. In the succeeding spring they begin to 

 feed vigorously, and in less than a month become mature 

 " foundresses," all this generation being virgin females, each 

 of which lays a number of eggs and then dies. The tree 

 responds to the stimulus of their sucking action and salivar}^ 

 secretion by forming a chambered gall like a small cone or 

 pine-apple within which the young develop from the eggs 

 which the foundresses laid. As these young grow and 

 develop within the gall chambers during summer they are 

 known as gall-dwellers (" gallicolae "), but before the last 

 moult they come out on to the leaves and ultimately acquiring 

 wings fly away from their native spruce to some other 

 coniferous tree — larch, pine, or fir — as " migrants." They 

 are all females and lay their eggs on the larch or other 

 " intermediate host " tree ; the young in their first stage 

 hibernate, with their piercers thrust into the bark or leaves, 

 and become mature in spring as '* colonists "—wingless, 

 virgin females which in their turn lay eggs ; from these are 

 hatched young which may develop quickly by midsummer 



