March 1896.1 



PSYCHE. 



361 



the (lak species, the eggs of which are 

 much more snugly ensconced than 

 those of the Pine Laciinus. In the 

 case of many bark-ilepositing species, 

 which develop on the leaves, it is easy 

 to see that the elimination of the unfit 

 is still taking place, and that there is 

 an enormous waste of individuals which 

 might be saved by a more perfectly 

 tleveloped old-fashioned 'instinct.' The 

 Apple Aphis, for example, perishes 

 in great numbers everv autumn by 

 the falling of leaves containing devel- 

 oping colonies of the oviparous form ; 

 such leaves not only bear immature 

 s|)ecimens of this form, but often also 

 adults which have not migrated to the 

 twigs with sutficient promptness. This 

 loss is due largely to the lateness 

 of the arrival of the return migrants 

 to the apple foliage, and would l>e 

 to a considerable extent at least 

 prevented by the earlier development 

 of the latter upon their summer host. 

 The oviparous t'orms also exhibit 

 even now considerable diversity in 

 sites chosen for oviposition, many 

 eggs being deposited upon smooth 

 bark, although the great majority are 

 placed about buds or in the interstices 

 of rough bark. The action of elimi- 

 nation must evidently tend toward 

 the preservation of the latter and the 

 Llestruction of the former. 



The case of the Beech Callipterus 

 described above is evideiitly an illus- 

 tration of a much more perfectly de- 

 veloped instinct than is exhibited by 

 the brdinaiy twig-depositing species. 

 In this case each egg is carefully placed 



in a specialh- chosen site, and is then 

 not only securely fastened in position, 

 but also concealed fiom view. 



The four species of Melanoxanthus 

 mentioned on preceding pages as living 

 upon willow twigs furnish an interest- 

 ing illustration of the gradual perfection 

 of habits of oviposition of species of one 

 genus. The first species (vl/. salicti) 

 oviposits on smooth bark and about the 

 buds, eggs being developed in great 

 numbers. The second species {iM. bi- 

 color) confines itself as a rule to the 

 region of the buds ; in both these the eggs 

 are plain black with no protective cover- 

 ing. In M. sah'a's a decided step in 

 advance has been taken : the oviparous 

 forms congregate upon the gray bark 

 of the trunk and larger branches and 

 deposit their eggs side by side over a 

 considerable area. The sticky cover- 

 ing with which most aphid eggs are 

 provided when first extruded is here 

 abnormally developed. It serves to 

 hold the eggs more firmly in place and 

 also, on drying, leaves a thin gray coat- 

 ing which gives an appearence so 

 .similar to the surrounding bark that 

 the eggs are visible only by the closest 

 scrutiny. In Af. Jlocculosus this is 

 carried a step farther, the protective 

 covering being more perfectly developed 

 anti the insects apparently choosing 

 rougher bark wheie there are more 

 interstices in which to conceal the eggs. 



It is less easy to account for the 

 origin of the method of hibernation 

 ado])ted by those species whose eggs 

 are kept through the winter by ants 

 in their nests. It seems most prob- 



