222 NATURAL HISTORY OF ARTHROPODS. 



midst of forms which are not more variable than many others of the typical aphides. 

 More than two patterns of form are stated to occur in the Phylloxera of the grape- 

 vine, and hence it is said to be polymorphic. In many species of the true plant-lice it 

 is now well established that there are two types of the same insect, the one inhabiting 

 the roots, and living there during the colder part of the year unwinged, the other in- 

 habiting the leaves and twigs throughout the spring and summer. This is notably the 

 case with a small black aphis which injures, and even destroys, the peach-trees of east- 

 ern Maryland, Delaware, and New Jersey. Other examples may be cited, such as the 

 Woolly Aphis of the apple-tree, Schizoneura lanigera, the Maize Aphis, A. maidis, 

 and the Grape Gall-louse, Phylloxera vastatrix. 



On the other hand, M. Lichteiistein, of Montpelier, France, has recently made and 

 verified some .remarkable discoveries with regard to the plant-louse of the European 

 elm, Tetraneura ulmi. He finds that this species, which produces little smooth green- 

 stalked galls on leaves of the elm, lives during its subterranean budding phase on the 

 roots of maize in Austria and Hungary, and in France on the roots of the dog's-tooth 

 grass. As late as the 10th of December he still found the wingless form alive upon the 

 roots, showing that " side by side with the winged form, which gives rise to the sexual 

 reproducers, there is an uninterrupted sequence of subterranean organic reproduction, 

 so that should any circumstance happen to destroy the winter-egg, there would be 

 always a subterranean provision ready " to replace the unfortunate sexual generation. 

 Besides this the mother does not expel the winter-egg, but keeps it protected within 

 her dried skin, settled deeply in a crevice of the bark of the tree. 



Morren tells us that the plant-louse of the apple tree produces one hundred young 

 ones in a single generation, and that each female of these brings forth others just as 

 numerous, so that by the completion of the tenth generation, which takes place before 

 the advent of cold weather, the original individual has become the mother of one qtiin- 

 tillion of her species. From this we can gain an idea of the countless swarms of these 

 creatures which arise from the multitudes of eggs which are found sticking in cracks 

 and wrinkles of the bark, twigs, and buds of every kind of plant and tree. 



Occasionally after a mild, dry winter they become more numerously winged than 

 usual, and in such a case become overcrowded, migrate to other localities, and fill the 

 air with their dense swarms. This has occurred in notable instances, when whole 

 crops of cereals and vegetables have been destroyed by their combined attacks. 

 Nearly all of the aerial species have the power to secrete and expel from their honey- 

 tubes a sweet fluid, which serves to nourish the young shortly after they are born. 

 It is this liquor which so greatly attracts the ants and other insects, so that the 

 former may often be seen coursing over the branches and twigs in search of aphides, 

 and, when found, stroking them with the antennae, thus causing them to give forth a 

 drop of the substance, which they greedily lap. 



In the southern and central parts of North America little colonies of brown or yel- 

 lowish plant-lice may be seen congregated in almost every underground ants' nest. 

 There the ants carefully tend them, protect their eggs, secure to them a comfortable 

 home where they can enjoy the sap of the tender roots, and are then rewarded by the 

 sweets which flow forth at their bidding. 



The family as now constituted includes four sub-families, Aphidina, Pemphigina, 

 Rhizobiina, and Chermesina, of which the first and second include the largest and 

 most conspicuous forms, and those which are most commonly seen about the farm 

 and garden. Among the former, the genus Lachnus embraces the largest and most 



