152 TRANSACTIONS OF THE ILLINOIS 



according to Packard, also considers this mode of generation a process 

 of budding similar to that seen in vegetables, and that the whole series, 

 from the spring egg until the end of the last fall brood, is but a single 

 generation resulting from the union which produced the egg in the fall, 

 just as the leaves of the tree, which are renewed each year, are said to be 

 contained in the germ of the acorn, or to result from the union of the 

 staminate and pistillate elements that produced the acorn. The theory 

 is the same as Huxley's in substance ; but the illustration is wholly inap- 

 plicable, as the leaves are but the organs of the individual, as are the legs 

 and antennae of each individual Aphis. Parthenogenesis and agamogene- 

 sis are terms which, upon their face, are indicative of our ignorance of 

 their meaning, and are used to express a fact without conveying an idea, 

 if we can suppose such a thing possible. Balbiani has revived the old 

 theory of Leuroenhoek and Cestoni, that of internal impregnation or 

 hermaphrodism. He maintains that the embryo viviparous Aphidians 

 are hermaphrodites. He also finds a similar condition in other species 

 considered parthenogenetic, thus striking a heavy blow at this incompre- 

 hensible theory. 



There are numerous facts in the life-history of some of the lower 

 animals, as strange as it would be for the male principle to be retained 

 under certain circumstances in the female Aphis, and separated under 

 others. This is the simplest explanation that can be found, and it will 

 explain how it is possible to protract indefinitely the agamic reproduction. 



But it is not necessary for me to discuss this question further here ; 

 for although very interesting to the physiologist and naturalist, it has but 

 little practical value to the farmer or horticulturist, the fact of the rapid 

 multiplication of these insects being the chief point in connection with 

 their reproduction with which he is concerned. 



In reference to this point it has been truly said, "There is no tribe 

 of insects so universally distributed or exceeding in numbers the plant- 

 lice." One single individual hatched in the spring, if reproduction 

 should go on unchecked, would by the end of the season produce hun- 

 dreds of millions; but fortunately for vegetation and for man, nature, as 

 we shall hereafter show, has provided numerous checks and restraining 

 influences. 



In reference to the effect they produce upon plants, it is only neces- 

 sary to state, when numerous, those on the leaves by constantly withdraw- 

 ing the sap cause these essential organs to curl, blister, and gradually 

 losing their vitality to die and drop ; the constant drain of sap from the 

 bark by those that reside on the trunk and branches not only gradually 

 lessens the vitality, but causes it to split, wrinkle or become roughened, 

 thereby rendering the plant liable to disease and injury by climatic influ- 

 ences. Any one who has watched the operations of the Woolly Aphis of 

 the apple tree, are aware how rapidly a little colony causes a little crevice 

 in the bark to expand. Not only is it true, as has been stated by Curtis, 

 that " probably there is not a plant, from the smallest grass to the most 

 stately tree, that is altogether exempt from the visits of these pigmies ; " 

 but there is not a part of the plant, from the bud and leaf to the root. 



