138 TRANSACTIONS OF THE ILLINOIS 



I also take pleasure in acknowledging my indebtedness to the writ- 

 ings of Mr. Walsh, Dr. LeBaron, Prof. Riley, Dr. Shimer, Dr. Packard, 

 and others. 



Permit me to add further, that I am endeavoring to seize upon all 

 such opportunities as the present to place in reach of our farmers, horti- 

 culturists, teachers and entomological students the description of insects 

 found, or likely to be found, in Illinois, which are injurious to vegetation. 

 These descriptions, as a general rule, are scattered through numerous works 

 inaccessible to most readers, and usually in language with which unscien- 

 tific readers are not familiar. 



The very common method of giving the descriptions and history of 

 a few species of a group, as illustrations of the group, and leaving the 

 reader entirely ignorant as to the rest, is exceedingly tantalizing to 

 those in search of practical information. 



Trusting that this may be of some benefit to the horticulturists of 

 Illinois, I remain. 



Yours very respectfully, CYRUS THOMAS, . 



State Entomologist. 



PLANT-LICE. 



There is scarcely an individual, old or young, who has paid any 

 attention to plants during the summer, that has not observed upon the 

 leaves or stems, twigs or bark, minute, soft-bodied insects busily engaged 

 in sucking the sap, usually without wings and generally huddled together 

 in little groups. These insects, on account of their resemblance to cer- 

 tain other insects, and their parasitic habits, have, by common consent, 

 received the very appropriate Yid^xn^ oi Plant- Lice. Scarcely a plant in 

 the vegetable or flower garden, a bush or tree in the yar^, orchard, nursery, 

 vineyard, grove or forest, but what at some time appears to be infested by 

 them in greater or less number. And not only is this true in the tem- 

 perate climate of our own latitude, but if we move south to the tropics, 

 or pass to the cold regions of the north, we find the same thing true 

 there. 



On account of their general similarity in appearance and habits, 

 ordinary observers have been content with simply designating them by 

 their common name. That they somewhat closely resemble each other 

 in general appearance, as well as in the habit of subsisting upon the juices 

 of the plants on which they reside, is true ; but when we come to exam- 

 ine them closely and compare them carefully one with another, we find 

 there are marked differences in the details of their external anatomy as 

 well as their habits, showing not only that there is a vast number of 

 different species, but also that the species readily admit of being aggre- 

 gated or arranged into groups by common characters; or, in other words, 

 that there are generic and family differences, as well as specific differences, 

 among them. 



For example : A number of species possess the power of leaping, all 

 of which have the antennae terminated by two minute bristles, in both 

 of which respects they differ from all the other species ; some are always 



