154 TRANSACTIONS OF THE ILLINOIS 



are short and larger than the others ; the third usually much longer than 

 either the first or second. The beak (apparently) but three-jointed, short, 

 arising apparently from the breast and standing perpendicular. Wings, 

 when present, placed against each other in repose in the form of a very 

 steep roof; they are of the typical form heretofore described, the fore- 

 wings with the three discoidal veins or main oblique branches, the third 

 twice forked ; the hind wings with two discoidal veins. (See Fig. 3, No. i . ) 

 The abdomen is furnished near the extremity with two distinct honey- 

 tubes, which stand out obliquely like short cylindrical horns, and which 

 are never reduced to mere tubercles. The posterior legs are much longer 

 than the others. The body is always naked, or nearly so, at least it is 

 never covered with down or the cottony substance common on the woolly 

 lice. They always operate on. the surface, never inclosing themselves in 

 galls. They are found chiefly on the leaves of plants or on the tender 

 shoots of the current year, and are extremely common, being diffused 

 upon almost every kind of vegetation. They resemble each other so 

 closely that it is very difficult to detect any marks or differences by which 

 to distinguish them from each other, hence some of the older authors 

 described them simply by stating what plant and the part thereof they 

 infested. As a general rule, those which reside upon the leaves select 

 the under surface, doubtless for the purpose of avoiding the rays of the 

 sun, the rain, and also to escape the observation of birds and other ene- 

 mies. It is also probable they find the under surface of the leaves more 

 easily penetrated by their tiny beaks. 



The constant withdrawal of sap from the tissue on one side of the 

 leaf necessarily dwarfs the growth of this side, and causes the leaf to curl 

 in that direction ; or, if confined to a central point, that point of the leaf 

 puffs up, giving the upper side a blistered appearance. 



13. Aphis mall, Fabr. The Apple-tree Plant-louse, or Apple Aphis. 



This, as its name indicates, is the common plant-louse, which is so 

 frequently met with on the tender twigs and leaves of the apple tree. It 

 is more than probable that two or three different species are often con- 

 founded under this common name ; but practically this is of but little 

 importance, as the habits of the closely allied species are generally very 

 similar, and the remedies to be used against them the same ; still, it is 

 well for the horticulturist to be able, if possible, to identify the different 

 species which injure his trees. 



It is often quite difficult to determine whether the lice upon Ameri- 

 can plants are the same as the European species which occur on the same 

 plants, as the descriptions by the early authors are very brief, and drawn 

 up without proper examination and care ; but a comparison of the Apple- 

 louse of America with that of Europe shows that they are identical, there- 

 by making it evident that it has been imported into this country. But a 

 change of locality has produced a slight change in the color, which for a 

 time casts some doubt over this question. The following somewhat full 

 description of this species will enable the reader to identify it with suffi- 

 cient certainty : 



