148 TRANSACTIONS OF THE ILLINOIS 



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Mr. Walsh alludes to a species found by himself and Baron Osten- 

 sacken, forming true galls on the hackberry ; and Dr. Harris alludes 

 incidentally to species found on birch and alder, which have the young 

 covered with a cottony substance, as is usually the case in this genus. 



Aphid.k. I True Plant-lice, i 



This family comprises the true Plant-lice, of which Cabbage-lice, 

 Wheat-lice, Woolly-aphis of the apple tree and the Phylloxera of the grape- 

 vine are representatives. 



It includes some of the most destructive species belonging to the 

 entite group, and presents one of the strangest physiological facts apper- 

 taining to the entire insect class. 



The species belonging to it may be distinguished from the Psyllidce 

 by their antennte, which never contain more than seven joints (the num- 

 ber ranging from three to seven), and being without the two bristles at 

 the tip. From the Aleurodidce they are distinguished by their naked, 

 membraneous, and usually transparent wings. The beak, or elongated 

 mouth, consists of three or four joints, and varies considerably in length, 

 reaching, in some species, to and even beyond the tip of the abdomen, 

 while in others it is quite short. Sometimes it remains more or less per- 

 pendicular in repose, while in other species it is pressed against the 

 underside of the body. The antennse also vary considerably in length, 

 in some extending to and even beyond the tip of the abdomen, while in 

 others they scarcely reach the middle of the thorax. The legs are usually 

 long and slender, with two-jointed tarsi or feet, the hind pair being some- 

 times elongated, as though formed for leaping, although none of the 

 species seem to possess this power'. The head is not so broad as in the 

 preceding family, nor is the thorax as greatly developed in proportion to 

 the size of the abdomen. Occeli, or little eyes, are usually wanting, but 

 are present in a few species. 



In one group or section of the family, the abdomen is furnished with 

 two tubes, usually called "honey-tubes," or "nectaries," which arise like 

 little horns from the sixth segment. These appendages ^erve for the pass- 

 age of a sweet fluid from the stomach, and appear to act as escape pipes 

 for the excessive amount of sap imbibed by these incessant pumpers. If 

 we examine a properly arranged series of species, we shall find that these 

 tubes gradually decrease in length until the orifice alone remains, and at 

 last this also disappears. This decrease appears to correspond, to a cer- 

 tain degree, with the degradation of the species. 



The wings are thin, and usually almost perfectly transparent ; they 

 are traversed by few veins, the- variation in these affording excellent 

 generic characters; usually, when at rest, they are placed against each 

 other above the abdomen, in the form of a very steep roof, but in many 

 species they are placed horizontally on the abdomen. In the typical 

 si)ecies, the fore wings have a strong mid-rib or rib-vein, which runs 

 along very near the front border for about two-thirds the length of the 

 wing, where it comes to the margin; beyond this, where the margin 

 curves, and close to it, are two short arcs ; from the posterior side of the 



