PLANT LICE. 215 



fond.^ They are generally green or brown, and the fully mature 

 sexual forms have large wings ; but they are also propagated 

 asexually. Some species feed on the leaves, while others attack 

 the roots of plants ; and one form of the same species will some- 

 times live in one situation, and another form in another. In some 

 parts of the country they are called Smotherflies, and their 

 sudden appearance was formerly attributed to the blighting in- 

 fluence of the east wind, and they are therefore still frequently 

 called Blight. The extraordinary cycle of their development has 

 lately been studied in several species by M. Lichtenstein of Mont- 

 pellier and others. It has long been known that wingless Aphidce 

 propagate their race asexually, but the anomalies presented by 

 their different stages are very curious. In some species there arr 

 two winged forms, in addition to the two wingless forms, of plant- 

 feeders and root-feeders. One winged form is perfect, and does 

 not feed, being destitute of a rostrum ; but the other, like the 

 wingless forms, is sexless, and is provided with a rostrum. 



One of the most destructive insects of this family is the Aphis ol 

 the vine {Phylloxera Vastaf/rix, Planch.), which has committed great 

 ravages in most of the vine-producing countries of the Continent 

 for several years past. They attack both the roots and the 

 leaves, on which they produce small galls. The various forms are 

 yellowish or reddish, more or less shading into brown or green. 



Family X. — Aleyrodidce. 



Both sexes Avith four wings of nearly equal size, opaque ; fore 

 wings with a single nervure ; body covered with a white powder ; 

 antennse filiform, six-jointed ; pupa inactive, enclosed in the dried 

 skin of the larva. 



The typical species Aleyrodes Proletella, Linn., is a very small 

 reddish- yellow insect, which the older writers on Entomology 

 considei'ed to be a moth. 



Family XI. — Cocndce. 



Antennae filiform, eight- to eleven- jointed ; tarsi with one or 

 two joints ; male with two wings, destitute of a rostrum, but pro- 

 vided with anal setse, as in the Epheinerce ; female wingless, scale- 

 like. 



The Coccidce or Scale-Insects are sometimes very injurious to 

 cultivated plants ; but they are also useful, producing cochineal, 

 1 Compare p. 11". 



