INSECTA. 433 



Leaf-lice, Plant-lice, Pucerons. These animals live together on 

 different plants and trees, and mostly in very large numbers. They 

 do not leap, but run commonly very slowly. From the hind part 

 of the body there often drops a transparent honey-sweet fluid, of 

 which ants are very fond and on that account are often found 

 in the neighbourhood of the leaf-lice. These suck the sap of the 

 plants, and some species, by puncturing the leaves or leaf-stalks, 

 produce excrescences or swellings occasionally hollow, which are 

 filled with a great number of these animals and often with a 

 considerable quantity of the sweet sap. The sticky fluid on leaves, 

 known by the name of honey-dew, is caused by leaf-lice. Most 

 species are covered with a powdery substance or with white threads, 

 (a peculiar secretion of their body). 



To counteract the excessive multiplication of leaf-lice, in the great 

 economy of nature, a number of enemies are on the watch, not 

 merely ichneumons (see above, p. 378), but insects especially which 

 feed on them and devour large quantities, the pupae of some diptera, 

 of hemerobii (p. 418), of beetles (Coccinellce), &c. 



LEEUWENHOECK had already noticed that plant-lice are viviparous, 

 that they also lay eggs was first discovered by LYONET j but the 

 succession of generations, the descendants of mothers and grand- 

 mothers which are viviparous and fruitful without copulation, was 

 first discovered by BONNET ; see above, pp. 263, 264. 



Comp. on this group of insects, as numerous in species as interesting in 

 their economy, KEAUMUR, Ins. in. Him. ix. pp. -281 350; C. BONNET, 

 Traite d'Insectol. Tom. i. Paris, 1745 ; DE GEEK, Mem. p. servir a 1'IIist. 

 cTIns. in. pp. 19 129, and for the systematic arrangement, HAKTIG, 

 Versuch einer Eintheilung der Pflanzenlause in GERMAR'S Zeitsclir. f. die 

 Entomol. in. 1841, s. 359 376, and especially J. H. KALTENBACH, Mono- 

 graphic der Familien der Pflanzenlaiise. Mit Abbild. Aachen, 1843, 8vo; 

 also T. WALKER, Descriptions of Aphides, Ann. of Nat. Hist. sec. Ser. rv. 

 p. 202, v. 1850, pp. 1428, 269281, 388395, vi. pp. 41 48, 

 118 122. 



Some species live on the roots of plants. They have no wings ; BOUCHE, 

 however (according to RATZEBURG, Forst-Ins. in. s. 216), discovered two 

 species of Rhizol>ius that were winged. These species may be collected 

 provisionally under the name of : 



Hhizopthiridium nob. 



Here belongs the genus Rkizobius BURM. (a name already given to a 

 genus of Coleoptera), and the genera Paradetus, Trama and Forda V. 

 HEYDEN, Entomol. Beitrage in Alkandl. der Senclcenl. Geselhch. n. 1837, 

 s. 291 295. Rhizoterus HARTIG, according to KALTENBACH, does not 

 differ from Forda V. HEYDEN. 

 VOL. T. 28 



