The active nymphs of the Hemiptera, like those of the locusts, resemble closely the imago, differing mainly in possessing the rudiments of wings, which are acquired after the second molting. After two changes of skin (four in all) they assume the pupa state, which differs mainly from that of the larva in having larger wing-pads. While the development of the imago ordinarily occupies the summer months, in the Aphides it takes but a comparatively few days, but in the 17-year cicada as many years as its name indicates. An exception to this mode of development is seen in the nymph of the male coccus, which, somewhat as in the higher orders, spins a silken cocoon, and changes into an inactive pupa. Apterous individuals, especially females, sometimes occur, especially in the aquatic Hydrometra, Velia and Limno bates, and in many other genera the hind pair of wings are often absent. There are about 50,000 species living and fossil. Some species are of great size, especially the Hydrocores. a division containing the aquatic genera, Vella, Nepa, Belostoma and Notonccta, and which first appeared in the Jurassic formation. But
the oldest known fossil insect (Protocimes silurica) was apparently a bug; traces of one wing having been found in the Upper Ordovi cian beds of Sweden.' Consult Packard, 'Guide to Study of Insects> (1889) ; 'Entomology for Beginners' (1899) ; Comstock, 'A Manual for the Study of Insects' (1895) ; Sharp, 'Insects (1899).
BUG, two rivers in European Russia. One rises near the confines of Volhynia, in the northwest of government Podolsk, and pro ceeds first east and then southeast to Oliviopol, where it enters government Kherson, which it traverses almost centrally from north to south, and falls into the estuary of the Dnieper, near Kherson. Its chief affluents are the Ingul, Balta, Tchertal and Solonicha. It has a course of 500 miles, but its navigation is greatly ob structed by rocks and sandbanks. The second river rises in Galicia and joins the Vistula at the fortress of Novogeorgieosk, about 20 miles north-northwest of Warsaw. It is navigable for nearly 300 miles.