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POPULAR SCIENCE REVIEW. 
wings, become more apparent, and when the insect finally leaves the water, 
these rudimentary wing-cases cover the first two or three abdominal segments. 
The external sexual differences commence to manifest themselves at the 
eighteenth stage. They consist principally of the rudiments of the pillared 
eyes, and of the processes on the under side of the penultimate abdominal 
segment, both of which are characteristic of the male sex. 
The Birds of Siberia . — In an important treatise, published under the 
patronage of the Imperial Geographical Society of St. Petersburg, and 
which is the second of a series intended to be issued on Siberian zoology, the 
author, Herr Radde, not only records the species, but gives an account of the 
periods of the migration of Siberian birds. He gives a list of 368 species, which 
he refers to the following orders : — Rapaces, 36 ; Scansores, 19 ; Oscines, 140 ; 
Gallinacese, 18 ; Grallatores, 74 ; and Natatores, 81. Concerning the mi- 
gration of birds, Herr Radde confirms the result arrived at by Yon 
Middendorf in his learned memoir “ Die Isepiptesen Russlands ; ” the most 
important of them being, (1) that the high table-land of Asia and the 
bordering ranges of the Altai, Sajan, and Dauria retard the arrival of the 
migratory birds ; (2) eastward of the upper Lena, towards the east coast of 
Siberia, a considerable retardation of migrants is again noticeable ; and 
(3) the times of arrival at the northern edge of the Mongolian high steppes 
are altogether earlier than those of the same species on the Amoor. 
