A REVISION OF THE GENUS LOCUSTA, L. 143 



Another case of sexual dimorphism is observed in the adults of migyatoria during the ^ 

 period of copulation and concerns the general coloration.* The males gradually 

 assume a bright yellowish general coloration, most pronounced on the pronotum, 

 while the females become moYQ dull coloured, the pronotum tm'ning to dark brown. 

 These changes doubtless depend on some physiological processes connected with the 

 maturation of the sex product^. As for danica, it is not yet known whether it 

 exhibits any change of coloration during adult life. 



Biology. 



While danica seems to have no marked preference for any particular type of 

 locality for its permanent habitation, except vast waterless tracts and forests, 

 migyatoria, on the contrary, has strictly defined permanent breeding grounds. In 

 Russia, where much attention has been paid by entomologists to the biology of 

 migratoria, its breeding grounds are all confined to the basins of the Caspian and 

 Aral Seas and of Lake Balkhash, and more definitely to the deltas of the rivers 

 discharging into them, namely, the Volga, Ural, Kuma, Terek, Arax, Syr-Darya, 

 Amu-Darya, Hi, etc. These deltas, as a rule, extend over vast areas irrigated by 

 numberless channels which change their course almost every year, some of them 

 forming temporary pools and small lakes. The shores of these channels and even 

 the beds of the shallower ones, as well as all the less elevated portions of land 

 separating the channels from each other, are covered with a dense growth of the 

 gigantic cane, or reed grass {Phvagmites communis, Trin.), sometimes 10-15 feet high, 

 which form almost impenetrable jungles extending over hundreds of square miles. 

 These reed-beds, however, are not uninterrupted, since many of the islands between 

 the channels are more elevated above the level of the water than is suitable for the 

 growth of the reeds, which require a very damp soil. The soil of such islands 

 mostly contains a large proportion of sand, and a peculiar flora of low, not very 

 dense, mesophilous and xerophilous grasses covers them. It is in such localities 

 that migratoria lays its eggs, as a rule,| while its larval swarms wander all over the 

 valley, penetrating through the reed-beds, and even swimming across the streams. 

 The leaves of the reeds, which contain a very large percentage of silica, represent 

 the most preferred food of the larvae. The chmatic conditions in these reed-beds 

 are very peculiar and differ very much from those prevailing in the adjoining steppes 

 and deserts ; since the _ waters of the river are there spread over a vast surface, the 

 evaporation is very extensive, and the damp, hot air amidst the reeds at midday 

 vividly recalls the tropics. 



All the sufhciently investigated breeding grounds of migratoria m Russia are of 

 the character described, and nowhere except in these grounds does this insect live 

 and breed permanently. Naturally the locusts are not always equally numerous 

 in the breeding regions, and in the years of minimum development only a few swarms 

 or even but single individuals may be found ; during the next few years their number 

 mcreases, the swarms become more and more dense, and a maximum is in this 

 way attained. 



The newly-hatched larvae collect in small groups, each of which is, as a rule, 

 formed by larvae hatched from one egg-mass. These groups soon begin to move 

 irregularly to and fro ; if two groups meet they form one larger group and in this way 

 larval swarms are formed, and their movements become more and more regular. 

 Most authors regard want of food as the direct cause of these movements, but this 

 explanation is entirely wrong, at least so far as concerns Locusta migratoria and 



* This fact was first observed by my assistant, Mr. Th. Gliniuk, in 1912, in Stavropol province, 

 and was recorded by me in the paper : " The Fight against I-ocusts in the Government of Stavropol 

 in the years 1907-1912," St. Petersburg, 1913 (in Russian). 



t The matter is really more complicated than this, since the selection of spots for oviposition 

 is usually closely connected with the more minute character of the soil and vegetation. 



