148 B. p. UVAROV. 



larvae comes across a solitary specimen of the same form, the latter immediately 

 joins the movement, but when a larva of danica is overtaken by a swarm of migratoria, 

 it tries to escape by leaps as quickly as it can. At the same time, the larvae of danica 

 seem to possess thermotropism of the same kind as that exhibited by migratoria, 

 their time of feeding being restricted to the evening and early morning, while 

 during the day they are probably also on the move, though I have no reliable 

 observations on this point. As for the adults of danica, the only point of their biology 

 that we know for certain is that they do not form swarms and hardly migrate at all ; 

 a study of their behaviour, as well as of their anatomy (air-sacs) is of the greatest 

 importance for the solution of the whole problem. 



Some very interesting indications of further biological differences between 

 danica and migratoria have been obtained at the Turkestan Entomological Station 

 by V. Plotnikov in his breeding experiments.* This entomologist obtained from 

 typical individuals of danica, kept in the laboratory, a second generation of larvae 

 in only 16-30 days after oviposition, instead of in the following spring as is usually 

 the case with migratoria. In one particular experiment even three generations 

 were bred in one year. That this unusually short period of hatching was not due to 

 the unnatural conditions of the experiment is shown by the fact that eggs laid in 

 the same laboratory by individuals of migratoria did not hatch before the following 

 spring. Dissections of eggs showed that the development of the embryo begins 

 in eggs of both migratoria and danica shortly after oviposition ; but in the case of 

 migratoria, when the embryo reaches a rather advanced stage, development is 

 suspended for several months, corresponding to the period of hibernation, though 

 in the laboratory there is no change in the conditions to account for this. At the same 

 time and under exactly the same conditions, the embryos in eggs of danica develop 

 without any interruption. These experiments suggest an explanation of the fact 

 that, while the larval stages of migratoria may be found only in spring and the 

 beginning of summer and the adults during the summer and autumn, there is no such 

 strict regularity about the occurrence of the stages in danica, though many eggs of 

 this form probably hibernate as well. 



Field Observations on the Transformation of migratoria into danica. 



During the great invasion of locust swarms which occurred in the Stavropol 

 province in the autumn of 1912 (see p. 147), I used the opportunity for studying, 

 from the systematist's standpoint, as large a series of specimens as possible. All 

 the insects collected, which were taken from the swarms without any selection and 

 amounted to many hundreds, proved to be quite typical migratoria. Nothing in the 

 least like danica was observed in field, either by myself or by my assistants, whom I had 

 previously instructed to look out for all aberrant forms and who knew danica perfectly 

 well ; the number of individuals thus studied without collecting them is difficult to 

 estimate, but it doubtless amounted to many thousands. I beheve, therefore, that 

 I am right in assuming that the swarms consisted purely of migratoria, and that 

 danica, or even intermediate forms, were entirely absent. The measurements of 

 the specimens from those swarms are given in the fifth line of Table I (p. 139), 

 and the following conclusions may be drawn from them : the specimens are rather 

 uniform, the extent of their variability (0-16) being less than the average for migra- 

 toria (0-18 — see line 1) ; the average figures for the pronotal (0-79) and the femoral 

 (0-45) proportions are extremely near to the average for migratoria (0-80 and 0-46, 

 respectively). The colour characters, though not very rehable, were very constant, 

 which is not the case in danica. If we consider also that the locusts kept in close 

 swarms which had no tendency to disperse, we must conclude that the swarms were 

 formed exclusively by typical individuals of migratoria. 



* Report on the work of the Turkestan Entomological Station in 1912, 1913, 1914, and part of 

 1915; pp. 28, 55-59; Tashkent, 1915 (in Russian) ; see also Rev. Appl. Entom., iv, p. 211. 



