i88i.] Probable Extension of Solar Physics. 483 
parlance denoting an inseCt of the Orthopterous class, it will 
be found that various records of locust increase, about the 
year 1810, along a line extending from Egypt to India, are 
extant : 1823 was a year when they commenced their depre- 
dations in the Crimea ; in 1833 they were destructive about 
Marseilles ; 1845 and 1866 mark ever memorable years of 
havoc in Algeria ; all and each of these epochs indicating, 
as will be seen by reference to Wolf’s tables, the minimum 
period of sun-spots as now more or less established. 
But it must not therefore be assumed that all destructive 
insects have the same period of multiplication ; for while 
the corn weevils of our granaries, or certainly the more 
destructive sort which is imported from the marts of the 
South, have shown a tendency to increase about the mini- 
mum of spots, it has likewise in measure been ascertained 
that the wheat flies ( Diploxis ), of which there are said to be 
two varieties, affect in Germany a decennial period recurring 
towards the maximum years. This phenomenon, I think, is 
in some cases doubtless attributable to the circumstance of 
the greater uniformity in the solar action within the zone of 
the tropics, and in others to the circumstance that insects 
are variously adapted to conditions of climate. 
If, however, species thus show a tendency to increase at 
different periods, and there be recurring times when certain 
families and certain individuals find themselves in the most 
congenial of circumstances for multiplication, there yet re- 
main fixed epochs when a general move is witnessed in the 
mass of the in'sect fauna. These, north of about 45 0 N. lat., 
alternate rather than agree with the extremes of solar 
energy, so that the great European migrations, as obtained 
by tabulation of pamphlet notices, may be stated to have 
recurred regularly every eleven years since 1846, or at times 
when the sun-spot average has been 48*6 according to Mr. 
Norman Lockyer’s method. The set of these migrations 
has been north and west, and in this direction rare butter- 
flies, sphinx moths, and locusts, whose home has been traced 
to southern Asia and northern Africa, travel periodically ; 
the inauguration of the occurrence being made known by 
their vanguard, so to speak, sweeping over the eastern shore 
of our island. That this track is not voluntary or chosen 
by instinCt, as some have supposed, but due rather to a 
prevailing south-easterly direction of the wind, is a matter 
that now-a-days rests on a great amount of experience and 
observation. 
It is likewise of interest to note, in connection with these 
flights, that although they traverse Europe between the 
