18 
at certain intervals from the date of egg-laying, and, 
after feeding for a known length of time, turn to chrysa¬ 
lids, from which the insect develops also at a known in¬ 
terval. 
In the case of many of our double-brooded insects, 
the duration of the time of the whole series of changes 
is much less in summer than in winter, and usually 
(although not invariably) with regard to the insects 
of this country a certain amount of warmth and of 
moisture in the surrounding earth or air are the con¬ 
ditions most favourable for rapid and healthy develop¬ 
ment. 
This progress, however, may be modified by a great 
many circumstances, sush as temporary drought, or 
excess of wet, heat, or cold, and also by the effect 
which the condition of the plant-food, altered by these 
influences, exerts on the rapidity of growth, or the 
healthiness of development, and it is in a great degree 
in the fact of these various circumstances occasionally 
occurring in such succession as may suit the require¬ 
ments of the insects in their successive stages that 
we find the explanation of the extraordinary appear¬ 
ances of insect-life that we occasionally suffer from ; 
whereas in the usual order of events they so influence 
one another that we are not devastated by one kind 
of insect, or the crops especially choked by one kind 
of weed. In American observation it is said that it 
takes at least two years to foster a great outbreak, 
and we may trace how a widespread attack is or may 
be produced by precise coincidence of suitable weather 
to the period of the different stages of the insect, from 
the notes given in 8th Report of the State Entomologist 
of Missouri regarding one of the greatest American 
insect-pests, the grass-and corn-feeding caterpillar known 
as the “Army Worm.” This Northern Army Worm 
is the caterpillar of Leucania impunctata, Haw., a 
reddish-brown or fawn-coloured moth, about two inches 
in the spread of the wings, which is considered to lay 
its eggs mainly in old meadows or grass that has not 
