420 
LEPIDOPTERA. 
grasshopper and a migrating locust; this insect occurs over a large part 
of India in small numbers as an ordinary member of the fauna not occur¬ 
ring in specially large numbers. In certain areas it becomes extremely 
abundant in occasional years, packs into swarms and migrates over long 
distances. In this case, the change of habits is associated with a change 
of colour, the insect becoming suffused with brilliant red. Normally 
this occurs in November or early December when the insect migrates ; 
yet specimens with the normal colouring are found elsewhere or when in 
small numbers right through until June and no colour-change takes 
place. One would hesitate to attribute the colour-change to the mere 
change of habits did one not also feel that the habit of migration is one 
that must exert a profound effect on the insect itself. We have elsewhere 
suggested that the red colour facilitates migration in swarms since it 
renders the swarm visible at a distance and enables stragglers to come 
up (Mem., Agric. Dept., India, Entom. Vol. I, No. 1); that the 
commencement of the migration induces the colour-change is striking, 
but the observation of this insect leads one to believe it to be true. The 
question that naturally arises is, why do locusts migrate ? Why does 
this impulse come upon them and impel them to move in swarms over 
long distances, or even, as the Bombay Locust does in its early flights, 
to fly solitarily and steadily in one direction at night till they have 
covered a hundred or as much as 200 miles. We believe it is due to 
two motives : first the need of food ; second, the need of finding satisfac¬ 
tory places to lay eggs. In the year 1903, the Bombay Locust gathered 
in immense quantities in the forests of the Western Ghats before the 
winter ; this was probably for food since only in these forests would they 
find a sufficiency of green leaves ; afterwards they moved out in swarms, 
as they have done before and after; this was, we believe, to enable 
them to lay eggs in places where grass abounded rather than trees, 
since the hoppers live in moist grasslands among low vegetation. The 
same two motives would appear to apply to the Migratory Locust which 
first migrates in search of food and has a brilliant u migration ” colour, 
and then moves further in search of sandy wastes and gets a protective 
yellow “ Sand colour ” when it is going to lay eggs; only in this species 
the hoppers too have a migrating habit since they are born and live, not 
in the midst of plenty in rainy places as does the Bombay Locust-hopper, 
but in arid lands where the drought-resisting bushes afford less food. If 
then these views are correct, the migrating habit has arisen in the latter 
species as a necessity of food getting and reproduction, and is so habitual 
as to be instinctive, while in the former it arises only in the adult when 
it is surrounded by many of its kind and the assumption of the habit 
produces peculiar colour-changes as a physiological result. Out of all 
the many grasshoppers in India, we know of this habit in only two 
species and one may wonder why it should occur in these only; but 
it is necessary to think back one stage and wonder why these two 
should reproduce so abundantly. This we cannot answer, save by 
saying that Nature is full of variety and makes one species prolific, 
another always a rarity. There is reason to believe that, like the 
