OF PAPILIONACEOUS INSECTS. 59 
Those of the Meadow Brown Butterfly (Hip- 
parchia Janira) are crowned by imbricated scales 
like the tiles of a roof, as in the following figure : 
The period of hatching varies according to the 
state of the atmosphere. A certain degree of heat 
is also necessary to the exclusion of the caterpillar. 
This heat is in most instances derived from the state 
of the air ; but other causes sometimes produce it. 
Those species which have several broods in the year, 
—such as the Nettle Tortoise-shell Butterfly, are 
hatched in a few days after they are laid ; but should 
the female lay late in the autumn, the eggs remain 
in a state of hybernation till the succeeding spring. 
That this condition and difference are attributable 
to the influence of temperature, has been proved by 
numerous experiments. These late laid eggs may be 
hatched by placing them in the temperature of sum- 
mer heat. The Silkworm is never hatched till six 
weeks after its extrusion. However, by artificial 
means, the ordinary laws of nature may be altered ; 
for in countries where they are much propagated 
on account of the silk, it is the practice for women 
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