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dom increase in size, from the time they 
have been deposited by the parent, till they 
are hatched ; those of the tentliredo, how- 
ever, and of some others, are observed to 
increase in bulk. At first there is nothing 
to be perceived in the eggs of insects but a 
watery fluid ; after some little time, an ob- 
scure point is observable in the centre, 
which, according to Swammerdam, is not 
the insect itself, but only its head, which 
first acquires consistence and colour: and 
the same author alleges, that insects do not 
increase in bulk in the egg, but that their 
parts only acquire shape and consistence. 
Under the shell of the egg, there is a thin 
and very delicate pellicle, in which the in- 
sect is enveloped, which may be compared 
to the chorion, and amnios, which surround 
the foetus in quadrupeds. The little insect 
remains in the egg till the fluids are dissi- 
pated, and till its limbs have acquired 
strength to break the egg, and make its es- 
cape ; the different species of insects re- 
main inclosed in the egg for very different 
periods ; some continue enclosed only a few 
days, others remain for several months. 
The eggs of many insects remain without 
being hatched during the whole winter, and 
the young insects do not come forth from 
them, till the season at which the leaves of 
the vegetables, on which they feed, begin 
to expand. When the insects are ready to 
break their prison, they commonly attempt 
to pierce the shell with their teeth, and 
form a circular hole, through which they 
put forth first one leg, and then another, 
till they extricate themselves entirely. 
Insects are by far the most numerous class 
of animals : about eleven thousand species 
have been described by Gmelin, in the last 
edition of the “ System of Nature a great 
many more have been described by other 
naturalists since the publication of that 
work; and a very considerable number are 
to be met with in the cabinets of the cu- 
rious, which have not as yet been described 
by any author. In those parts of the world 
which we are best acquainted with, we may 
easily suppose that many species of insects 
exist, which have hitherto escaped no- 
tice. The minuteness of some insects makes 
them easily overlooked ; the agility of 
others renders the catching of them diffi- 
cult; the retired situations which many of 
them haunt, favour their concealment. In 
the unexplored parts of America, Africa, 
and Asia, many hundred species must exist 
utterly unknown to naturalists: all these 
circumstances render it very probable, that 
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not one half of the insects which exist in 
the world have hitherto been described. 
Insects afford nourishment to a great 
number of the superior animals : many of 
the fishes, reptiles, and birds, draw the 
principal part of their sustenance from that 
source. The immense swarms of different 
species of crab, which abound in every sea, 
directly or indirectly form the principal 
part of the food of the cod, haddock, her- 
ring, and a great variety of fishes. The 
snake, lizard, frog, and many other reptiles, 
feed both on land and aquatic insects. Gal- 
linacious fowls, and many of the small birds, 
&c. feed on insects. Swallows, indeed, 
feed entirely on winged insects. They af- 
ford food, likewise, to many of the mamma- 
lia, viz. to many species of the bat, to the 
ant-eater, &c., and even to man himself. 
Many species of crab, viz. lobster, common 
crab, shrimp, prawn, land-crab, &c. are 
reckoned delicacies. The larvae of some 
coleopterous insects and locusts form part 
of the food of man. Insects, likewise, by 
consuming decayed animal and vegetable 
matter, which, if left to undergo the putre- 
factive process, on the surface of the 
ground, might taint the atmosphere with 
pestilential vapours, preserve the air pure 
for the respiration of man and other ani- 
mals. 
INSERTION, in anatomy, the close 
conjunction of the vessels, tendons, fibres, 
and membranes of the body with some other 
parts. 
INSOLATION, in chemistry, a term 
sometimes made use of to denote that ex- 
posure to the sun, which is made in order 
to promote the chemical action of one sub- 
stance upon another : one of the most strik- 
ing experiments of this kind is that of the 
exposure of vegetables, as fresh-gathered 
cabbage-leaves, in a glass jar of water, to 
the rays of the sun, by the action of which 
a large quantity of pure oxygen gas is ob- 
tained. 
INSOLUBILITY, in chemistry. The 
insolubility of a substance in a fluid, which 
is the medium of chemical action, has an in- 
fluence on that action somewhat similar to 
that of cohesion, and is nothing but a modi- 
fication of it, in relation to the fluid in 
which it is exerted. If substances in their 
liquid state be made to act on each other, 
their action will meet with little foreign re- 
sistance, and will be, in a great measure, 
proportioned to their affinity and quantity; 
but if one of them be solid, and be farther in- 
soluble in the fluid, which is the medium of 
