104 
STRUCTURE OF INSECTS. 
restricted to one part of the body, but are diffused either on 
its outside or through its substance. 
96. The organs of sense, in this group, are less numerous 
than in Yertebrata, and are inferior in perfection; those of 
sight are the most developed, and are formed upon a very 
peculiar plan (§ 573); but all organs of special sense appear 
wanting in the lowest tribes. Yet we find that the muscular 
power is very great; for the animals of this group, taken as a 
whole, can move faster in proportion to their size, and possess 
greater strength, than those of any other. We observe, too, 
that with little or no intelligence, they are prompted to the 
most remarkable actions by instinct alone. They seem to act 
like machines, doing as they are prompted, without choice, or 
knowledge of the end to be gained; and consequently the dif¬ 
ferent individuals of the same species have not that difference 
of capacity and of disposition, which we see in animals whose 
endowments are higher. 
97. In the highest division of the Articulated series, we 
easily recognise, as forms quite distinct from each other, the 
Insects , the Spiders , the Crustaceous animals (crabs, lobsters, 
&c.), and the Centipedes. The class of Insects is distinguished, 
for the most part, by the presence of wings; but to this there 
are exceptions. It includes those of the higher Articulata, 
which breathe air by means of air-tubes distributed through 
the body (§ 320), which have no more than six legs, and 
whose body, in its perfect form at least, manifests a division 
into three distinct parts—the head , thorax , and abdomen 
(fig. 45). To the thorax alone are attached the six legs, as 
well as the wings; and its cavity is principally occupied by 
the muscles that move them : the abdomen contains the 
organs of digestion and reproduction, as in vertebrated animals. 
In the greater part of this class, the young animal comes forth 
from the egg in a condition very different from that which it 
is ultimately to possess; and it undergoes a complete meta¬ 
morphosis , the larva which the egg produces bearing a close 
resemblance in form to the lower Articulata, and only attain¬ 
ing the condition of the imago or perfect insect by passing 
again into a state of inactivity, during which the store of 
nutriment wdiich it has acquired is applied to the development 
of new organs. This pupa or chrysalis condition may be 
considered as a sort of postponed completion of the embryonic 
