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 Crustaceans 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 which 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 



