208 LECTURE XVI. 



of the trunk. And if decapitation or amputation of the prothorax be 

 neatly performed on the living insect, while in its natural and ordinary 

 position, perched by its middle and hinder pairs of legs upon a twig, 

 the rest of the trunk does not fall to the ground, but is maintained 

 for a certain period in that posture, which it even recovers by actions 

 of the wings, when the balance is slightly and purposely disturbed. 



The supra-CEsophageal or cerebral mass in insects obtains its largest 

 development in the dragon-fly, which from the size and perfection of 

 its organs of vision, its great and enduring powers of flight and pre- 

 datory habits, may be regarded as the eagle of insects. From the 

 side of each of the superior lobes of the brain, the optic nerve is con- 

 tinued of equal breadth, so as to seem rather as a lobe of the brain. 

 It expands, and, like the stalk of a mushroom, forms the stem of a 

 very large reniform ganglion, the convexity of which is turned for- 

 wards and outwards, and the free concave projecting margin de- 

 veloped at the under part. Thousands of branches to the divisions of 

 the compound eye are given off from the convex surface of this 

 ganglion. The brain presents a single median inferior lobe ; the 

 oesophageal chords sent downwards to the maxillary ganglion are 

 short and thick. This ganglion is succeeded by three large equi- 

 distant thoracic ganglia, of which the last two, corresponding with 

 the elytral and alar ganglions of the preceding insect, are, as might be 

 expected, from the development of the muscles of the wings (both of 

 which are alike organised for flight), considerably the largest. Of the 

 ganglia of the abdomen, the terminal one resulting from the con- 

 fluence of two, and which supplies the organs of generation, is remark- 

 able for its large size. 



In the white butterfly (Papilio hrassiccB) the brain is a thick trans- 

 verse rounded mass, indented by a longitudinal furrow along the 

 median line. From its sides proceed the large optic nerve, now greatly 

 surpassing the other cerebral nerves in size. The oesophageal collar 

 is triangular, leaving a very small interval for the passage of the 

 alimentary gullet. The maxillary ganglion is relatively much smaller 

 than in the dragon-fly, the blatta, and other mandibulate insects. 

 The first two thoracic ganglions are blended into one, and the third 

 thoracic and first abdominal ganglions have coalesced to form a 

 similar mass in the thorax, connected with the preceding by short 

 chords, separated by an interval to allow the passage between them 

 of certain processes of the thorax giving attachment to the muscles of 

 the legs. The ganglions of the thorax have been observed in some 

 species (as the Bomhyx Neustria) to present a reddish tint. They are 

 succeeded in the Lepidoptera by four other ganglions in the abdomen, 

 of which the last, as usual, is the largest. 



