182 PHYSIOLOGY OF INSECTS. 



means of describing the result of observation ; whereas, in 

 perfect insects, several segments are liable to become 

 united, and the exact bomidaries of each are thus rendered 

 subjects of doubt. It must also be observed, that the 

 internal systems of organs following and adapting them- 

 selves to the wonderful changes on the surface, require the 

 utmost caution in the anatomist who attempts to charac- 

 terize them. Let us then examine the catei"pillar of a com- 

 mon butterfly, and trace its system of brains and nerves : 

 in this we have the advantage of being able to examine 

 it in all its stages, and to compare and ascertain, with to- 

 lerable certainty, both the mode and extent of change. 

 In the larva of a butterfly the spinal cord has thirteen 

 knots, one in each of its segments ; these knots, from their 

 similarity in ofiice to brain, may be called cerebroids or 

 cerebroicla ; and each one may be distinguished by the 

 name of the segment in which it is situated. The first 

 cerebroid or capitis cerebi'oida, is composed of two hemi- 

 spheres ; from these arise, in perfect insects, several pairs 

 of nerves ; there are two optic nerves or nervi oculoriim ; 

 two antennary nerves or nervi antennarum ; two mandi- 

 bulary nerves or nervi mandibulartayi ; two maxillary nerves 

 or ne7-vi maxillarum ; and two labial nerves or nervi labii. 

 To return, however, to the caterpillar, the first cerebroid is 

 situated above the cavity of the mouth ; from each hemi- 

 sphere of the cerebroid a cord proceeds ; these unite on 

 leaving the head, just above the opening of the throat, 

 forming the second cerebroid or prothoracis cerehroida ; 

 they then again divide, and, passing one on each side of 

 the gullet, reunite below it, and enclose it in a ring. At 

 the union they form the third cerebroid or mesothoracis 

 cerehroida ; and from this arise four principal nerves, two 

 of which, the mesopedum nervi, descend to the feet, and 

 two others, the prothoracis nervi dorsales, ascend to the 



