PHYSIOLOGY OF INSECTS. 139 



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 characterise them. 



520. Let us then examine the caterpillar of a 

 common butterfly, and trace its system of brains 

 and nerves : in this we have the advantage of 

 being able to examine it in all stages, and to com- 

 pare and ascertain, with tolerable certainty, both 

 the mode and extent of change. 



521. In the larva of a butterfly the spinal cord 

 has thirteen knots, one in each of its segments ; 

 these knots, from their similarity in office to brain, 

 may be called cerebroids (cerebroidce] ; and each 

 one may be distinguished by the name of the 

 segment in which it is situated. 



522. The first cerebroid (capitis cerebroida), is 

 composed of two hemispheres ; from these arise, 

 in perfect insects, several pairs of nerves ; there 

 are two optic nerves (nervi oculorum) ; two anten- 

 nary nerves (nervi antennarum) ; two mandibulary 

 nerves (nervi mandibularum) ; two maxillary nerves 

 (nervi maxillarum) ; and two labial nerves (nervi 

 labii). 



523. To return, however, to the caterpillar, the 

 first cerebroid is situated above the cavity of the 

 mouth ; from each hemisphere of the cerebroid a 

 cord proceeds; these unite on leaving the head, 

 just above the opening of the throat, forming the 

 second cerebroid (prothoracis cerebroida) ; they 



