INTERNAL STRUCTURE OF THE BUTTERFLY LARVA. 29 



From the ganglionic mass in the head arise the nerves which supply 

 the eyes and antennae, so that this is evidently the nerve-centre for 

 such special senses as insects may possess. If now we turn to the 

 vertebrates, we shall find that their central nervous system consists of 

 two distinct parts : (1) The brain and spinal cord, placed in the skull 

 and vertebral column. (2) The sympathetic nerve system, which is 

 composed of a double chain of ganglia running through the neck 

 thorax and abdomen in front of the spine. The nerves which arise 

 from this double chain of ganglia supply chiefly the digestive canal 

 and the walls of the blood-vessels, but some of them join the nerves 

 which spring from the brain and spinal cord, and thus connect the two 

 systems together. The nerves from the brain and spinal cord are 

 principally distributed to the skin and muscles of the body and convey 

 impressions from them to the centre, giving rise to sensation, and from 

 the centre to the muscles, giving rise to movement ; those from the 

 spinal cord are provided near their origin with independent ganglia 

 by means of which certain automatic movements, and others called 

 "reflex," .can be carried on without the intervention of the brain. 

 The ganglia and the nerves in the thorax and abdomen of an insect 

 are similarly automatic in their action. When the thorax is pinched 

 the nervous tissue in it is crushed and its functions abolished, so 

 that no pain is felt when a pin is thrust through it. The insect, 

 however, still moves the abdominal segments and the antennae, 

 because the muscles, which effect this movement, derive their supply 

 of nervous force from the ganglia situated respectively in those parts 

 which have not been included in the pinch, and which act independ- 

 ently of each other. It must not be supposed that the ganglia in 

 the head are, to any considerable extent, comparable with the brain of 

 the higher animals ; there is no evidence that they are in any special 

 degree the centres for sensation, other than sight or smell, and it is 

 highly improbable that insects feel pain even in the slightest degree. 

 This hurried sketch of the nervous system of insects will help us 

 to understand why a wasp, whose abdomen has been severed from 

 its thorax, will go on sucking up juices, even those exuding from 

 its wounded body, for a long time after the mutilation has occurred. 

 Although, as noted above, apparently so different, the development of 

 the nervous system in the embryo is analogous with that of vertebrates, 

 and, although the nervous system of insects is apparently ventral, 

 whilst that of vertebrates is dorsal, the ventral part of an insect 

 corresponds with the dorsal part of a vertebrate, i.e., in reality, opposite 

 parts of the body are placed ventrally in insects and vertebrates 

 respectively, owing to the limbs being turned in opposite directions in 

 the two cases. 



(7) The reproductive system : Herold, as long ago as 1815, figured 

 the changes that he observed the essential reproductive glands to 

 undergo in the larva and succeeding stages of Pieris brassicae, but, up 

 to the present time, there appear to have been no external openings, 

 in connection with the sexual organs, discovered in any lepidopterous 

 larva. The internal glands, however, are not difficult to observe in 

 some larvae, and can usually be obtained by a little careful dissection. 

 The testes and ovaries are placed just beneath the skin of the 5th 

 abdominal segment. They exist in pairs, one on either side of the 

 dorsal vessel, just above the position of the alimentary canal. The 



