240 TEXT-BOOK OF ENTOMOLOGY 



simplest condition yet known in the nervous system of Machilis 

 (Fig. 239 s). It consists of a fine, slender nerve, which extends 

 along the surface of the ventral chain of ganglia, and sending off a 

 pair of branches (accessory transverse nerves) in front of each 

 ganglion. These accessory nerves receive nerve-twigs from the 

 upper cord of the ventral chain, dilating near their origins into a 

 minute elongated ganglion, and then passing partly outwards to the 

 branches of the tracheae and the muscles of the spiracles, uniting in 

 the middle line of each segment of the body behind the head, i.e. of 

 those segments containing a pair of ganglia. 



e. The supraspinal cord 



In the adult Lepidoptera has been detected, continuous with and 

 on the upper side of the abdominal portions of the ventral cord, a 

 longitudinal cord of connective tissue forming a white or yellowish 

 band, and which seems to be an outgrowth of the dorsal portion of 

 the neurilemma of the ventral cord. Muscles pass from it to the 

 neighboring ventral portions of the integument. Its use is unknown, 

 and attention was first called to it by Treviranus, who called it "an 

 unknown ventral vessel " (Bauchgefass). Afterwards it was re-dis- 

 covered by Newport, who described it as "a distinct vascular canal." 

 But Burger has proved by cross-sections that it is not tubular, but 

 a comparatively solid cord composed, however, of loose connective 

 tissue. Newport found it in the larva of Sphinx lignstri, but Cattie 

 states that it is not present in that of Acherontia atwpos. It has 

 not yet been observed in insects of other orders, but its homologue 

 exists in the scorpion and in the centipede, and it may prove to 

 correspond with the far more complete arterial coat which, with the 

 exception of the brain, envelops the nervous system of Limulus. 



/. Modifications of the brain in different orders of insects 



There are different grades of cerebral development in insects, and 

 Viallanes claimed that it was no exaggeration to say that the brain 

 of the locust (Melanoplus) differs as much from that of the wasp 

 as that of the frog differs from that of man. He insists that the 

 physiological conditions which determine the anatomical modifica- 

 tions of the brain are correlated with 1, the food ; 2, the per- 

 fection of the senses; and 3, with the perfection of the psychic 

 faculties. For example, in those which feed on solid food and 

 whose oesophagus is large (Orthoptera and Coleoptera), the con- 

 nectives are elongated, the subcesophageal commissure free in all 



