io4 



HARDWICKE'S SCIENCE-GOSSIP. 



ledge of the anatomy of an insect, and, with regard 

 to the nervous system, we noticed that the most 

 anterior pair of ganglia, which is placed in the fore 

 part of the head, is joined by two large commissures 

 to the second pair, which is placed lower down, and 

 towards the back part of the head. Through the 

 ring thus formed the gullet or oesophagus passes, and 

 hence the anterior ganglia, being above, are termed 



' 2 * Lh 



Fig. 91.— Brain of mole-cricket, after Dietl.— id, upper division 

 of the brain, or supra-cesophageal ganglia ; lb, lower division 

 of brain, or infra-cesophageal ganglia ; cm, commissures be- 

 tween upper and lower divisions of brain ; x, a cross-band of 

 fibres peculiar to mole-cricket and some other insects ; an, 

 antennary nerve ; op, optic ganglion ; o, ocelli ; fg, frontal 

 ganglion of stomato-gastric nerve ; 1, 2, 3, nerves to mouth 

 organs. 



From the lower pair of ganglia the nerves are given 

 off which supply the mouth appendages. The re- 

 searches of Faivre, in 1857 ("Du Cerveau des 

 Dytisques dans ses rapports avec la locomotion," 

 "Ann. d. Sci. Naturelles Zool." tome viii. p. 245) 

 seem to show that the power of co-ordinating the 

 movements of the body is lodged in the infra-ceso- 

 phageal ganglia, and, therefore, it is not without 

 reason that some authors regard these as a part of 

 the brain. What follows in this communication refers 

 only to supra-cesophageal ganglia, or, as I should 

 prefer to call them, the upper division of the 

 brain. 



The general arrangement of the internal structures 

 will, perhaps, be best understood by reference to the 

 figure given by Leydig, of the brain of Formica rufa 

 (fig. 92). (" Tafeln zur vergleichenden Anatomie," 

 1864, t. viii.) Upon each side there is a large 

 central ovoid mass (pi), which has been termed the 

 primary lobe, and this abuts in the middle line upon 

 its fellow of the opposite side, while the optic nerve, 

 with its ganglion (op), is given off from the outer or 

 opposite end. The optic ganglion itself is a very 

 complex structure. The antennary lobes (al) consist 

 of a number of large rounded masses, which have 

 been called cells, but are really made up of a network 

 of fine fibres. Above the primary lobe are seen the 

 peculiar bodies, having the appearance of half-rings 

 (nib), which have been called convolutions, by 

 Dujardin. (" Sur le systeme nerveux des Insectes," 

 1850, "Ann. Sci. Naturelles Zool." ser. 3, tome xiv. 

 p. 195), and have been compared to mushrooms. 



the supra-asophageal ganglia, and the second being 

 below, are called the infra-cesophageal ganglia. The 

 positions of these parts is 

 very well shown in the 

 diagram of these ganglia, 

 taken from a mole-cricket 

 (fig. 91). The upper ganglia 

 present two rounded pro- 

 minences above, from the 

 sides of which the optic 

 nerves are given off (op), 

 while at the top are seen 

 two ocelli. Somewhat lower 

 down, and towards the front, 

 are two other prominences, 

 from which the antennary 

 nerves pass off (an). A little 

 lower down a nerve is given 

 off from each side, the two 

 joining in the middle line to 

 form the frontal ganglion 

 (fg) ; from this a single 



nerveDassesbackvvardsalor.tr Fig - 92 ~ Bra!n of For > n ^ rufa, adapted from Leydig.— fl primary lobe ; al, antennary lobe; 

 nerve passes DacKwaras along an> nerve t0 antenna . ^_ optic g ang H on ; / e> facetted eye ; o, ocelli ; mi, mushroom-bodies ; 



the upper surface of the 



alimentary canal. Below and 



behind the large commissures (cm) pass to the lower 



ganglia (lb), and, being long in the mole-cricket, the 



two pairs of ganglia are well separated. In some 



insects they are much closer together. 



st, stems of mushroom-bodies ; c, cap of cells covering the mushroom-bodies ; ma, optical 

 section of the anterior mass of nervous matter. 



Each of these mushroom -bodies is supported upon 

 a stem (st) which passes downwards into the primary 

 lobe, where the two lie close to each other, if they 

 do not join. 



