952 



INSECTA. 



centratcd structure, although the cords still 

 remain more distinct from each other tlian in 

 tlie higher forms of Coleoptera. In the Ac/ictidte, 

 of which the Mole-cricket affords us an exam- 

 ple, as combining amazing strength and activity 

 with apparently highly developed instinct, there 

 is a more complete general form of the nervous 

 system than in the Acri'la. The ganglia of 

 the thorax are particularly large, but the ganglia 

 of the pro-thoracic segment, in which, and in 

 the enormous limbs, nearly the whole strength 

 of the insect, as we have before seen, is con- 

 centrated, does not equal in size the meta- 

 thoracic ganglion, which is nearly one-third 

 larger than either of the others, although the 

 wings unto which it is given, as well as to the 

 legs, are only of secondary importance as 

 organs of locomotion, and although a fifth and 

 much smaller ganglion is also attached to the 

 meta-thoracic. The cord of the thorax is also 

 large in proportion to the size of the ganglia. 

 In the abdomen the relative size of the cord 

 is less than in the Acrida, and there are only 

 three small oval ganglia in it besides the large 

 terminal one, so that the cord is extended 

 scarcely half way through the abdomen, and 

 yet the whole of the segments, and more 

 especially the posterior ones, are capable of 

 the most free and extensive motion. Thus, 

 then, although in these and other forms of 

 insects, particularly in the Hymenoptera, Lepi- 

 doptera, and Diptera, the ganglia are usually 

 aggregated together in certain segments, appa- 

 rently as a means of concentrating the energies 

 of the animal when one particular region of its 

 body is more actively employed than another, 

 the presence of ganglia in the different segments 

 is not more indispensable to the mobility than 

 to the sensibility of these parts to external im- 

 pressions, since the nerves that convey both 

 motion and sensation to them may be derived 

 from ganglia in distant segments, and yet the 

 freedom of motion be not less than when each 

 segment contains its own ganglion, and derives 

 its nerves immediately from it. 



The structure of the cords in the perfect 

 insect is almost as distinct as in the larva, 

 although the whole of the parts have become 

 more opaque and closely connected together. 

 In some instances it is more strongly marked 

 than in others after the cord has remained for 

 some time in spirits of wine, which is necessary 

 before an examination of its structure is 

 attempted. In many of the Coleoptera the 

 motor column is seen passing in almost a direct 

 line over the ganglia of the sensitive, but the 

 transverse nerves are less easily detected, and 

 in many instances appear to have become 

 united with the other structures. We have, 

 however, seen what we regard as such in the 

 (irifl/itlic, and more distinctly in Gryllotalpa, 

 lying upon and above the motor column. In 

 some specimens we have not found them from 

 their being easily detached in those insects, 

 and, probably, removed during dissection. But 

 in these families we have always found the 

 motor column strongly marked, particularly 

 while passing over the ganglia of the thorax. 

 In the Carubidif (fig. 411) the course of the 



motor column (6) is distinctly indicated as it 

 passes over the surface of a ganglion () by a 



Fig. 411. 



A portion of the gangliated abdominal cord of Carabut 

 monilis. 



a, a ganglion of the external or sensitive column ; 

 li, the upper or motor column; c, a ganglion of 

 the transverse nerves. 



longitudinal sulcus. Just as it is entering upon 

 and also as it is leaving the surface of the 

 ganglion, the motor column gives off a minute 

 branch to join with the large branch from the 

 ganglion of the sensitive column, and with it 

 form a compound nerve. At a part of the 

 cord corresponding to the anterior margin of 

 each ganglion, lying upon and attached to the 

 motor column on each side, is a minute gangli- 

 form mass (<), which we regard as the analogue 

 of the plexus of the transverse nerves. It is of 

 an obtusely angulated shape, and is attached 

 to the motor column by a minute filament from 

 its base on either side, and which passes out- 

 wards in the direction of the anterior pair of 

 nerves. From its upper part in the median 

 line extends another filament, the course of 

 which we have not been able to follow. In 

 Lucanus cervm the motor column is slightly 

 elevated while passing over the ganglia, and at 

 the anterior margin of each gives off a filament 

 to join with the nerve from that part of the 

 cord. We have sometimes observed attached 

 to the motor column, just as it had passed over 

 the meta-thoracic ganglion, on each side a 

 little gangliform mass, which may possibly be 

 part of a series of nerves like those on the cord 

 in the Carabus. In the aculeate Hymenoptera, 

 in which the ganglia of the thorax are large, the 

 motor column is readily observed, but in some 

 of the Terebrantia, as in the Turnip-fly, At/m/ia 

 centif'oliie, when the cord is examined by a 

 strong light, the motor column is most distinctly 

 seen both on the ganglia of the thorax and 

 abdomen, and in this insect exhibits an appear- 

 ance which we have not observed in any other. 

 This is a slight increase in the diameter of the 

 column when it has passed about half-way 

 over a ganglion, and a decrease to its original 

 size when leaving it. Two filaments appear to 

 be given off from the column to join the nerve 

 fi'om the ganglion, one as usual at the anterior 

 margin of the ganglion, and the other, which 

 appears to be the analogue of the transverse 

 nerves, united to the motor column when 

 about half-way over the ganglion. This en- 

 largement of the motor column is greatest 

 where it is passing over the thoracic ganglion, 

 hut is best seen on the abdominal onfs. 

 Tins fact has appeared particularly interesting 



