INSECTA. 



951 



passage between the two for the esophagus. 

 Each segment of the thorax contains a ganglion, 

 the meta-thoracic one, which gives nerves to 

 the wings, being the largest. In the abdominal 

 portion of the cord, which extends as far as 

 the penultimate segment, there are six double 

 ganglia very distinct from each other. This is 

 exactly the number found by us in the same 

 portion of cord in the Carabidte, although, 

 according to Burmeister, there are only five. 

 Thus, then, in the Forftcutiefo, in which there 

 is the most extensive motion of the segments 

 of the abdomen, there is the same number of 

 ganglia as in the Carabida, in which most 

 of the abdominal segments are anchylosed 

 together and immovable, as in the Lamelli- 

 cornes, in which the whole of the cord is 

 situated within the region of the thorax. In 

 the oil-beetles, as in Proscarabaus vulgaris, 

 there are as usual three thoracic ganglia, the 

 largest being the meta-thoracic, although the 

 proper wings, and scarcely even the elytra, do not 

 exist. In the abdominal region there are five 

 ganglia, but smaller than those of the thorax, 

 although it is stated by Burmeister* that these 

 ganglia of the thorax are larger than those of 

 the abdomen when perfect organs of flight are 

 developed, but smaller when they are absent. 

 So far as our own observations have extended, 

 we have invariably found the thoracic ganglia 

 larger than the abdominal, whether organs 

 of flight exist or not, a condition that might 

 naturally be expected whether the ganglia be 

 connected with the production of nervous energy 

 in the parts, or be only the centres of sensation. 

 In the full-grown larva of this insect, of which 

 we have examined a considerable number, but 

 which at present appears to be scarcely if at all 

 known to naturalists, we have found twelve per- 

 fectly distinct sub-oesophageal ganglia. Of these 

 the fifth was the largest and separated from the 

 fourth only by a very short cord, as were also 

 the eleventh and twelfth, besides which the 

 twelfth was larger than the eleventh, and ap- 

 peared as if formed at an early period of two 

 approximated ganglia. That in the earliest 

 state of this insect there are thirteen sub- 

 resophageal ganglia seems highly probable. On 

 watching the changes that take place it is found 

 that this d ouble terminal ganglion becomes united 

 to the eleventh, and that a similar union takes 

 place between the fourth, fifth, and sixth, so 

 that only five separate abdominal ganglia exist 

 in the perfect insect. In the Forficula, which 

 does not undergo a perfect metamorphosis, a 

 similar change appears to take place at a much 

 earlier period, the terminal ganglion being dis- 

 tinctly formed of two masses, and the ganglion 

 of the meta-thorax or wing-bearing segment of 

 three. A similar change appears also to occur 

 in the Staphylinidtf . In Creophilus maxillosus 

 (Jig. 341), in which the abdominal segments are 

 as freely moveable as in the Earwig, but which 

 undergoes a more complete metamorphosis, 

 there are nine sub-oasophageal ganglia, only 

 the last three of which are abdominal, the last 

 five segments being entirely without ganglia. 



* Op. cit. p. 281. 



There is exactly the same form and position of 

 the nervous system in the common species, 

 Goerius olens. On- comparing these circum- 

 stances it is found that a much smaller number 

 of ganglia in general exists in those perfect 

 insects which have undergone a complete meta- 

 morphosis than in the larva state, and than in 

 those that scarcely change their form. In the 

 Gryllida:, Acrida viridissima (fg. 410), there is 



Fig. 410. 



Nervous system of Acrida viridissima. 

 A, brain; D, antennae; B, optic nerves ; d, man- 

 dibular nerve ; e, auxiliary connecting nerve ; 

 g, nerve of prothoracic legs ; t, of second pair 

 of legs ; k, third pair ; a, tendon of flexor mus- 

 cle with its nerve accompanying it to its insertion 

 at extremity of the femur ; c, second head of the 

 same flexor muscle at the end of the tibia. 



the same number of ganglia as in the Forficula, 

 and a similar difference in the size of the 

 thoracic ganglia. But in this insect there is 

 also a closer lateral approximation of the abdo- 

 minal cords, and a comparatively smaller size 

 and more elongated form of their ganglia, 

 evidently shewing a tendency to a more con- 



