g. Functions of the nerve-centres and nerves
As we have seen, the central seat of the functions of the nervous system is not the brain alone (supraœsophageal ganglion), but each ganglion is more or less the seat of vital movements, those of the abdomen being each a distinct motor and respiratory centre. The two halves of a ganglion are independent of each other.
According to Faivre, the brain is the seat of the will and of the power of coördinating the movements of the body, while the infraœsophageal ganglion is the seat of the motive power and also of the will.
The physiological experiments of Binet, which are in the line of those of Faivre, but more thorough, demonstrate that an insect may live for months without a brain, if the subœsophageal ganglion is left intact, just as a vertebrate may exist without its cerebrum. As Kenyon says: “Faivre long ago showed that the subœsophageal ganglion is the seat of the power of coördination of the muscular movements of the body. Binet has shown that the brain is the seat of the power directing these movements. ‘A debrained hexapod will eat when food is placed beneath its palpi, but it cannot go to its food even though the latter be but a very small space removed from its course or position. Whether the insect would be able to do so if the mushroom bodies only were destroyed, and the antennal lobes, optic lobes, and the rest of the brain were left intact, is a question that yet remains to be answered’” (Kenyon).
In insects which are beheaded, however readily they respond to stimulation of the nerves, they are almost completely wanting in will power. Yet insects which have been decapitated can still walk and fly. Hymenoptera will live one or two days after decapitation, beetles from one to three days, and moths (Agrotis) will show signs of life five days after the loss of their head.
That the loss of will power is gradual was proved by decapitating Polistes pallipes. A day after the operation she was standing on her legs and opening and closing her wings; 41 hours after the operation she was still alive, moving her legs, and thrusting out her sting when irritated. Ichneumon otiosus, after the removal of its head, remained very lively, and cleaned its wings and legs, the power of coördination in its wings and legs remaining. A horse-fly, a day after decapitation, was lively and flew about in a natural manner.[[44]]
When the abdomen is cut off, respiration in that region is not at first interrupted. The seat of respiratory movements was referred by Faivre to the hinder thoracic ganglion, but Plateau says that this view must be entirely abandoned, remarking: “All carefully performed experiments on the nervous system of Arthropoda have shown that each ganglion of the ventral chain is a motor centre, and in insects a respiratory centre, for the somite to which it belongs” (Miall and Denny’s The Cockroach, p. 164).
The last pair of abdominal ganglia serve as the nervous centre of the nerves sent to the genital organs.
The recurrent or stomatogastric nerve, which, through the medium of the frontal ganglion, regulates digestion, has only a slight degree of sensibility; the insect remains quiet even when a powerful allurement is presented to the digestive tract (Kolbe).
Faivre states that the destruction of the frontal ganglion, or a section of the commissures connecting it with the brain, puts an end to swallowing movements; on the other hand, stimulation results in energetic movements of this nature.