102 A BOOK OF INSECTS 



of these marvellous creatures to the world in which they 

 live. 



It will be convenient to consider first the nervous 

 system of the insect, and the organs which minister to 

 it — the receivers and wires, so to speak, by means of which 

 impressions are taken up and transmitted to the centres 

 of perception. We have already seen that the central 

 nervous system differs in a marked degree from that of 

 a vertebrate animal : that it consists of twin nerve-cords, 

 which extend along the ventral floor of the body, and 

 connect a series of paired knobs, or ganglia. Typically, 

 there is a nerve-centre (paired ganglia) to each segment, a 

 condition which is most nearly realised in certain larvae, 

 and adult insects of the order Aptera. But in all other 

 adult insects the ganglia tend, as it were, to draw together, 

 while the nerve-cords frequently become one. In the 

 head, the ganglia are always united to form two masses. 

 The first is the brain proper, which innervates the eyes 

 and antenna?, and directs the movements of the legs and 

 wings ; the second may be regarded as a kind of supple- 

 mentary brain which governs the activities of the mouth- 

 parts. 



The consolidation or linking up of the originally separate 

 nerve-centres of the thorax and abdomen is most marked 

 in highly developed insects. " In the stag-beetles " (writes 

 Professor Carpenter) " the three nerve-centres of the fore- 

 body are distinct, though the second and third are nearly 

 united, but the number of nerve-centres in the hind-body 

 is reduced to three. In bees there are only two nerve- 

 centres in the thorax, and five in the hind-body. In gad- 

 flies the three thoracic nerve-centres are fused into a single 

 mass ; those of the abdominal chain, though still distinct, 

 are moved close together and far forward. In chafers and 

 in house-flies and their allies all the nerve-centres behind 



