THE INSECT: ITS INTERNAL STRUCTURE 



21 



Excretory Organs. — The organs which ehminate the nitrogenous wastes 

 from the body and correspond in function to the human kidneys, are 

 known as Malpighian tubes. These are bhnd-ended tubes, the walls 

 of which consist of a single layer of cells surrounding a central channel 

 which at one end opens into the hind-intestine, usually near its front, 

 just behind the stomach (Fig. 27). When blood containing nitrogenous 

 waste matter washes over the outer surface of a Malpighian tube, the 

 cells of which it is composed have the power of taking this matter out of 

 the blood into their own substance and passing it through themselves into 

 the channel between them, down which it moves 

 until it enters the mid-intestine, from which it 

 is finally expelled at the anus. 



The Malpighian tubes may be few or many; 

 long or short (see Figs. 22, 23, 24). They show 

 a tendency to collect in groups and to unite near 

 the hind-intestine, so that their outlets into this 

 are much fewer than the number of tubes. It 

 seems possible that a certain amount of poison 

 entering the body by way of the stomach can 

 be eliminated by the Malpighian tubes, which 

 may explain the varying degree of resistance to 

 such poisons by different insects. 



Nervous System. — The nervous system of insects 

 is located along the middle line of the body quite 

 near its under surface (Fig. 22). As in animals 

 generally, it is composed of cells and fibres. The 

 former are for the most part gathered together in 

 clusters which are called ganglia, and from each of 

 the cells in a ganglion, one or more nerve fibres 

 pass out, either to connect with some other nerve 



cell or with some structure of the body. The larger nerves are really bundles 

 of these fibres running side by side like the wires of a telephone cable. 



Apparently each segment of the insect body once had a nerve ganglion, tnit 

 with the fusion of the segments, many of these have also fused, reducing the 

 separate ganglia in adult insects to a smaller number, which varies in different 

 kinds. This fusion has been produced by the hinder ganglia moving forward until 

 in some cases none are found in the abdomen. Different degrees of this are 

 shown in Fig. 28. 



Each ganglion is connected to the one in front and the one behind by one or 

 two bundles of nerve fibres which are called commissures. Each consists of 

 numerous fibres and these taken together form the means of communication 

 between the different parts of the system. 



In the head, in front of or above the oesophagus, is the largest gangUon of the 

 body, called the brain, produced by the fusion of several ganglia. In addition to 

 its two commissures, which connect it with the ganglion next behind, it has nerves 

 which lead to the eyes, to the antenna) and to other parts of the front of the head. 



Fig. 27. — Portion of the 

 Malpighian tube of a fly, 

 greatly enlarged, k, cell 

 nucleus; /, lumen of the 

 central canal; tr, trachejE. 

 (Modified from Gcgcnbaur.) 



