112 INJURIOUS AND USEFUL INSECTS 



afterwards enlarges, and then tapers to the anus. A pair 

 of salivary glands lie one on each side of the gullet, and 

 communicate with the mouth by long and slender ducts, 

 which unite into a common duct, and finally discharge into 

 the mouth. The nervous system can also be made out in 

 the living larva. There are no ganglia in the small head, the 

 brain and sub-oesophageal ganglia being retracted into the 

 prothorax. Behind them comes the usual gangliated chain, 

 with somewhat short and thick connectives. The last ganglion 

 lies in the seventh abdominal segment, the tenth from the 

 head. The tracheal tubes, which in most insects supply every 

 organ of the body with a copious network, are reduced almost 

 to nothing in the larva of Chironomus. By attentive examina- 

 tion of the thorax in side-view, a few delicate and branching 

 tubes can be seen, which constitute almost the only vestige 

 of a tracheal system. It is completely closed, but on each side 

 of the body branches lead to two sealed spiracles. The 

 vestigial system of air-tubes and spiracles is some proof 

 that remote progenitors of the blood-worm lived in air, and 

 breathed after the fashion of ordinary insects. But the air- 

 tubes are now completely closed, and some other means of 

 aerating the blood must be looked for. The blood-worm, 

 having become completely adapted to a submerged life, 

 breathes by gills, tbin-walled, hollow filaments, which stand 

 out into the water, and allow the carbonic acid formed in the 

 body to pass out, while oxygen is passed in. The gills of 

 the blood-worm are the two pairs of long tubules on the last 

 segment but one, and the two pairs of short prominences 

 on the last segment of all. All these gills are distended with 

 blood, which is pumped in and out by the action of the 

 heart, and they have thin, transparent walls to facilitate the 

 exchange of gases. 



The blood-worm, in order to avoid its enemies, keeps to 

 the mud at the bottom of the stream, and there constructs 

 a tube or sheath in which it lies hid. Now the bottom-water 

 of a slow stream contains very little oxygen, particularly when 

 decaying organic matter, such as forms the food of the blood- 

 worm, is plentiful. How then does it manage to aerate its 

 blood ? The difficulty, though serious, is not insurmountable. 

 By day, when fishes and predatory insects are active, the 

 blood-worm stays at the bottom, but at night it comes to the 



