54 INJURIOUS AND USEFUL INSECTS 



leads to a rectum which when full has a considerable capacity, 

 but contracts when empty. On each side of the alimentary 

 canal may be seen the long, winding silk-glands. These are 

 usually much longer than the body, and are therefore repeatedly 

 folded. They are of unequal thickness ; and consist of a long, 

 very slender, tubular thread, a wider reservoir, and a narrow 

 outlet or duct. The ducts cannot be completely followed at 

 present, but the glands should be unravelled and laid out with 

 pins. The salivary glands, which are of minute size, open 

 directly into the mouth. They resemble in structure the 

 corresponding glands of the cockroach, but consist of a single 

 pair only. Observe the fine, convoluted Malpighian tubules, 

 which overlie the intestine, and the hinder end of the stomach. 

 When unravelled, they are found to consist of three pairs of 

 delicate tubes, which open into the beginning of the intestine 

 by a pair of common ducts. The Malpighian tubules act as 

 kidneys, separating waste nitrogenous matter from the blood, 

 and discharging it into the intestine. 



Draw the dissection, after displaying the various parts. 



Cut through the intestine near the anus, and turn it forwards^ 

 cutting through the tracheal tubes which hold it in place. 

 Look for the ducts and the silk-glands on the under side of 

 the crop and oesophagus, and find the common duct in which 

 they terminate. Follow this towards the head. A pair of very 

 minute branched glands are often seen on the common duct 

 or its branches near the junction. The fluid contained in 

 each silk-gland is drawn out into a thread as required, and 

 the two threads so formed may possibly be bound together 

 by a cementing substance derived from the small glands. 



By neatly halving a larva lengthwise, removing the ali- 

 mentary canal, soaking the halves in dilute potash solution 

 for a day or two, and then mounting in balsam, beautiful 

 preparations of the respiratory system can be made; some 

 experience in microscopic mounting is of course requisite. 



Clear the nerve-cord, which Hes in the middle line on the 

 ventral surface. Besides the brain and the sub -oesophageal 

 ganglion which occupy the head, there are three thoracic 

 ganglia, and seven abdominal ganglia, united by double con- 

 nectives. In the thoracic region the connectives are more 

 widely separated than elsewhere. 



Separate the silk-glands as completely as possible from the 



