THE STRUCTURE AND LIFE OF INSECTS 243 



composed of a substance resembling horn in texture, 

 but differing from horn in composition ; this substance 

 is called Chitin. It is one of the very few components 

 of the bodies of animals which can resist the action of 

 boiling alkalies, however strong. For the sake of flexi- 

 bility the chitinous armour is divided into segments, and 

 these segments are united by membranous junctions, 

 where the chitinous covering, though not interrupted, 

 becomes thin and flexible. The segments are grouped 

 into three regions, head, thorax and abdomen. There 

 are three pairs of legs, one pair to each segment of the 

 thorax. The head is furnished with a pair of feelers, 

 a pair of compound eyes, and biting jaws, which will be 

 seen very indistinctly in the whole cockroach. Along 

 the sides of the body, in the thin membranes which 

 unite the segments, are the breathing-holes, or spir- 

 acles, but these cannot be well seen without special pre- 

 paration. 



We can next show by means of the lantern-microscope 

 further details, which require enlargement. The head of 

 a cockroach may be prepared for demonstration in this 

 way. Cut it off, hold it between the finger and the thumb, 

 pass a scalpel into the mouth, press the edge upwards 

 arid thus divide the head into a front and a back half. 

 Boil these in a solution of caustic potash (10 per cent.) for 

 a quarter of an hour or more, then soak in water, changing 

 the water now and then until the potash is completely 

 removed. Get rid of the water by soaking in methylated 

 alcohol, afterwards in absolute alcohol, and lastly in 

 turpentine. Mount in balsam, and the preparation is 

 ready. Some days should however be allowed for harden- 

 ing before any balsam preparation is put. into the lantern. 

 Watch-glasses may be used to hold costly fluids like 

 absolute alcohol. 



Put the front half of the head into the lantern. Observe 

 the large compound eyes, made transparent by the potash, 

 the long, many- join ted antennae, the mandibles, with their 



