THE MICROSCOPE IN ZOOLOGY. 8* 



ramifications, is conveyed to every part of the system. 

 " The structure of the air-tubes," as Dr. Carpenter 

 says, " reminds us of that of the spiral vessels in 

 plants, which seem destined (in part at least) to perform 

 a similar office ; for. within the membrane that forms 

 that outer wall, an elastic fibre winds round and round, 

 so as to form a spiral closely resembling in its position 

 and functions the spiral wire-spring of flexible gas- 

 pipes ; within this again, however, there is another 

 membranous wall to the air-tubes, so that' the spire 

 winds between their inner and outer coats." There is 

 much difference in the form of the spiracles, or stigmata, 

 as they are also called ; but in most cases the opening 

 is protected by a sieve or grating formed by hairs or 

 branches of the integument; these prevent particles 

 of dust, &c., from getting into the air-vessels. Some 

 of the large caterpillars of the Sphinx-moths show the 

 stigmata very clearly even to the naked eye. 



CHAPTER VIII. 

 THE MICROSCOPE IN ZOOLOGY (continued). 



You will find much to admire in the examination of 

 the eggs of insects, which are often of great beauty. 

 The butterfly tribe (Lepidopterd) furnish some of the 

 most interesting forms, and those ot the garden white 

 or cabbage butterfly are too common on the leaves of 

 that vegetable. The eggs of the water-scorpion (Nepa 

 cinered) are very curious ; they are of an oval form, 

 and one end is surmounted by seven stiff reflexed hairs 

 or filaments. The eggs of the mangold-worzel fly 

 (Ant homy ia beta) have their surfaces very symmetri- 

 cally marked. In the summer of 1861 these flies 



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