88 A HALF-HOUK WITH THE 



and the skin of all should be examined for the 

 purpose of observing these differences. The web 

 of the spider should also be examined. The cords 

 of these beautiful structures, which run from the 

 centre to the circumference of the web, are plain, 

 as seen at figure 214 ; whilst those which form 

 the concentric lines are beaded with drops of a 

 glutinous substance. It is by means of this adhe- 

 sive matter that the webs are held together. Nor 

 should the microscopist neglect examining the 

 spinnarets of the spider, by which these beautiful 

 threads are elaborated. 



The breathing organs of insects are well de- 

 serving attention. Their bodies are perforated at 

 the sides, and the openings thus formed, called 

 spiracles, lead into tubes which are branched, and 

 are called trachece. These air-tubes are composed 

 of a delicate membrane, which is supported on a 

 series of delicate rings, which are easily traced into 

 the more minute branches. They are well seen in 

 the larvae of most of the lepidopterous insects, and 

 represented from a caterpillar in figure 222, 

 plate 8. The spiracle is not an open hole. In the 

 common house-fly, seen at figure 212, plate 7, and 

 the water-beetle (Dyticus), in figure 213, it is 

 covered over with irregular branched processes 

 from the sides of the opening. The object of this 

 obstruction is probably to prevent particles of dust, 

 and other foreign substances, from entering the air- 

 passages, and thus choking the animal. 



The legs of insects will afford an almost un- 

 limited supply of objects for examination. The 

 spoilt specimens of a summer's capture may well 

 supply materials for a winter's examination. The 

 legs of insects are composed generally of five parts, 

 jointed together. The lowest of these is called the 

 tarsus, or foot. It is variously formed to adapt it 



