DO A HALF-HOUR WITH THE 



which form the concentric lines are beaded with 

 drops of a glutinous substance. It is by means 

 of this adhesive 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 trachea. 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 (Fig. 212, PL 8). The 

 spiracle is not an open hole. In the common 

 house-fly (Fig. 205, PL 7), and the water-beetle 

 (Dyticus) (Fig. 206, PL 7), 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 sub- 

 stances, 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 

 to the locomotive habits of the insect. In the 

 common fly it is terminated with a pair of disks, 

 which are covered with suckers, called pulvilli 

 (Fig. 198, PL 7). By means of these suckers the 

 animal is enabled to lay hold of smooth surfaces, 

 and thus to crawl up them. They also exude a 

 glutinous matter, which assists in this process. 

 The same kind of arrangement is observed in the 



