SUB-ORDER— THYSANURA. 15 



composing vegetable and animal matter. The 

 metamorphosis is imperfect, and most of these 

 creatures cannot stand a dry atmosphere; they can 

 bear heat and cold, but not dryness. 



There are two Sub-Orders: — (i) Thysanura or 

 Silver Fish group; (2) Collembola or Spring-tails. 



(i) SUB-ORDERr-THYSANURA. 



Family — Lepismidae. 



These insects are about Jin. to under i inch in 

 length, and are covered with scales, giving the 

 body a dull silvery metallic appearance. 



The thoracic segments differ from those of the 

 abdomen and also from one another; they are 

 rounded at the edges; there are 10 segments of 

 the abdomen ; the tracheal system is well 

 developed. The body tapers towards the end 

 and there are five appendages. There are 

 three long appendages easily distinguished with 

 the naked eye ; the middle one is twice as long as 

 the two lateral cerci, as they are called. Between 

 these there are two much shorter appendages (seen 

 with a lens). All these five caudal (belonging to 

 a tail) appendages are jointed with tufts of hair 

 at each segment. Some of the hairs (when exam- 

 ined with the low power of microscope) are seen to 

 stand out at an angle to the appendages, while the 

 majority of them lie close to the sides. 



The antennae are long, and consist of a number 

 cf segments similar to the caudal appendages but 

 smaller. The hair tufts are seen here too. Hair 

 tufts occur at each segment of the body of the 

 silver fish. When the scales are examined with 

 the microscope, they are seen to be very beautiful, 



