334 ZOOLOGY 



straight tube stretching between the mouth and anus. It 

 is at present unsettled whether any salivary glands or Mal- 

 pighian tubules exist. 



The heart, which lies in the middle dorsal line in Macro- 

 stoma, is said to have five pairs of valves. The blood is 

 yellowish and corpusculated. It seems doubtful if a tracheal 

 system exists in many of the CoUembola, but in the larger 

 Sminthurinae a pair of stigmata open on the under side of 

 the head, a very unusual position ; from these bundles of 

 tracheae radiate. 



Like all other Insects, the CoUembola are dioecious ; there 

 is very little external difference between the sexes. The 

 developement is direct. 



Sub-order 2. Thysanura. 



The Thysanura are of larger size than the CoUembola, and 

 have to a much greater extent the , appearance of insects. 

 One of the most familiar genera is Lepisma, often termed the 

 "silver-fish," a quickly -running, silvery-gray insect, which 

 infests old chests of drawers and disused cupboards. It is a 

 nocturnal insect, and hides away during the winter. The 

 genus Machilis is found in woods, etc., or on rocky sea-coasts, 

 where it lives between stones or in clefts of the rock, but it 

 loves to run on warm sunny places. Campodea is found under 

 fallen leaves or stones in shady places and in loose earth. Japyx 

 also shuns the light ; it is found widely distributed in Europe, 

 but not in cold places. 



The number of abdominal segments is always ten, and the 

 abdomen ends in three long many-jointed processes or cerci. 

 The three thoracic segments each bear a pair of legs ; in 

 Machilis the coxae of the two posterior pairs of legs bear pecu- 

 liar processes, which externally resemble certain paired processes 

 found on the ventral surface of the abdomen in different 

 species of Thysanura. These processes, however, are regarded 

 by some authorities as the representatives of abdominal limbs ; 

 in Machilis they are found on the second to the ninth seg- 

 ments, in Japyx on the first to the seventh, in Lepisma only 

 on the eighth and ninth — in the last-mentioned insect the 



