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 les 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 Collembola, 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 Collembola 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 Collembola, 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 Japyzx on the first to the seventh, in Lepisma only 
on the eighth and ninth—in the last-mentioned insect the 
