TRACHEATA 333 
stones or dried leaves, on roofs, ete., and some of them live on 
the surface of the water, upon which they move actively. 
The head bears a pair of antennae with few joints (4-8). 
The prothorax is usually compressed; there is no trace of 
wings, and no evidence that the group ever possessed them. 
Each of the three thoracic segments bears a pair of legs with 
four or five joints. 
The abdomen is short; the Sminthurinae have apparently 
only two or three segments, the Podurinae six. On the ventral 
side of the first abdominal segment is a structure known as 
the “ventral tube”; this in Podura and Lipura is a simple 
tubercle, but in other genera it takes the form of a tube, which 
in Sminthurus is divided into two halves at its end, from each 
half a long delicate tube can be protruded at the will of the 
animal. The ventral tube is essentially a protrusible part of 
the integument, it may be compared with somewhat similar 
structures in the Thysanura. Its function is possibly an ad- 
hesive one. The springing apparatus consists of a forked 
process borne on the fifth, or, as is stated in Podwra, on the 
fourth segment; this process is directed forwards, and in those 
species which jump the best, it is retained in position by 
two chitinous hooks which form the “catch”; these hooks in 
Tomocerus are borne on the third abdominal segment. The 
spring acts by the process pressing violently against the 
ground, and the insect is thus propelled into the air; the 
process is then folded under the abdomen again, and retained 
in position by the catch. This saltatorial apparatus is absent 
in some species, as Lipura, Anura, ete. 
The nervous system consists of the usual chain of ganglia, 
but the number in the abdomen is reduced. In Sminthurus 
and others there is only one abdominal ganglion. Eyes may 
be absent or present, in the latter case they consist of two 
little groups of at most eight simple eyes. 
The mouth appendages are, as is usual in insects, a pair 
of mandibles and two pairs of maxillae; the first pair are 
provided with palps, and the second are partially fused into 
an under lip. All the mouth appendages can be withdrawn 
into a cavity, and in this respect the Collembola resemble 
the Myriapod Scolopendrella. The alimentary canal is a 
