TRACHEATA 333 



stones or dried leaves, on roofs, etc., 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 Podura, 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, etc. 



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 All the mouth appendages can be withdrawn 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 CoUembola resemble 

 the Myriapod Scolopendrella. The alimentary canal is: a 



