24 THYSANURA. 



while these latter have a suctorial moutli, that of the 

 Bird-lice is formed for biting, consequently the true louse 

 with its sharp retractile proboscis, will come under the 

 Order Rhyncota, from which the other is excluded on 

 account of the structure of its mouth ; so difficult a 

 matter it often is to form a classification that shall 

 include and exclude all that is required. But not- 

 withstanding the difference between the mouths of the 

 true lice and the bird-lice, the general similarity of the 

 whole structure of the animals will authorize us to place 

 them both in the division of the Rhyncota. 



The Spring-tails {Poduridce) and Silver-scales {Lepis- 

 midce), first grouped together by Latreille under the 

 name of Thysanura, from the fringed tails of some of 

 the species, have lately been formed into two distinct 

 orders by Sir John Lubbock, under the names of 

 Collemhola and Thysanura, in his valuable "Mono- 

 graph for the Ray Society." They frequent dark places, 

 but while the former, for the most part, prefer moist 

 situations, and can endure great cold — I have seen 

 numbers on ice under stones on the Swiss glaciers — the 

 latter like dry walls and warm rooms. In the Collem- 

 bola there is a remarkable organ underneath the ab- 

 domen, called the ventral tube or sucker, whose function 

 is to enable the creature to adhere to surfaces by the 

 emission of some viscous fluid. From this Sir John 

 Lubbock has proposed the name of the order.* In most 

 of the CoUembola the tail, which is forked, is bent under 

 the body, forming a jumping organ, by its sudden ex- 

 tension ; in some species there is no saltatory organ. 



* From fcoXXj; " glue," and tn^oXoq " a peg." 



