THYSANURA. 59 



This appendage consists of a basal segment and two appendages ; 

 which are sometimes two-jointed, and sometimes consist of a single 

 piece each. This appendage constitutes the spring. On the ven- 

 tral side of the third abdominal segment in certain genera is also an 

 appendage, which is termed the catch. This catch passes between 

 the branches of the spring and holds it in place. By relieving the 

 spring and extending it suddenly the insect is able to throw itself to 

 a considerable distance. 



Some of the Collembola are clothed with scales, resembling in 

 this respect butterflies and moths. These scales are very minute, 

 and wonderfully sculptured. The markings on them are so minute 

 that it requires a very good microscope to resolve them. On this 

 account they have been much used as test objects for microscopes 

 under the name of " Podura scales." 



The most remarkable feature in the structure of the Collembola 

 is that in nearly the entire sub-order tracheae are wanting; while in 

 certain members, SmyntJirus, there is a well-developed respiratory 

 system. The difference is a great one to exist between genera other- 

 wise so closely allied. 



In certain respects the Collembola is the lowest of the three sub-orders of 

 the Thysanura ; and it is so placed in some of the standard works on entomol- 

 ogy. The mouth-parts are very rudimentary in this sub-order; and the tra- 

 cheae are rarely developed. But it has seemed more natural to me to follow 

 the Myriapoda by Scolopendrilla ; and to place next the sub-order Cinura, 

 containing, as it does, forms with rudimentary abdominal legs. We have thus 

 an unbroken series from the Myriapodous to the Hexapodous condition. In 

 the Collembola we have insects which, while retaining a very primitive form 

 of the mouth-parts and of the respiratory system (the result of their con- 

 tinuing to feed on soft substances, and to live in damp situations), present a 

 much modified form of the body, and the development of peculiar special or- 

 gans, the spring and the ventral tube or sucker. We thus see well illustrated 

 the difficulties, referred to elsewhere, of any attempt to arrange animals in a 

 linear series. 



The Collembola is divided by Lubbock into six families. These 

 can be separated by the following table : 



A. Saltatorial. 

 B. Body globular. 

 C. Terminal segment of abdomen short, with whorls of hair. 



1. Papiriid.e. 

 CC. Terminal segment long, ringed. 2. S.mvxthurid/E. 



BB. Body cylindrical. 



