THYSANURA. 



135 



Oedee I. — THYSANUEA. 



This order comprises the lowest, simplest six-footed insects. They are called Thy- 

 sanura (from the Greek thusaneros, hairy or bristly, and oura, tail), in allusion to the 

 hairy or bristly abdominal appendages of some of the typical forms. They are all 

 wingless, and undergo no metamorphosis. There is a great variation in their form 

 and structure, especially in the parts of the mouth, and it is difficult to put into few 

 words a general account of them. They are divided into three groups of sub-ordinal 

 rank. 



Jis. 196. — Popmas. «. spring. 5. sucker. 



StJB-OkDEE I. — COLLEMBOLA. 



These forms, for which the common name spring-tails may be adopted, are small, 

 often of microscopic size; their mouth-parts are partially abortive, while they are 

 enabled to leap by means of a curious spring be- 

 neath the abdomen. A distinctive feature of the 

 CoUembola, and one which gives the name to the 

 group, is the remarkable organ situated on the lower 

 surface of the basal segment of the abdomen. It 

 is a small tubercle, consisting of two valves, from 

 between which is thrust a flesliy sucker, or in 

 Smynthurus a pair of long tubes, which are capable 

 of being darted out on each side of the body, en- 

 abling the insect to attach itself to small objects, 

 and even to stand upside down. There are two families. These forms are often col- 

 lectively known as podurans, and furnish the " podura scales " used by microscopists 

 as test objects. 



The curious little creatures, forming the family Podttbidje, occur everywhere 

 except in dry places. Besides a tracheal respiration they undoubtedly breathe directly 

 through the skin, and therefore inhabit moist places. They take up their abode under 

 stones in damp, grassy, shaded spots, live in damp cellars, or hide under mushrooms 

 and logs ; we have seen them in greatest abundance in hot-beds, leaping in the air, 

 and appearing like a shower of sand. One species (^Anurida maritima) is often found 

 under stones at tide-marks. A species of Achoreutes is sometimes found floating in 

 large masses on the surface of roadside puddles, or under the bark of old trees ; and 

 one species sometimes abounds on the surface of the snow. On 

 the glaciers of the Alps there is a species peculiar to that unusual 

 habitat. 



The podurids, as well as the sminthurids, leap by means of a 

 curious two-forked ' spring,' which bends under the body, and when 

 at rest is retained in place by a sort of catch ; when the muscles of 

 this catch are relaxed it lets the spring fly back, and this sends the 

 little creature into the air, out of harm's way. The facility with 

 which they leap is a constant source of vexation to the zealous col- 

 lector, but this trait may be turned to advantage by cautiously 

 placing over them a vial, into which they may be made to leap. The 

 podurids are thick-bodied ; the segments, though in the Thysanura 

 cyrtus gibbuius. generally of equal size, are in this family inclined to become unequal. 



