3o8 THE BIOLOGY OF INSECTS 



individuals of certain species or in a whole group. This 

 is a preliminary division of the insects which seems truly to 

 indicate such differences as express degrees of relationship. 

 It is instructive to recall that in classifications of a century 

 and a half ago bristle-tails and springtails were grouped with 

 Crustacea and Centipedes as *' Aptera." No doubt now 

 remains that, although they show certain crustacean afEni-' 

 ties they must be definitely included in the same class as 

 the winged insects, but recognised as a distinct sub-class. 

 This systematic treatment of the Apterj^gota is now generally 

 followed by entomologists. 



The bristle-tails and springtails form the vast majority 

 of the Apter}'gota, and the features which distinguish the 

 one group from the other may serve as samples of the 

 discrimination between two exceedingly well-defined orders 

 of insects. Bristle-tails have long, many-jointed feelers, 

 usually possess compound eyes, and a typical insectan 

 abdomen of ten segments the last of which bears a pair of 

 conspicuous appendages, usually long and many-jointed 

 like the feelers ; as many as eight of the other abdominal 

 segments may carry paired limbs — typically unjointed 

 stylets — and usually there are prominent reproductive 

 processes. Springtails, on the other hand, have relatively 

 stout feelers with four (rarely six) segments, their eyes are 

 always of the simple ocellar type, and the number of 

 abdominal segments is reduced to six, of which only the 

 third, first, and fourth bear appendages concerned with 

 locomotion and forming respectively the ventral tube, the 

 catch, and the spring. The bristletails (Fig. 52, b) are 

 therefore separated as an order (Thysanura) from the 

 springtails (Collembola, see Figs. 66, 70). 



Our review of the development of winged insects after 

 hatching (Chap. VII) showed a strildng divergence in the 

 method of wing-growth, between such groups as cock- 

 roaches, grasshoppers, dragon-flies, and bugs, in which the 

 wing- rudiments appear early outwardly on the thorax, 

 increasing in size at each moult, and on the other hand, such 

 groups as the beetles, two-winged flies, butterflies and 



