328 THE BIOLOGY OF INSECTS 



(Figs. 52, 79). Comparison of Thysanura with the Col- 

 lembola, the other important order of the Apterygota, 

 indicates that the latter have departed far from the primitive 

 type mainly through a degenerative specialisation, for the 

 number of abdominal segments is reduced to six, three of 

 v^^hich normally carry paired appendages, partly fused 

 basally and greatly modified to form respectively the ventral 

 tube, catch, and spring (Fig. 66). That the Collembola 

 are, despite their extreme modification, truly akin to the 

 Thysanura is evident from the fact that one of their most 

 remarkable features — the retraction of the jaws into the 

 head-capsule, a condition associated with a striking speciali- 

 sation of the maxillae and tongue — is found also in two 

 families of the Thysanura, the Campodeidae and lapygidae, 

 which are often relegated to a special sub-order and 

 sometimes even to a distinct order, as in the scheme of 

 C. Borner (1904). 



When the naturalist tries thus to deduce relationships 

 between groups of animals by a comparative study of their 

 structure, he desires to check his conclusions by knowledge 

 of the past history of the groups as revealed by fossils. As 

 far as the Apterygota are concerned this is impossible because 

 such frail terrestrial creatures as bristle-tails and springtails 

 have little chance of preservation in sedimentary rocks 

 accumulated under water, and our only certain knowledge 

 about extinct Apterygota is derived from some Collembola 

 closely like living types, embalmed in the Baltic amber of 

 early Tertiary (Oligocene) Age. Fragmental remains in 

 rocks of Devonian Age have been referred to the Thysanura, 

 but no reliance can be placed on them as evidence that 

 bristle-tails lived in that remote period. No doubt, how- 

 ever, can be felt that thysanuroid insects were living then, 

 because the existence of winged insects of various types in 

 the succeeding Carboniferous period is abundantly attested 

 by fossil remains. 



In our discussion on the growth and transformation of 

 insects in Chap. VII (pp. 160-194) reasons were given for 

 the belief, now generally accepted by all students of the 



