364 The Pedigree of Insects 



Branch can be solved only by means of the facts 

 drawn from a study of structure and life-history. 

 There are a few points in the embryonic growth of 

 insects which help us to realise the form of their 

 ancestral stock a little earlier in the history of life's 

 unfolding, than the evidence of fossils can carry us 

 back. The vestigial appendages between the feelers 

 and mandibles, and on the abdominal segments, to- 

 gether with the manner of growth of the cercopods 

 (pp. 93-4), show that the progenitors of six-legged 

 insects had a head bearing five pairs of appendages, 

 and a body of fifteen segments, whereof all but the 

 last carried limbs. The limbs of the first three 

 body-segments (the walking-legs of modern insects) 

 and of the fourteenth (the cercopods) must, at an 

 early period, have become more important than the 

 intervening pairs, which now survive only as the 

 vestigial limbs of Bristletails. These many-legged 

 animals which, we must believe, preceded our six- 

 legged insects, can now be compared with other 

 classes of Arthropods. 



Symphyla. — The closest general likeness to 

 insects among animals not actually classed with 

 them is to be seen among some small, fragile, 

 white creatures, living in damp earth and similar 

 concealed situations, with a head carrying a pair 

 of feelers, a pair of mandibles, and two pairs of 

 maxillce, and an evenly segmented body with fourteen 

 pairs of limbs. These little animals make up the 

 single genus Scolopetidrella, and they are usually 

 reckoned as a distinct class — Symphyla — though 

 their affinity to the Thysanura is probable (2). The 

 head and its appendages probably correspond with 

 those of a typical insect, the original five pairs being 

 reduced to four, though both pairs of maxillse are 

 to a great extent fused together j while the number 



