326 Proceedings of the Royal Irish Academy. 



three tlioracic segments, each with, its pair of legs, are succeeded by- 

 ten abdominal segments. Of these latter, the second to the ninth 

 bear short unjointed appendages inMachilis ; while the tenth, in many 

 of the' more generalized insects, carries a pair of jointed cercopods. 

 Then comes a small terminal anal segment. But the researches of 

 Heymons ('95) have shown that the segment on which the cercopods 

 arise in the embryo, is in reality the eleventh abdominal, which, as 

 growth proceeds, becomes fused with the tenth. It has long been 

 known that rudiments of limbs appear on the abdominal segments of 

 many insect-embryos. This fact, in conjunction with the abdominal 

 appendages of Machilis and other Thysanura, leads us naturally to 

 conclude that the ancestors of insects had limbs on all the segments of 

 the body, except the anal segment. With confidence, therefore, we 

 can believe that the most primitive insects possessed a head with five 

 post-oral limb-bearing segments, completely fused, a " neck " segment, 

 undergoing fusion with the head, three thoracic segments with well- 

 developed legs, and an abdomen of twelve segments, whereof the first 

 ten carried poorly-developed limbs, the eleventh a well-developed pair 

 of cercopods, while the twelfth or anal segment had no appendages. 

 As no insect is hatched in the winged stage, and as the young of so 

 many insects are Thysanuriform, there need be no hesitation in 

 concluding that the ancestral insects were wingless. And it is 

 reasonable to conclude that the pedigree might be traced farther back 

 still to animals with a head with paired eyes and five limb-bearing 

 segments, and a trunk with sixteen undifferentiated segments, whereof 

 all but the last carried paired appendages. (See Table, pp. 354-5.) 



Relationships hetween Insects, Centipedes, and Millipedes. 



But it may readily be objected that Centipedes and Millipedes are 

 less highly organized than Insects — to which class nevertheless they 

 are related — and that they possess a larger number of limb-bearing 

 segments than the Insects have. Therefore, it may be argued, Insects 

 must have been derived from ancestors with numerous segments. 

 This objection, however, is by no means serious, and rests largely on 

 the assumption that "rich segmentation" must, of necessity, be a 

 primitive character among Arthropods. The absence of wings in 

 Centipedes, and the similarity of most of the body-segments and their 

 appendages, are doubtless primitive characters. But it is quite as 

 likely that, compared with the ancestral stock, the number of segments 

 should have increased as that they should have suffered reduction. 



