::s2 



J. D. Tothill — The Ancestry of Insects. 



In some respects, therefore, the trilobites, especially such 

 generalized forms as Mesonacis (fig. 6) and Paradoxide's, fulfill 

 the requirements of our hypothetical insect ancestor. The 

 absence of spiracles and the general lack of strong convincing 

 evidence of close relationship indicates, however, that if a 

 relationship exists, as seems quite likely, it is scarcely as close 

 as supposed by Handlirsch. 1 will return to the group later. 



Fig. 7. 



Fig. 7. A. Lithobius adult. A representative chilopod. (After Koch.) 

 B, Lithobius ; a, Antennae ; b, Maxillipedes ; c, Brain ; d, Salivary glands ; 

 e, Legs ; /, Ventral nervous system ; g and h, Malpighian tubes ; i, Vesicula 

 seminalis ; j and k, Accessory glands. (After Vogt and Yung.) 



The Progoneata, characterized by the Diplopoda, are by 

 general agreement of myriapod students less closely related to 

 the Opisthogoneata (Chilopoda) than they are to insects. They 

 throw little or no light upon the present discussion. 



Turning to the Chilopoda, there are no fossil forms yet 

 known that throw much light upon their lineage, even Palceo- 

 carnpa is distinctly disappointing. Recent forms are, how- 

 ever, plentiful and illuminating. A dissection of Scolopendra 

 shows that the nervous system is almost identical with that in 

 generalized insects. In Lithobius (fig. 7, A and B) the num- 

 ber of trunk segments is reduced to 16 (compare fig. 3) ; a 

 tracheal system histologically identical with that of insects is 

 well developed ; also the last pair of legs is larger than the 



