VI 



ANTENNA TA—PHYLOGENY 



503 



ones are gi-adually added posteriorly during tlie many moults undergone by the 



animal. The young of the Scutigcridcc and Litliobiidce [Ohilopoda) have 7 pairs 



of legs. The number then increases to 15. The young Diplopoda.{ Fig. 357), on 



the contrary, have only 3 pairs of feet on the 3 anterior trunk segments, and a few 



posterior segments still limbless. They 



thus recall the type of the Insect larvfe. 



New segments gradually appear posteriorly 



and the number of legs increases. After 



each moult the number of rings is greater ; 



the increase generally takes, place very 



irregularly, so that [e.g. Polydesmidce) 



stages Avith 7, 9, 12, 15, 17, 18, 19, and 



finally 20 rings, succeed one another. 



From the above we see, firstly, that a sort 



of metamorphosis takes place in many 



Ilyriapoda, and, secondly, that the body 



there differentiates from before backward, 



a point which can no longer be made out 



in the Iiisecta. 



XIV. Phylogeny. 



Of the Antennata now living, the 

 Symphyla, perhaps, stand nearest the 

 common racial form. Yet even they are 

 one-sidedly developed, and many of their 

 organs, and above all, the tracheal system, 

 by no means show a primitive arrange- 

 ment. From the common racial form of 

 the Antennata, the Myricqmda branched 

 off to the one side, and the Hexapoda to 

 the other. The different orders of ilyria- 

 poda perhaps developed polyphyletically, 

 while for all Hexapoda we can assume one 

 common racial form, resembling the now 

 living A'ptcryyoia, and especially the Thysanura. There is thus no special reason 

 for considering the A^itcrygota as originally winged insects, which became sexually 

 mature at progressively earlier stages of development, and finally at a larval stage. 

 At least one reason against such a supposition is the occurrence of the protrusible 

 vesicles in the abdomen of the Thysanura, which is present in the Myriapoda 

 [Lysiopetalidce and Symphyla), but almost entirely wanting in the Pterygota, 

 only one pair appearing temporarily on the foremost abdominal segment in the 

 embryo. 



The racial form of the Pterygota is to be derived from the Apterygota-like racial 

 form of all Hexapoda, fi-om which the various orders of insects have been produced. 

 These have of course developed independently of one another. Those orders, how- 

 ever, whose members undergo a gi'adual or incomplete metamorphosis, have retained 

 the original characteristics to a larger extent than the rest. Of the remaining 

 orders, again, it is the Lepidoptera, HyinenopUra, and Diptera which are furthest 

 removed from the racial form, and which reach the highest development among 

 the Insecta. 



Regarding the relation of the Antennata to the Protracheata, there can be no 



Fig. 367. —Larva of Polydesmus oom- 

 planatus, just hatched (after y. Bath). Ibr, 

 Upper lip ; a, aatenna ; 6f, sides of the head 

 (cheeks) ; gch, gnathochilarium ; &i, &2, &3, the 

 three pairs of legs of the larva ; sd, glands 

 (saf tdriisen) ; an, anus. 



