442 T/-XT-BOOK OF ENTOMOLOGY 



so that the air in the trachea can directly communicate with the 

 external air. Usually one end of the muscle is attached to the clos- 

 ing peg, and the other end to the closing bow. Where, as in Melo- 

 lontha, the closing apparatus is provided with two levers, then natu- 

 rally the muscle binds these two together and brings about by 

 powerful contractions a firm closure of the trachea"; but, remarks 

 Krancher, " this is not the only kind ; there are numerous modifica- 

 tions. Besides the form just described, the levers assume the form 

 of valves (Sirex), or of a brush (Pulex) ; or of a ring (larvae of 

 Diptera) with a circular muscle attached to it ; or of a ring which 

 simply becomes compressed (thoracic stigmata of Diptera)." 



c. Morphology and homologies of the tracheal system 



As first shown by Butschli, the tracheal system is a series of seg- 

 mentally arranged tubular invaginations of the ectoderm ; a pair of 

 stigmata primitively occurring on every segment of the body except 

 perhaps the most anterior, and the last two or last one, a reduction 

 in their number having since taken place, until in the Podurans none 

 have survived. In the supposed ancestor of myriopods and insects, 

 Peripatus, there are tracheae ; but they are very fine, simple, not- 

 branched chitinous tubes which are united into tufts at the base of 

 a flask-shaped depression of the integument, the outer aperture of 

 which depression is regarded as a stigma. In one species (P. ed- 

 wardsii) these tufts and their openings are scattered irregularly over 

 the body ; but in another kind (P. capensis) some of the stigmata at 

 least show traces of a serial arrangement, being disposed in longi- 

 tudinal rows two on each side, one dorsally and one ventrally, 

 those of each row, however, being more numerous than the pairs of 

 legs. (See p. 9 and Fig. 4, D.} 



It should be observed that in Peripatus, which does not possess 

 urinary tubes, the segmental organs or nephridia are well developed, 

 hence the tracheal tubes coexisting with them cannot be their homo- 

 logues. We are therefore compelled to regard the tracheal system 

 as of independent origin, arising in the earliest terrestrial air-breath- 

 ing arthropod, and not indebted for its origin to any structure found 

 in worms, unless perhaps, as both Kennell and Lang suggest, to 

 dermal glands, since, according to Kennell, certain Hirudinea and 

 many Turbellariaii worms possess long, mostly unicellular, glands 

 which spread far through the parenchyma of the body. (Kennell.) 



Thus Kennell supposes that the ancestors of the Tracheates had spiracles on 

 every segment of the body where the internal organization allowed them to exist. 



