441 TEXT-BOOK OF ENTOMOLOGY 



Both Kennell and Lang derive the coxal glands of Arthropoda from the seti- 

 pamus or parapodial glands of annelid worms, and the recent endeavor of Bernard 

 to show that the tracheae arose from setiparous glands seems to be disproved by 

 tin- fact that in insects as well as in other Arthropoda coxal glands with their 

 outlets exist in the same segments as those bearing stigmata. Keasoning by 

 exclusion, we are led to regard Kennell's original view as the soundest. 



Patten, however, regards the tracheae as modified ends of nephridia, remark- 

 in- : " Since in Acilius some of the abdominal tracheae at first communicate with 



o 



the cavities of the mesoblastic somites, it is probable that all the tracheae repre- 

 sent the ectodermic portions of the uephridia." (Origin of Vertebrates from 

 Arachnids, p. 355.) 



It is probable, therefore, that the trachea? first arose as modifica- 

 tions of dermal glands, as in mites and Peripatus, and that at first 

 they were not provided with tsenidia (as in Chilopoda), while in later 

 forms taenidia were developed. In the earliest tracheate forms the 

 stigmata were not segmentally arranged, probably appearing irregu- 

 larly anywhere in the body, but afterwards in the myriopods and 

 insects became serially arranged. 



d. The spiral threads or taenidia 



It is generally supposed that the so-called " spiral thread " forms 

 a continuous thread from one end of a tracheal branch to the other. 

 This was first shown not to be the case by Platner in 1844. Minot 

 has proved that " there is not a single spiral thread, but several, 

 which run parallel to one another and end after making a few turns 

 around the trachea." 



The tsenidia we have found to be in some cases separate, indepen- 

 dent, solid rings, though when there is more than one turn the 

 thread necessarily becomes spiral. The taenidia of a main branch 

 stop at the origin of the smaller branches, and a new set begins at 

 the origin of each branch. The taenidia at the origin of the branch 

 do not pass entirely around the inside of the peritoneal membrane ; 

 in the axils they are short, separate, spindle-shaped bands (Fig. 409). 



At one point in the main trachea of the larva of Datana the taenidia were 

 seen to end singly on one side (at a considerable distance from any branch or 

 axil) at intervals, with a tfenidium situated between them, making four or five 

 turns; then there is only one band situated between two ends; this band or 

 thread is succeeded by a set with five turns between the two ends, this set being 

 succeeded by one complete ring situated between two ends ; in all cases the ends 

 vary in length, some threads being short and others long, so that they apparently 

 end anywhere along the circumference of the trachea, and this arrangement is 

 seen to apparently extend along the whole length of the trachea. Hence it is 

 seen that as a rule the taenidia vary much in length, and never, as generally 

 supposed, pass continuously from one end to another of a tracheal branch, for 

 there are many spirals in a branch, each making only from one to five turns, 



