OF THE TRACHEA IN INSECTS. 25 



Bombus, wliere tufts are present in most of the organs, the tubules are particularly 

 numerous on the ovary. On the other hand, this rule is not without numerous excep- 

 tions — as, for instance, Ophion (PL III. fig. 7), Acheta (PL III. fig. 12), A-phrophora 

 (PL III. fig. 1) ; and it would perhaps not be safe to generalize on the subject. 



In every insect which I have examined, the ganglia were well supplied with tracheae, 

 and the type was very uniform, showing, I think, that the minute structure of these 

 organs is very similar, not only in different parts of the nervous column, but also in 

 different insects. 



As regards the commissures, the case is quite the reverse : not only does the distribu- 

 tion of the tracheae difii'er much in different groups, but in some, as, for instance, in the 

 Neuroptera and Lepidoptera, they are, at least in the perfect insects, almost entirely absent. 

 This difference probably arises from the difference in the respective properties and func- 

 tions of the ganglia and commissures. It is an interesting fact, that while in butterflies 

 and moths I have generally found the commissures free from tracheae, in the larvae they 

 are richly supplied. Is this to be accounted for by supposing that the relative functions 

 and structures of the different parts belonging to the nervous system are not so com- 

 pletely differentiated in the larva as they afterwards become in the perfect insect ? 



The presence of tracheae on the commissures is not, however, always a sign of low 

 development, since they are present in many Coleoptera. 



Both the larva and perfect insect of Acheta have tracheae on the thoracic, but hardly 

 any on the abdominal commissures. The presence of a few tracheae cannot, however, be 

 of any great functional importance, since in insects with double commissures I have more 

 than once seen instances in which one only was so provided. 



Dr. Williams*, in a paper on the respiration of insects, enu.nciates very confidently 

 the following proj)ositions, which he considers to be true without any exception, and 

 which he has since reiteratedf : — - 



1st. That the larger tracheae never anastomose ; that " in the sinral tracheae no 

 plexiform union of the branches ever anywhere occurs." 



2ndly. " That the ' spiralled ' or larger tracheae are mere conduits, like arteries or 

 veins, and have nothing to do with, take no part in, the ultimate act of respiration." 



Srdly. " That the peripheric or extreme distribution of the tracheal system is conform- 

 able in plan to that of a blood-vascular system ; that is, the capillary or membranous 

 tracheae are always placed intermediately between larger trunks, the branches of which 

 they serve to connect, — standing to the larger trunks in the same relation as the capil- 

 laries of a blood-vascular system do to arteries and veins." 



4thly. " That the tracheae can be discovered in no single instance to end in caecal 

 terminations — always in mutual inosculations." 



I might have passed over the fu'st assertion as a mere momentary slip of the memory 

 if it were not repeated more than once in 1854, and again reaffirmed in 1856. Several of 

 the figures given by Straus-Dtirckheim, Leon Dufour, and other writers, show inos- 

 culations of the tracheae ; and there is hardly a single insect in which they do not 



* Annals and Mag. of Nat. Hist. 1854, vol. xiii. p. 194. f lb. 1856, vol. svii. p. 347. 



VOL. XXIII. E 



