26 MR. LUBBOCK ON THE DISTRIBUTION 



occur in certain organs. It will be sufficient here to mention the dorsal tracheae in 

 certain Coleopterous larvae ; the ganglia, anterior part of the stomach, oviduct, and heart 

 of Bomhus ; the crop and ganglia of Carabtis ; the ca^ca and stomach of Acheta ; the 

 stomach of Tentliredo, Athalia, and lAbellula. 



These instances, and many others which might be given, show that inosculations of the 

 larger tracheae, so far from never occurring, are, on the contrary, by no means uncommon. 

 The second principle enunciated by Dr. Williams, viz. that the spiral tracheae are mere 

 conduit pipes and have nothing to do with the act of respiration, is less easily disproved, 

 though I believe quite as erroneous as the preceding. Of course I do not mean to deny 

 that the absorption of air is probably more rapid at the thin ends of the tracliea3, where 

 the spiral is absent or rudimentary and the tracheal walls thinner. Still upon this Dr. 

 Williams probably relies, since he scarcely gives any reason for the statement. 



On the ovary of Aphrophora a few large tracheae are given to the posterior end, and 

 there divide into about 200 branches. These branches (PL III. fig. 1) run the whole 

 length of the egg-tubes, a distance of xoth of an inch, without giving off a single branchlet. 

 They are o-o-outh of an inch in diameter at the base, and gradually taper until they become 

 excessively fine. At the base the spiral filament is very distinct ; gradually it becomes 

 less and less so, but it does not cease suddenly, and it can be traced for a considerable 

 distance up the branch. If, therefore, Dr. Williams's view were correct, no respiration 

 could take place in the ovary, except at its anterior end. 



Moreover, while I readily admit that in all probability very little respiration can take 

 place through the spiral filament itself, still it must be remembered that the coils of the spiral 

 leave between them a considerable space, which is occupied by a dehcate membrane. Why 

 Dr. Williams should suppose that no interchange of gases can take place through this 

 membrane, I know not ; and until some reason is given, I shall think myself justified in 

 concluding that the spiralled trachea3 are not mere conduit pipes, and that, although 

 respu'ation may proceed more rapidly at the fine extremities of the branchlets, still it is 

 not confined to these parts, but is carried on also in the larger vessels. It is true (as Dr. 

 Williams was, I believe, the first to observe) that in some cases the spiral rib ends suddenly; 

 this is generally the case where a trachea gives off at one point several small tubules, as 

 for instance in PI. I. fig. 8 ; but the sudden change does not occur in all insects, which led 

 me for some time to think that there must be some error in Dr. Williams's observations. 



We now come to Dr. Williams's third proposition, that the capillary or membranous 

 tracheae are always placed intermediately between larger trunks, standing to them in the 

 same relation as the capillaries of a blood-vascular system do to arteries and veins. 



I must confess that I am unable in any way to confirm this statement. I have never 

 seen the minute tubules gradually collect into efferent branches, nor have I been able to 

 find any evidence that there are two sorts of large branches — one afferent, and the other 

 efferent — as there ought to be according to Dr. Williams's hypothesis. In the blood- 

 vascular system we have arteries, capillaries, and veins ; in the aeriferous system of 

 insects we have only air-tubes, becoming, as a general rule, gradually smaller and smaller 

 as they proceed further from their origin. 



In most cases also, each trachea supplies a definite area, which receives no branchlets 



